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Synthesis of novel salts with HF, AsF3 and XeF2 as ligands to metal cations
Journal of Fluorine Chemistry 127 (2006) 1275–1284 www.elsevier.com/locate/fluor
Review
Synthesis of novel salts with HF, AsF3 and XeF2 as ligands to metal cations Melita Tramsˇek, Boris Zˇemva * Department of Inorganic Chemistry and Technology, Jozˇef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia Received 17 March 2006; received in revised form 12 May 2006; accepted 16 May 2006 Available online 23 May 2006
Abstract A review of all known compounds of the type [Mn(L)m](AF6)n (M is a metal in the oxidation state n; A = P, As, Sb and Bi; L = HF, AsF3 and XeF2) is given with the emphasis on the compounds isolated and characterized by our group. The synthetic routes for the preparation of these compounds are given together with a brief analysis of their structures. In the case of L = XeF2 the influence of the properties of the cation and the anion on the structural diversity of these coordination compounds is discussed. A brief analysis of their Raman spectra is also given. # 2006 Elsevier B.V. All rights reserved. Keywords: Ligands to metal ions; HF; AsF3; XeF2; Syntheses; Structures
Contents 1. 2.
3. 4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Coordination compounds with HF as a ligand to metal cation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Synthesis and structure of [La(HF)2](AsF6)3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Synthesis and structure of [Pb(HF)](AsF6)2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3. Synthesis and structures of [Ca(HF)](AsF6)2 and [Cd(HF)](AsF6)2 . . . . . . . . . . . . . . . . . . . 2.1.4. Synthesis and structures of [Mg(HF)2](SbF6)2 and [Ca(HF)2](SbF6)2 . . . . . . . . . . . . . . . . . 2.1.5. Synthesis and structures of other coordination compounds with HF as a ligand to metal ion . 2.2. Coordination compounds with AsF3 as a ligand to metal cation . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Synthesis and structure of [M(AsF3)3](AsF6)3 (M = Ce and Pr) . . . . . . . . . . . . . . . . . . . . . 2.2.2. Synthesis and structures of [M(AsF3)2](AsF6)2 (M = Fe, Co and Ni). . . . . . . . . . . . . . . . . . 2.2.3. Synthesis and structure of (H3O)4La2F(AsF3)2(AsF6)9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4. Synthesis and structure of [Sn(AsF3)2](SbF6)2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5. Synthesis and structure of [Ca(AsF3)](AsF6)2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Coordination compounds with XeF2 as a ligand to metal cation. . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Structural features in the coordination compounds of the type [Mn+(XeF2)p](AF6)n . . . . . . 2.3.3. Raman spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction
* Corresponding author. Tel.: +386 1 477 35 40/33 01; fax: +386 1 477 31 55. E-mail address: [email protected] (B. Zˇemva). 0022-1139/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jfluchem.2006.05.014
Metal hexafluorometalates, Mn+(AF6)n (A = P, As, Sb and Bi), where n is the oxidation state of the metal M, have rather low lattice energy as a consequence of the anion volume
M. Tramsˇek, B. Zˇemva / Journal of Fluorine Chemistry 127 (2006) 1275–1284
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˚ 3) [1]. Thus, almost ‘‘naked’’ metal centers (V > 100 A surrounded with weakly coordinating AF6 anions can interact with different, even weak ligands. Therefore, the compounds Mn+(AF6)n represent excellent starting materials for the preparation of novel salts of the type [Mn+(L)m](AF6)n with AsF3 and XeF2 as possible ligands (L). Anhydrous HF (aHF) was used as a solvent for the preparation of Mn+(AF6)n. In some cases the metal center can coordinate also weak ligands of the solvent forming coordination compounds of the type [Mn+(HF)m](AF6)n. In this paper, a review of all known coordination compounds with HF, AsF3 and XeF2 as ligands is presented together with the reaction systematics and available structures. In the case of XeF2 as a ligand also Raman spectra are included, providing a good insight into the strength and the type of XeF2 interactions with the cation (non-bridging or bridging XeF2 molecules). The compound [Cd(XeF2)](BF4)2 [2], so far the only fully characterized compound with BF4 anion and XeF2 as a ligand, is included in this study for comparison with compounds having AF6 anions. The anion BF4 is smaller and the negative charge on its fluorine ligands is higher than in the case of AF6. 2. Results and discussion 2.1. Coordination compounds with HF as a ligand to metal cation It has been known for a long time that metal fluorides dissolve in aHF acidified with Lewis acids (e.g. PF5, AsF5 and SbF5) yielding solution of solvated cations Mn+(HF)m and corresponding anions. The solid compounds of the type [Mn+(HF)m](AF6)n (A = P, As and Sb) were not isolated until recently [3]. In this chapter only coordination compounds with HF ligand coordinated directly to the metal cation are described. The compounds in which HF is coordinated to the metal center and further with strong hydrogen bonds connected to F ions forming poly-hydrogen-fluoride anions like HF2, H2F3 and H3F4 are not the subject of this discussion. 2.1.1. Synthesis and structure of [La(HF)2](AsF6)3 The first isolated coordination compound in which HF is bound directly to the metal cation was [La(HF)2](AsF6)3 [3]. The compound was prepared by the reaction between LaF3 and stoichiometric amount of aHF in AsF5 as a solvent at the temperature above critical temperature of AsF5 [4]: AsF5 ; 393 K
LaF3 þ 2HF ! ½LaðHFÞ2 ðAsF6 Þ3 t¼14 days
The polyhedron around the metal center is basically a tetracapped trigonal prism (Fig. 1), whose trigonal faces are formed by two fluorine ligands provided by AsF6 units and one F ligand provided by HF molecule. All rectangular faces of the prism are capped by fluorine ligands, being like the La cation within the crystallographic mirror plane. However, while one type of fluorine ligand provides common apexes with AsF6 octahedra,
Fig. 1. Coordination sphere of La in the [La(HF)2](AsF6)3 [3].
the other type of fluorine ligand provides common edge, respectively. Thus, coordination number 10 is finally achieved for the La atom. The hydrogen bonding with the distance F(As)–H at 192(7) pm and the angle F(As)–H–F at 166(9)8 is quite strong as these values are very similar to those in crystalline HF [5]. Although HF is certainly coordinated to metal centers in an aHF solutions of Mn+(AF6)n, attempts to isolate such species have long not been successful, probably because HF is quite a weak ligand. Keeping this in mind it is rather surprising that the F(H)–La distance (246.6(3) pm) is equal to most other La–F bonds (from 243.3(2) to 282.6(4) pm) in the present structure and also to those in LaF3 [6]. 2.1.2. Synthesis and structure of [Pb(HF)](AsF6)2 The reaction between PbF2 and excess of AsF5 in aHF as a solvent yields a colorless solution of solvated Pb2+ cations and AsF6 anions: aHF
PbF2 þ nAsF5 ! ½PbðHFÞx 2þ þ 2AsF6 þ ðn 2ÞAsF5 ; 295 K
n2 [Pb(HF)](AsF6)2 [7] was obtained as a white solid after removal of excess of AsF5 and aHF. It is very well soluble in aHF. When it is redissolved in aHF partial solvolysis always takes place and besides of the bulk of compound [Pb(HF)](AsF6)2 also some shiny crystals of PbF(AsF6) are obtained. The coordination number of the central Pb atom is 10 considering bond distances up to 306 pm (Fig. 2). There is one fluorine atom from HF molecule and nine fluorine atoms from AsF6 units. In the structure there are three crystallographically different AsF6 units. Around each Pb atom there are two As(1)F6 units, four As(2)F6 units and two As(3)F6 units. Each AsF6 unit interacts with four Pb atoms. In As(3)F6 unit the two non-coordinating fluorine atoms are in trans position while in As(1)F6 unit they are in cis position. As(1)F6 and As(3)F6 units bridge Pb atoms forming 16-membered heterocycles. As(2)F6 unit acts as a bidentate ligand forming a four-membered ring. Two of these units form eight-membered heterocycles. The distance Pb-F(HF) is the shortest distance in the structure
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Fig. 2. Coordination sphere of Pb in [Pb(HF)](AsF6)2 [7].
(248(4) pm) and is only slightly longer than the shortest Pb–F distance in PbF2 (241 pm) [8]. 2.1.3. Synthesis and structures of [Ca(HF)](AsF6)2 and [Cd(HF)](AsF6)2 The compounds [Ca(HF)](AsF6)2 [9,10] and [Cd(HF)](AsF6)2 [11] are isostructural. The crystals of [Ca(HF)](AsF6)2 were found in the same crystallization batch as crystals of [Ca(XeF2)4](AsF6)2, only several months later, after they had been isolated and stored in the dry box. The compound [Cd(HF)](AsF6)2 was found during the preparation of Cd(AsF6)2. First the coordination compound with HF is obtained which is slowly losing HF at room temperature in a dynamic vacuum finally yielding Cd(AsF6)2. In [M(HF)](AsF6)2, M atoms (Ca, Cd) are coordinated by six bulky AsF6 units and one HF molecule forming distorted tricapped trigonal prism (Fig. 3) with Ca–F(As) distances in the range of 231.5–246.2 pm and Cd–F(As) distances in the range of 226.3–232.6 pm. All Cd–F distances to the fluorine at
Fig. 3. Coordination sphere of Cd in the [Cd(HF)](AsF6)2 [11].
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the apexes of the coordination prism are shorter than those in the structure of CdF2 (Cd–F = 233.3 pm) [12] indicating that the Cd2+ cations bear a higher positive charge than those in CdF2. The HF molecule caps a trigonal-prismatic face at a distance of 239.6 pm (Ca), which is among the shortest Ca–F distances in the compound (Ca–F in CaF2 is 236.6 pm [13]), and a distance of 237.6(5) pm (Cd). Additionally, two AsF6 units interact with the metal atom via second fluorine atom placed over rectangular faces of the trigonal prism at slightly longer distances, Ca–F of 267.8(11) and 277.3(11) pm and Cd– F of 268.1(4) and 292.1(4) pm. The metal cation is thus located in an environment of nine fluorine atoms. Basic units in the structure of [M(HF)](AsF6)2 are square rings formed of two M atoms connected via two crystallographically different AsF6 units. Similar rings can be found in the structure of [Pb(HF)](AsF6)2 [7]. The rings are inter-connected via trans fluorine atoms forming zig-zag ribbons. Neighboring rings running almost perpendicular to each other share common M atoms finally resulting in a 3D-network. In accordance with the much higher electron affinity of Cd2+ (16.91 eV) than that of Ca2+ (11.87 eV), the M–F bond is expected to be less ionic in the Cd compound. In the Cd compound, fluorine atoms from the AsF6 units capping the side faces of the basic trigonal-prismatic polyhedra are at longer distances from the metal than those in the Ca compound, although the other Cd–F(As) distances are shorter (average 229.5 pm) than those in Ca–F(As) (average 237.6 pm). This indicates that Cd2+ cations bear a lower positive charge than Ca2+ cations, which is probably a consequence of the higher covalency of the Cd–F bonds. 2.1.4. Synthesis and structures of [Mg(HF)2](SbF6)2 and [Ca(HF)2](SbF6)2 The coordination compounds [Mg(HF)2](SbF6)2 and [Ca(HF)2](SbF6)2 [14], were prepared by the reaction of MgF2 or CaF2 with excessive SbF5 in aHF. The stability of these compounds depends among others also upon effective nuclear charge of the central atoms. The interactions are stronger when the central atom has a high effective nuclear charge and a small radius. In our case the compound [Mg(HF)2](SbF6)2 is more stable than [Ca(HF)2](SbF6)2 because the effective nuclear charge on Mg2+ is higher than on Ca2+. This was evident from X-ray powder diffraction patterns of the products after prolonged pumping under dynamic vacuum. In the case of Mg compound some lines which could be attributed to HF compound were consistently found while in the case of Ca only the lines which could be attributed to Ca(SbF6)2 were found. In Mg compounds metal atom is octahedrally coordinated to six fluorine atoms (Fig. 4). Two cis equatorial fluorine atoms are from two HF molecules, the other two cis equatorial fluorine atoms are from two trans bridging SbF6 units, while two axial fluorine atoms are from two cis bridging SbF6 units. The Mg–F distances in the equatorial plane are identical within experimental error (Mg–F(HF) is 197.1(3) pm and Mg–F(Sb) is 197.3(3) pm), while the axial Mg–F(Sb) distances are 194.9(4) and 194.3(4) pm. The Mg–F distances in rutile MgF2 are 4 198.4 and 2 197.9 pm [15].
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there is only one short Ca–F(Sb) distance while the others are longer. It appears that the negative charge is localized mostly on one of the interacting F ligands while the other terminal and bridging F atoms bear lower negative charges. The Ca–F(HF) distances are 230.4(3) and 231.5(3) pm. The Ca–F(Sb) distances range from 232.8(3) to 243.0(3) pm, indicating that the interactions with F ligands from the HF molecules are stronger than the interactions with F ligands from SbF6 units.
Fig. 4. Coordination sphere of Mg in the [Mg(HF)2](SbF6)2 [14].
Although the compounds [Mg(HF)2](SbF6)2 and [Ca(HF)2](SbF6)2 have analogous composition, the structures of both compounds are different due to the different sizes of the central atoms and consequently different coordination numbers. Ionic radius of Mg2+ (CN = 6) is 86.0 pm and of Ca2+ (CN = 8) is 126 pm [16]. The coordination sphere around the calcium atom in [Ca(HF)2](SbF6)2 consists of eight fluorine atoms in the shape of an Archimedian antiprism (Fig. 5). One square face of the Archimedian antiprism is formed by three F ligands from SbF6 and one F ligand from HF molecule. The second square face of the Archimedian antiprism is similar except that the HF molecule is crystallographically different. There are three independent SbF6 units in the structure. Sb(1)F6 and Sb(2)F6 units provide four bridging F ligands while the Sb(3)F6 groups provide two bridging F ligands to the Ca centers. It is interesting that regardless of the number of contacts of the SbF6 groups with Ca
2.1.5. Synthesis and structures of other coordination compounds with HF as a ligand to metal ion Among metal hexafluoroantimonates coordinated with HF the compound [Hg(HF)](SbF6)2 should be mentioned [17]. The compound was obtained when colorless material [HgXe]2+[SbF6][Sb2F11] came into contact with HF which replaced Xe atoms. Hg atoms are coordinated by seven fluorine atoms from SbF6 units and one F ligand from HF molecule. In the compounds [OsO3F(HF)2]AsF6, [OsO3F(HF)]SbF6 [18] and [Au(HF)2](SbF6)22HF [19], HF acts as a ligand to a metal center. In these cases two HF molecules connected via hydrogen bond bridge the metal center with AF6 unit. The distance Os–F(HF) in [OsO3F(HF)2]AsF6 is 228.2(5) pm [18]. The distance Au–F(HF) is 213.5(3) pm [19] and is comparable with Au–F(Sb) distance of 212.6(3) pm. Another example of the metal cation coordinated with HF is Mg(HF)AuF4AuF6 [20]. The attempt to prepare single crystals of Mg(AuF6)2 resulted in the synthesis of single crystals of Mg(HF)AuF4AuF6 instead. The compound represents the first example of mixed valence Au(III)/Au(V) ternary fluoride. The structure is built up of MgF6 octahedra sharing four equatorial F ligands with four AuF4 units (Mg–F distances range from 196.4(8) to 199.5(8) pm). All four fluorine atoms in AuF4 units are bridging and connect Mg atoms. Coordination sphere of Mg is completed with one F ligand from AuF6 unit (Mg–F distance is 200.3(10) pm and one F ligand from HF molecule Mg–F(HF) is 197.9(10) pm). 2.2. Coordination compounds with AsF3 as a ligand to metal cation Arsenic trifluoride can coordinate to metal cations in two different ways: (a) either via its fluorine ligands or (b) via its electron lone-pair:
Fig. 5. Coordination sphere of Ca in the [Ca(HF)2](SbF6)2 [14].
(a) Examples of AsF3 coordinated to a metal center via one fluorine atom are as follows: [M(AsF3)3](AsF6)3 (M = Ce and Pr) [21,22] [M(AsF3)2](AsF6)2 (M = Fe, Co and Ni) [23] and (H3O)4La2F(AsF3)2(AsF6)9 [24]. The coordination of AsF3 via more than one fluorine atom is exemplified by [Sn(AsF3)2](SbF6)2 [25]. (b) The only coordination compound known so far in which AsF3 is coordinated to the metal cation via its electron lone-pair is F3As–Au+SbF6 [26]. During the reduction of AuF3 to Au2+ and further to Au+ under strong acidic conditions (SbF5/HF) and addition of AsF3 as a mild reducing agent, F3As– Au+SbF6 was obtained. The distances As–F are from 167.0(7) to 168.5(7) pm. The distance As–Au is 226.8(1) pm.
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2.2.1. Synthesis and structure of [M(AsF3)3](AsF6)3 (M = Ce and Pr) The single crystals of the compounds [M(AsF3)3](AsF6)3 (M = Ce and Pr) were prepared in a nickel reaction vessel with a Teflon liner by the reaction between MF3 and HF in supercritical AsF5 during 7 months. It seems that during the reaction some reduction of AsF5 to AsF3 occurred [21], which was the reason for the formation of the coordination compounds with AsF3 as a ligand. The coordination number of Ce and Pr in these compounds is nine (Fig. 6). Six fluorine atoms, from six AsF6 groups form a regular trigonal prism. All three rectangular faces of the trigonal prism are capped by fluorine atoms from three fluorine-bridged AsF3 molecules. Each AsF6 is trans connected to the other lanthanide centers forming tridimensional network. In the case of compounds [Ln(XeF2)n](AF6)3 (Ln = La and Nd; A = As and Sb) the cis connection of AF6 groups with other lanthanide centers resulted in the formation of chains ([Nd(XeF2)3](SbF6)3) or double chains ([Nd(XeF2)2.5](AsF6)3) [27,28]. Both compounds (Ce and Pr) are isomorphous. Due to lanthanide contraction the unit cell in the case of praseodymium is smaller than in the case of the cerium compound. The distance As–F(b) is from 178.9(Ce) to 180.4(Pr) and the distance As–F(t) is from 161.3(Pr) to 164.2(Ce). This is consistent with the stronger attraction of a fluoride ligand of AsF3 in the case of praseodymium than in the case of cerium.
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by six fluorine atoms. Four fluorine atoms in the equatorial plane are from AsF6 species, which bridge transition metal atoms via trans fluorine ligands. The consequence of such arrangement is the formation of a layer composed of a square network. Apical fluorine atoms are from AsF3 molecules. Distance As(III)–F(b) is 180.1(3) pm in the case of Co and 179.5(3) pm in the case of Fe while As(III)–F(t) are 168.8(4) and 169.1(4) pm in the case of Co and 169.3(4) pm (2) in the case of iron. The lone-pairs of the As(III) ions are directed towards the rather open ring centers in the neighboring layers. The X-ray powder diffraction pattern of the compound [Ni(AsF3)2](AsF6)2 is similar to the X-ray powder diffraction patterns of the iron and cobalt compounds.
The compounds [Co(AsF3)2](AsF6)2 and [Fe(AsF3)2](AsF6)2 are isostructural (Fig. 7). The metal is octahedrally coordinated
2.2.3. Synthesis and structure of (H3O)4La2F(AsF3)2(AsF6)9 The system Ln2O3–AsF5–aHF yields solutions of H3O+, solvated Ln3+, AsF6 ions and under special reaction conditions also AsF3 as the side product of the reduction of AsF5. This system is rather complex and many compounds could be formed. In this paper we would like to mention briefly the compound with AsF3 as a ligand to lanthanum metal. Colorless crystals of (H3O)4La2F(AsF3)2(AsF6)9 were prepared from La2O3 and aHF under solvothermal conditions in AsF5 above its critical temperature (Tc = 337 K) [4,24]. Although the reaction was carried out in a Teflon-lined nickel vessel, AsF3 was formed by the reaction of AsF5 with the metal walls in the reactor with a Teflon liner due to severe reaction conditions. The structure of (H3O)4La2F(AsF3)2(AsF6)9 is very closely related to the structure of (H3O)8La2F(AsF6)13 [24]. The difference is that two AsF6 units that are coordinated only on the La are exchanged for two AsF3. Accordingly there are also less cations (H3O+) needed. Non-bridging distances in AsF3 unit are 168.2(6) and 168.3(7) pm, which is close to distances in AsF3 (169.9–172.1 pm) [29]. The dramatic change of the bridging distance in AsF3 (177.6(5) pm) shows that the bonding to the metal center is rather strong. In accordance with this is
Fig. 6. Coordination of the metal atom (Ln = Ce and Pr) in the compound [Ln(AsF3)3](AsF6)3 [22].
Fig. 7. Coordination sphere of M (M = Fe and Co) in the [M(AsF3)2](AsF6)2 [23].
2.2.2. Synthesis and structures of [M(AsF3)2](AsF6)2 (M = Fe, Co and Ni) The compounds [M(AsF3)2](AsF6)2 (M = Fe, Co and Ni) were prepared by the oxidation of corresponding metal with AsF5 in AsF3 as a solvent at room temperature [23]: AsF3
M þ nAsF5 ! ½MðAsF3 Þ2 ðAsF6 Þ2 þ ðn 3ÞAsF5 ; 295 K
M ¼ Fe; Co and Ni; n > 4
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also the distance La–F(AsF3) = 249.8(5) pm, being in the range of distances La–F(AsF6). This proves that the bonding of AsF3 units in this compound is not weaker than the bonding of AsF6 units. 2.2.4. Synthesis and structure of [Sn(AsF3)2](SbF6)2 There is also coordination of AsF3 via more than one fluorine atom exemplified by [Sn(AsF3)2](SbF6)2 [25]. The compound was prepared by dissolving Sn(SbF6)2 in AsF3. Sn(SbF6)2 was prepared by direct interaction of SnF2 and SbF5. The structure shows two crystallographically different AsF3 groups. One is coordinated to the metal cation by only one fluorine atom (Sn–F(AsF3) = 246 pm), while the second one is coordinated to two metal centers by two fluorine atoms (the shorter Sn–F(AsF3) distance is at 267 pm and the longer Sn– F(AsF3) contact is at 302 pm). 2.2.5. Synthesis and structure of [Ca(AsF3)](AsF6)2 The only compound known up to now in which AsF3 acts as a bridging agent is [Ca(AsF3)](AsF6)2 [30]. The compound was prepared by the reaction of CaF2 with excess of AsF5 in AsF3 as a solvent: AsF3
CaF2 þ nAsF5 ! ½CaðAsF3 ÞðAsF6 Þ2 þ ðn 2ÞAsF5 295 K
Although AsF3 is a weak ligand, its interaction with practically naked Ca2+ cation is strong enough to form the coordination compound [Ca(AsF3)](AsF6)2, which does not decompose even when AsF3 solvent and excess of AsF5 are removed. The compound has negligible dissociation vapor pressure at room temperature. Calcium is coordinated to eight fluorine atoms (Fig. 8). Six fluorine atoms originating from six AsF6 units are located at the corners of a regular trigonal prism. Two rectangular faces of the trigonal prism are capped by fluorine atoms from two fluorine bridged AsF3 molecules (Ca–F(AsF3) = 241.1 and 243.2 pm). It was found for the first time that the bridging As–F distances in AsF3 are shorter (172.4 and 173.1 pm) than the terminal As– F distance (184.5 pm). As the AsF3 molecule can be easily polarized [31] this results in significant elongation of the terminal bond in comparison with the bridging ones. On the other hand, the terminal fluorine atom is disordered, which by itself should result in the shortening of the terminal bond. 2.3. Coordination compounds with XeF2 as a ligand to metal cation 2.3.1. Syntheses The first isolated compound with XeF2 as a ligand to metal ion was [Ag(XeF2)2](AsF6) [32]. It was formed during the oxidation of xenon with a solution of AgFAsF6 in aHF. It may also be made from AgAsF6 and excess XeF2 in aHF. A review of the coordination compounds with XeF2 as a ligand to metal cations of the type [Mn+(XeF2)p](AF6)n was published recently [33]. In this paper a general review of this subject is given with a special emphasis on the parameters determining the structural diversity of these compounds.
Fig. 8. Coordination sphere of Ca in the [Ca(AsF3)](AsF6)2 [30].
Mn+(AF6)n dissolve in aHF as a solvent yielding solvated cations Mn+(HF)m and AF6 (A = P, As, Sb and Bi) anions. When XeF2 is added to this solution it can, as a stronger Lewis base, replace HF molecules in the coordination sphere of the cation: aHF
Mnþ ðAF6 Þn þ mXeF2 ! ½Mnþ ðXeF2 Þ p ðAF6 Þn 295 K
þ ðm pÞXeF2 XeF2 as a linear molecule with its semi ionic character and its ˚ 3) competes with AF6 relatively small formula volume (65 A in the coordination of the central metal atom. Anions AF6 have different negative charge on their fluorine ligands. In the row P, As, Sb, Bi and F ligands in PF6 have the highest negative charge while the negative charge on F ligands of SbF6 and BiF6 is the lowest. In the compound [Cd(XeF2)](BF4)2 [2] the negative charge on F ligands of BF4 anion is higher than in the case of AF6 anions. XeF2 with its negative charge of 0.5e on each of its F ligands is less competitive with F ligands from BF4 in the coordination of the metal cation. This could be one of the reasons why attempts to prepare other [M(XeF2)m](BF4)2 compounds have failed so far. At the preparation of the coordination compounds with XeF2 as a ligand to metal cation two postulations should be implemented: (a) metal cations should be sufficiently strong Lewis bases so that they will not be able to withdraw F from XeF2 and to generate Xe2F3+AF6 or XeF+AF6 salts and (b) metal cations should not be oxidized by XeF2, which dissolved in aHF is a relatively strong oxidizing agent. In the case when metal cations are oxidized by XeF2, they become weaker Lewis bases and therefore the possibility that they will withdraw F from XeF2 molecule is greater. Different synthetic routes could be used for the preparation of the coordination compounds of the type [Mn+(XeF2)p] (AF6)n:
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Table 1 Compounds with XeF2 as a ligand to metal ions M+
M2+ (alkaline earth)
M2+
M3+
[Li(XeF2)3](AsF6) [Ag(XeF2)2](AsF6) [32] [Ag(XeF2)2](PF6) [34]
[Mg(XeF2)4](AsF6)2 [35] [Mg(XeF2)2](AsF6)2 [35] [Mg(XeF2)2](SbF6)2 [28] [Ca(XeF2)5](PF6)2 [36] [Ca2(XeF2)9](AsF6)4 [37] [Ca(XeF2)4](AsF6)2 [38] [Ca(XeF2)2.5](AsF6)2 [38] [Sr(XeF2)3](PF6)2 [Sr(XeF2)3](AsF6)2 [39] [Ba(XeF2)4](PF6)2 [Ba(XeF2)5](AsF6)2 [Ba(XeF2)5](SbF6)2 [40]
[Cu(XeF2)6](SbF6)2 [41] [Zn(XeF2)6](SbF6)2 [41] [Cd(XeF2)](BF4)2 [2] [Cd(XeF2)5](PF6)2 [36] [Cd(XeF2)4](AsF6)2 [11] [Cd2(XeF2)10](SbF6)4 [33] [Cd2(XeF2)6](SbF6)4 [Cd3(XeF2)4](SbF6)6 [Pb3(XeF2)11](PF6)6 [Pb(XeF2)3](PF6)2 [Pb(XeF2)3](AsF6)2 [39]
[La(XeF2)2.5](AsF6)3 [22] [Nd(XeF2)2.5](AsF6)3 [27] [Nd(XeF2)3](SbF6)3 [28]
(a) The reaction of Mn+(AF6)n with excess XeF2 in aHF. The coordination compounds with different p-values could be isolated, after the removal of the solvent, by pumping away excess XeF2 at different temperatures. (b) The reaction of stoichiometric amounts of XeF2 and Mn+(AF6)n in aHF. This synthetic route is convenient when the compound with determined p-value has still some decomposition vapor pressure of XeF2 at the temperature of isolation. (c) The direct synthesis from metal fluoride, XeF2 and appropriate Lewis acid. This synthetic route could be used in the cases when XeF2 and used Lewis acid do not form stable Xe2F3+ or XeF+ salts (e.g. PF5 and BF3). (d) The reaction between Mn+(AF6)n and liquid XeF2 at temperatures around 100 8C. The starting compounds Mn+(AF6)n should be thermally stable at these temperatures.
effective nuclear charge, effective volume, Lewis acidity of the cation, character of the M–F bond, coordination number, etc. The effective nuclear charge experienced by the upper-level electrons of a cation depends on the extent to which these electrons are screened from the nucleus. The screening effect of the neighboring electrons in the same energy level is low, only lower-level electrons act as a shield. The effective volume depends upon the effective nuclear charge. Higher is the effective nuclear charge, smaller is the effective volume of the cation. The coordination number of the central atom is
All coordination compounds with XeF2 as a ligand and with known structures are listed in Table 1. 2.3.2. Structural features in the coordination compounds of the type [Mn+(XeF2)p](AF6)n The coordination compounds of the type [Mn+(XeF2)p] (AF6)n exhibit a great variety of different structures from a molecular structure (e.g. [Mg(XeF2)4](AsF6)2 [35], Fig. 9) over a dimeric structure (e.g. [Cd2(XeF2)10](SbF6)4, Fig. 10) to different polymeric structures such as chain (e.g. [Ca(XeF2)5](PF6)2 [36], Fig. 11) layer (e.g. [Ca(XeF2)4](AsF6)2 [38], Fig. 12) and 3D-structure (e.g. [Ca(XeF2)2.5](AsF6)2 [38], Fig. 13). In all these structures (except in the molecular structure), the metal ions are connected in three different ways: only by XeF2 molecules, only by AF6 anions and by the combination of XeF2 and AF6. The number of XeF2 groups coordinated to the metal atom could vary from one XeF2 up to nine XeF2 molecules. Only the coordination compound with seven XeF2 molecules is not known. The properties of the central metal and the properties of the anion determine the structures of these coordination compounds. The impact of the central atom on the structural diversity of these coordination compounds is determined by the simultaneous influence of cations properties such as: electron affinity,
Fig. 9. Molecular structure of [Mg(XeF2)4](AsF6)2 [35].
Fig. 10. Dimeric type of structure exemplified with [Cd2(XeF2)10](SbF6)4 [33].
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Fig. 14. Coordination of the metal center (M = Zn and Cu) in the compound [M(XeF2)6](SbF6)2 [41]. Fig. 11. Chain in the structure of [Cd(XeF2)4](AsF6)2 [11].
Fig. 12. Layer in the structure of [Ca(XeF2)4](AsF6)2 [38].
Fig. 13. Three-dimensional structural type in the case of [Ca(XeF2)2.5](AsF6)2 [38].
determined among the others also by the effective volume of the cation. The electron affinity of the cation, which is illustrated by the corresponding ionization potential of the atom, is responsible for the strength of the interaction of the metal cation with the negative fluoride ligands either from XeF2 ligands or from AF6 anions. Higher is the electron affinity stronger is the interaction of the cation with the fluoride ions. Lewis acidity of the cation illustrates the power of the cation to attract F ligands. If the Lewis acidity of the cation is strong enough to withdraw the F from XeF2 molecule, the corresponding binary fluoride is formed besides XeF+ or Xe2F3+ salts instead of the coordination compound. The F ligand either from XeF2 molecule or from AF6 anions donates its electrons to the metal center. The covalency of this M–F bond determines the amount of the negative charge transferred from the ligands to the central atom. XeF2 as a ligand can be bridging—that is connecting two metal centers or non-bridging—that is interacting only with one metal center. The F ligands of the non-bridging XeF2 molecules are being pulled away from their Xe atoms. Each of these XeF2 molecules is on the ionization pathway: F–Xe–F ! F– Xe+ + F [42]. One F ligand moves closely to xenon atom becoming more XeF+ like, while the other F ligand, approaching to the cation, moves away from the xenon atom. In the non-bridging XeF2 molecule the electron charge from the XeF2 is delocalized towards the cation. Therefore, the rest of the molecule is less capable of interacting with another metal center. Clearly, the F ligands approaching the Mn+ must be more negatively charged in non-bridging XeF2 molecules than in the more symmetrical, bridging XeF2 molecules. The impact of the anion AF6 on the structural diversity of these coordination compounds is less important than the impact of the cation. The following properties of the AF6 anion should be considered: Lewis basicity of AF6, the charge on the F-ligands of AF6 and size of the anion. The F donor ability of AF6 is important. The gaseous ionization energies for the process AF6 ! AF5 + F are 4.08 eV (A = P), 4.42 eV (A = As) and 5.07 eV (A = Sb) [43]. This must be a consequence of greater effective nuclear charge at Sb center
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than at the As center or P center. It is therefore to be expected that the F ligands of the SbF6 will bear less charge than those in AsF6 or PF6 anion. The most interesting compounds in this series of the coordination compounds are compounds with homoleptic centers. The first such compound isolated and structurally characterized was [Ca2(XeF2)9](AsF6)4 [37], which has only one from its two crystallographically different Ca atoms homoleptically coordinated by XeF2 molecules. Recently two new compounds [Cu(XeF2)6](SbF6)2 and [Zn(XeF2)6](SbF6)2 [41] (Fig. 14) have been isolated. In these cases we can talk about real homolectic compounds because all metal centers are connected only by non-bridging XeF2 molecules. Here are some requirements that should be fulfilled in order to obtain homoleptic compounds. The electron affinity of the cation should be higher than 15 eV, the basicity of the anion should be low (e.g. SbF6), the coordination number of the cation should be 6. This is the best arrangement regarding the repulsions of the negative F-ligands of the coordinating XeF2 and the repulsions of their positive XeF+ tails. In some coordination compounds where two or more crystallographically different metal centers are available (e.g. [Ca2(XeF2)9](AsF6)4 and [Pb3(XeF2)11](PF6)6) it is possible, that one of such centers is homoleptically coordinated by XeF2 molecules. In these cases the requirements mentioned above are not necessarily to be fulfilled. The coordination number could be higher and also anions with more negative fluorine ligands are possible (e.g. PF6). The differences in the properties of the determined cation and anion, which govern the structural behavior of these coordination compounds, are sometimes very subtle therefore it is difficult or even impossible to predict what kind of the structure new isolated coordination compounds would adopt. 2.3.3. Raman spectroscopy Raman spectroscopy is a very important tool for the characterization of these coordination compounds. The reactions are performed in transparent FEP reaction vessels, which allow identifying the reaction products in the solution and during the isolation. In this way also less stable species, e.g. [Mg(XeF2)6](AsF6)2 could be detected before excess XeF2 and HF are pumped away. Xenon fluorides and their coordination compounds have rather strong Raman active Xe–F stretching modes while A–F and M–F vibrations are usually far less intense. The totally symmetric stretching mode for XeF2 [44] or symmetrical XeF2 in the complexes like XeF2(XeF5AsF6)2 [42] is at or near 497 cm1. When XeF2 is distorted due to the interaction with one or two metal centers, the band at 497 cm1 is replaced by two bands: the band at higher frequency (n(Xe–F)) that represents the vibration of the short Xe–F bond, and the band at lower frequency (n(Xe F)) that represents the vibration of the longer Xe–F bond. The frequency for n(Xe–F)+ should be in the range between 600 and 620 cm1 [45]. The strength of the interaction of nonbridging XeF2 molecules with the metal center could be seen from n(Xe–F) band which is in the range from 544 to 584 cm1. The stretching band of the bridging XeF2 molecules is in the range from 500 to 535 cm1. The higher frequency of this
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stretching mode can be explained by the interaction of XeF2 with metal cations as M F–Xe–F M. 3. Conclusions Mn+(AF6)n, A = P, As, Sb and Bi proved to be excellent starting compounds for the preparation of novel salts of the type [Mn+(L)m](AF6)n with L = HF, AsF3 and XeF2. [La(HF)2](AsF6)3 represents the first coordination compound with the HF ligand coordinated directly to the metal center. [Ca(AsF3)2](AsF6)2 represents the first coordination compound in which AsF3 acts as a bridging ligand between two metal cations. In the case of XeF2 as a ligand a whole series of new coordination compounds of the type [Mn+(XeF2)p](AF6)n was isolated and characterized. The properties of the cation and the anion are determining the structural features of these interesting compounds. 4. Preparation procedure The general experimental procedures about the synthesis and characterization of the coordination compounds mentioned in this review can be found in the corresponding literature cited in this review. Here, only crucial experimental techniques for this chemistry are given. The coordination compounds of the type [Mn+(L)m] (AF6)n with L = HF, AsF3 and XeF2 were prepared by the reaction of Mn+(AF6)n with the corresponding ligand in the appropriate solvent. In the case of HF as a ligand anhydrous HF was used as a solvent. Compounds with coordinated HF seem to be more stable in the form of crystals while during the quick removal of the solvent coordinated HF is usually lost. In the case of AsF3 as a ligand, again the best results were obtained in the system where AsF3 is a ligand and a solvent. The alternative method is the reaction with AsF5 at supercritical conditions. AsF3 is formed in the reaction vessel by the reduction on the walls of the reaction vessel. The solvent aHF is not appropriate because the solvolysis of the compound takes place, at least partially, losing AsF3 which yields an impure product. In the case of L = XeF2 the reactions were performed between Mn+(AF6)n and excess XeF2 in aHF. After the reaction was completed the solvent and excess XeF2 were pumped away. In the case of phosphates and borates also direct reaction of binary fluorides and XeF2 in aHF as a solvent acidified by PF5 or BF3 under pressure was used. Acknowledgements The authors gratefully acknowledge the Slovenian Research Agency for the financial support of the Research Program P10045 Inorganic Chemistry and Technology and Dr. Evgeny Goreshnik for the preparation of the figures of the structures. References [1] H.D.B. Jenkins, H.K. Roobottom, H.K. Passmore, L. Glasser, Inorg. Chem. 38 (1999) 3609–3620.
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[2] G. Tavcˇar, B. Zˇemva, Inorg. Chem. 44 (2005) 1525–1529. [3] Z. Mazej, H. Borrmann, K. Lutar, B. Zˇemva, Inorg. Chem. 37 (1998) 5912–5914. [4] B. Zˇemva, A. Jesih, K. Lutar, in: J.L. Luche, C. Balny, S. Benefice, J.M. Denis, C. Pe´trier (Eds.), Chemical Processes and Reactions Under Extreme or Non-Classic Conditions, COST Action D6, European Commission, 1998, pp. 153–156. [5] M. Atoji, W.N. Lipscomb, Acta Crystallogr. 7 (1954) 173–175. [6] B. Maximov, H. Schulz, Acta Crystallogr. B 41 (1985) 88–91. [7] M. Tramsˇek, E. Lork, R. Mews, B. Zˇemva, J. Fluorine Chem. 110 (2001) 123–131. [8] A.F. Wells, Structural Inorganic Chemistry, 5th ed., Clarendon Press, Oxford, 1986, pp. 273–274. [9] M. Tramsˇek, P. Benkicˇ, A. Turicˇnik, G. Tavcˇar, B. Zˇemva, J. Fluorine Chem. 114 (2002) 143–148. [10] M. Tramsˇek, P. Benkicˇ, Acta Chim. Slov. 51 (2004) 769–778. [11] G. Tavcˇar, P. Benkicˇ, B. Zˇemva, Inorg. Chem. 43 (2004) 1452–1457. [12] H.M. Haendler, J. Am. Chem. Soc. 73 (1951) 5218–5219. [13] B.G. Hyde, S. Anderson, Inorganic crystal structures, 1st ed., Wiley, New York, 1989, 187 pp. [14] A. Turicˇnik, P. Benkicˇ, B. Zˇemva, J. Fluorine Chem. 121 (2003) 245–251. [15] P. Hagenmuller, in: P. Hagenmuller (Ed.), Inorganic Solid Fluorides, Academic Press, New York, 1985, pp. 80–81. [16] R.D. Shannon, Acta Crystallogr. A 32 (1976) 751–767. [17] K. Seppelt, Z. Anorg. Allg. Chem. 629 (2003) 2427–2430. [18] M. Gerken, D.A. Dixon, G. Schrobilgen, Inorg. Chem. 41 (2002) 259– 277. [19] I.C. Hwang, K. Seppelt, Z. Anorg. Allg. Chem. 628 (2002) 765–769. [20] Z. Mazej, E. Goreshnik, Solid State Sci. 8 (2006) 671–677. [21] Z. Mazej, Ph.D. Thesis, University of Ljubljana, Slovenia, 2000. [22] K. Lutar, H. Borrmann, Z. Mazej, M. Tramsˇek, P. Benkicˇ, B. Zˇemva, J. Fluorine Chem. 101 (2000) 155–160.
[23] K. Lutar, H. Borrmann, A. Jesih, B. Zˇemva, Acta Chim. Slov. 46 (1999) 213–228. [24] Z. Mazej, P. Benkicˇ, B. Zˇemva, J. Fluorine Chem. 112 (2001) 173–183. [25] A.J. Edwards, K.I. Khalan, J. Chem. Soc., Chem. Commun. (1984) 50–51. [26] R. Ku¨ster, K. Seppelt, Z. Anorg. Allg. Chem. 626 (2000) 236–240. [27] M. Tramsˇek, E. Lork, R. Mews, B. Zˇemva, J. Solid State Chem. 162 (2001) 243–249. [28] A. Turicˇnik, Ph.D. Thesis, University of Ljubljana, Slovenia, 2004. [29] J. Galy, R. Enjalbert, J. Solid State Chem. 44 (1982) 1–23. [30] M. Tramsˇek, B. Zˇemva, J. Fluorine Chem. 125 (2004) 1919–1924. [31] J.F. Sawyer, R.J. Gillespie, Prog. Inorg. Chem. 34 (1986) 65–113. [32] R. Hagiwara, F. Hollander, C. Maines, N. Bartlett, Eur. J. Solid State Chem. 28 (1991) 855–866. [33] G. Tavcˇar, M. Tramsˇek, T. Bunicˇ, P. Benkicˇ, B. Zˇemva, J. Fluorine Chem. 125 (2004) 1579–1584. [34] K. Matsumoto, R. Hagiwara, Y. Ito, Solid State Sci. 4 (2002) 1465–1469. [35] M. Tramsˇek, P. Benkicˇ, B. Zˇemva, Inorg. Chem. 43 (2004) 699–703. [36] T. Bunicˇ, G. Tavcˇar, M. Tramsˇek, B. Zˇemva, Inorg. Chem. 45 (2006) 1038–1042. [37] M. Tramsˇek, P. Benkicˇ, B. Zˇemva, Angew. Chem. Int. Ed. 43 (2004) 3456– 3458. [38] P. Benkicˇ, M. Tramsˇek, B. Zˇemva, Solid State Sci. 4 (2002) 1425–1434. [39] M. Tramsˇek, P. Benkicˇ, B. Zˇemva, Solid State Sci. 4 (2002) 9–14. [40] A. Turicˇnik, P. Benkicˇ, B. Zˇemva, Inorg. Chem. 41 (2002) 5521–5524. [41] G. Tavcˇar, E. Goreshnik, J. Mazej, J. Fluorine Chem., doi:10.1016/ j.jfluchem.2006.04.007. [42] B. Zˇemva, A. Jesih, D.H. Templeton, A. Zalkin, A.K. Cheetham, N. Bartlett, J. Am. Chem. Soc. 109 (1987) 7420–7427. [43] I. Krossing, I. Raabe, Chem. Eur. J. 10 (2004) 5017–5030. [44] P.A. Agron, G.M. Begun, H.A. Levy, A.A. Mason, C.G. Jones, D.F. Smith, Science 139 (1963) 842–844. [45] F.O. Sladky, P.A. Bulliner, N. Bartlett, J. Chem. Soc. A (1969) 2179–2188.
Review
Synthesis of novel salts with HF, AsF3 and XeF2 as ligands to metal cations Melita Tramsˇek, Boris Zˇemva * Department of Inorganic Chemistry and Technology, Jozˇef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia Received 17 March 2006; received in revised form 12 May 2006; accepted 16 May 2006 Available online 23 May 2006
Abstract A review of all known compounds of the type [Mn(L)m](AF6)n (M is a metal in the oxidation state n; A = P, As, Sb and Bi; L = HF, AsF3 and XeF2) is given with the emphasis on the compounds isolated and characterized by our group. The synthetic routes for the preparation of these compounds are given together with a brief analysis of their structures. In the case of L = XeF2 the influence of the properties of the cation and the anion on the structural diversity of these coordination compounds is discussed. A brief analysis of their Raman spectra is also given. # 2006 Elsevier B.V. All rights reserved. Keywords: Ligands to metal ions; HF; AsF3; XeF2; Syntheses; Structures
Contents 1. 2.
3. 4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Coordination compounds with HF as a ligand to metal cation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Synthesis and structure of [La(HF)2](AsF6)3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Synthesis and structure of [Pb(HF)](AsF6)2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3. Synthesis and structures of [Ca(HF)](AsF6)2 and [Cd(HF)](AsF6)2 . . . . . . . . . . . . . . . . . . . 2.1.4. Synthesis and structures of [Mg(HF)2](SbF6)2 and [Ca(HF)2](SbF6)2 . . . . . . . . . . . . . . . . . 2.1.5. Synthesis and structures of other coordination compounds with HF as a ligand to metal ion . 2.2. Coordination compounds with AsF3 as a ligand to metal cation . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Synthesis and structure of [M(AsF3)3](AsF6)3 (M = Ce and Pr) . . . . . . . . . . . . . . . . . . . . . 2.2.2. Synthesis and structures of [M(AsF3)2](AsF6)2 (M = Fe, Co and Ni). . . . . . . . . . . . . . . . . . 2.2.3. Synthesis and structure of (H3O)4La2F(AsF3)2(AsF6)9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.4. Synthesis and structure of [Sn(AsF3)2](SbF6)2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.5. Synthesis and structure of [Ca(AsF3)](AsF6)2. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Coordination compounds with XeF2 as a ligand to metal cation. . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Syntheses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Structural features in the coordination compounds of the type [Mn+(XeF2)p](AF6)n . . . . . . 2.3.3. Raman spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preparation procedure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction
* Corresponding author. Tel.: +386 1 477 35 40/33 01; fax: +386 1 477 31 55. E-mail address: [email protected] (B. Zˇemva). 0022-1139/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jfluchem.2006.05.014
Metal hexafluorometalates, Mn+(AF6)n (A = P, As, Sb and Bi), where n is the oxidation state of the metal M, have rather low lattice energy as a consequence of the anion volume
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˚ 3) [1]. Thus, almost ‘‘naked’’ metal centers (V > 100 A surrounded with weakly coordinating AF6 anions can interact with different, even weak ligands. Therefore, the compounds Mn+(AF6)n represent excellent starting materials for the preparation of novel salts of the type [Mn+(L)m](AF6)n with AsF3 and XeF2 as possible ligands (L). Anhydrous HF (aHF) was used as a solvent for the preparation of Mn+(AF6)n. In some cases the metal center can coordinate also weak ligands of the solvent forming coordination compounds of the type [Mn+(HF)m](AF6)n. In this paper, a review of all known coordination compounds with HF, AsF3 and XeF2 as ligands is presented together with the reaction systematics and available structures. In the case of XeF2 as a ligand also Raman spectra are included, providing a good insight into the strength and the type of XeF2 interactions with the cation (non-bridging or bridging XeF2 molecules). The compound [Cd(XeF2)](BF4)2 [2], so far the only fully characterized compound with BF4 anion and XeF2 as a ligand, is included in this study for comparison with compounds having AF6 anions. The anion BF4 is smaller and the negative charge on its fluorine ligands is higher than in the case of AF6. 2. Results and discussion 2.1. Coordination compounds with HF as a ligand to metal cation It has been known for a long time that metal fluorides dissolve in aHF acidified with Lewis acids (e.g. PF5, AsF5 and SbF5) yielding solution of solvated cations Mn+(HF)m and corresponding anions. The solid compounds of the type [Mn+(HF)m](AF6)n (A = P, As and Sb) were not isolated until recently [3]. In this chapter only coordination compounds with HF ligand coordinated directly to the metal cation are described. The compounds in which HF is coordinated to the metal center and further with strong hydrogen bonds connected to F ions forming poly-hydrogen-fluoride anions like HF2, H2F3 and H3F4 are not the subject of this discussion. 2.1.1. Synthesis and structure of [La(HF)2](AsF6)3 The first isolated coordination compound in which HF is bound directly to the metal cation was [La(HF)2](AsF6)3 [3]. The compound was prepared by the reaction between LaF3 and stoichiometric amount of aHF in AsF5 as a solvent at the temperature above critical temperature of AsF5 [4]: AsF5 ; 393 K
LaF3 þ 2HF ! ½LaðHFÞ2 ðAsF6 Þ3 t¼14 days
The polyhedron around the metal center is basically a tetracapped trigonal prism (Fig. 1), whose trigonal faces are formed by two fluorine ligands provided by AsF6 units and one F ligand provided by HF molecule. All rectangular faces of the prism are capped by fluorine ligands, being like the La cation within the crystallographic mirror plane. However, while one type of fluorine ligand provides common apexes with AsF6 octahedra,
Fig. 1. Coordination sphere of La in the [La(HF)2](AsF6)3 [3].
the other type of fluorine ligand provides common edge, respectively. Thus, coordination number 10 is finally achieved for the La atom. The hydrogen bonding with the distance F(As)–H at 192(7) pm and the angle F(As)–H–F at 166(9)8 is quite strong as these values are very similar to those in crystalline HF [5]. Although HF is certainly coordinated to metal centers in an aHF solutions of Mn+(AF6)n, attempts to isolate such species have long not been successful, probably because HF is quite a weak ligand. Keeping this in mind it is rather surprising that the F(H)–La distance (246.6(3) pm) is equal to most other La–F bonds (from 243.3(2) to 282.6(4) pm) in the present structure and also to those in LaF3 [6]. 2.1.2. Synthesis and structure of [Pb(HF)](AsF6)2 The reaction between PbF2 and excess of AsF5 in aHF as a solvent yields a colorless solution of solvated Pb2+ cations and AsF6 anions: aHF
PbF2 þ nAsF5 ! ½PbðHFÞx 2þ þ 2AsF6 þ ðn 2ÞAsF5 ; 295 K
n2 [Pb(HF)](AsF6)2 [7] was obtained as a white solid after removal of excess of AsF5 and aHF. It is very well soluble in aHF. When it is redissolved in aHF partial solvolysis always takes place and besides of the bulk of compound [Pb(HF)](AsF6)2 also some shiny crystals of PbF(AsF6) are obtained. The coordination number of the central Pb atom is 10 considering bond distances up to 306 pm (Fig. 2). There is one fluorine atom from HF molecule and nine fluorine atoms from AsF6 units. In the structure there are three crystallographically different AsF6 units. Around each Pb atom there are two As(1)F6 units, four As(2)F6 units and two As(3)F6 units. Each AsF6 unit interacts with four Pb atoms. In As(3)F6 unit the two non-coordinating fluorine atoms are in trans position while in As(1)F6 unit they are in cis position. As(1)F6 and As(3)F6 units bridge Pb atoms forming 16-membered heterocycles. As(2)F6 unit acts as a bidentate ligand forming a four-membered ring. Two of these units form eight-membered heterocycles. The distance Pb-F(HF) is the shortest distance in the structure
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Fig. 2. Coordination sphere of Pb in [Pb(HF)](AsF6)2 [7].
(248(4) pm) and is only slightly longer than the shortest Pb–F distance in PbF2 (241 pm) [8]. 2.1.3. Synthesis and structures of [Ca(HF)](AsF6)2 and [Cd(HF)](AsF6)2 The compounds [Ca(HF)](AsF6)2 [9,10] and [Cd(HF)](AsF6)2 [11] are isostructural. The crystals of [Ca(HF)](AsF6)2 were found in the same crystallization batch as crystals of [Ca(XeF2)4](AsF6)2, only several months later, after they had been isolated and stored in the dry box. The compound [Cd(HF)](AsF6)2 was found during the preparation of Cd(AsF6)2. First the coordination compound with HF is obtained which is slowly losing HF at room temperature in a dynamic vacuum finally yielding Cd(AsF6)2. In [M(HF)](AsF6)2, M atoms (Ca, Cd) are coordinated by six bulky AsF6 units and one HF molecule forming distorted tricapped trigonal prism (Fig. 3) with Ca–F(As) distances in the range of 231.5–246.2 pm and Cd–F(As) distances in the range of 226.3–232.6 pm. All Cd–F distances to the fluorine at
Fig. 3. Coordination sphere of Cd in the [Cd(HF)](AsF6)2 [11].
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the apexes of the coordination prism are shorter than those in the structure of CdF2 (Cd–F = 233.3 pm) [12] indicating that the Cd2+ cations bear a higher positive charge than those in CdF2. The HF molecule caps a trigonal-prismatic face at a distance of 239.6 pm (Ca), which is among the shortest Ca–F distances in the compound (Ca–F in CaF2 is 236.6 pm [13]), and a distance of 237.6(5) pm (Cd). Additionally, two AsF6 units interact with the metal atom via second fluorine atom placed over rectangular faces of the trigonal prism at slightly longer distances, Ca–F of 267.8(11) and 277.3(11) pm and Cd– F of 268.1(4) and 292.1(4) pm. The metal cation is thus located in an environment of nine fluorine atoms. Basic units in the structure of [M(HF)](AsF6)2 are square rings formed of two M atoms connected via two crystallographically different AsF6 units. Similar rings can be found in the structure of [Pb(HF)](AsF6)2 [7]. The rings are inter-connected via trans fluorine atoms forming zig-zag ribbons. Neighboring rings running almost perpendicular to each other share common M atoms finally resulting in a 3D-network. In accordance with the much higher electron affinity of Cd2+ (16.91 eV) than that of Ca2+ (11.87 eV), the M–F bond is expected to be less ionic in the Cd compound. In the Cd compound, fluorine atoms from the AsF6 units capping the side faces of the basic trigonal-prismatic polyhedra are at longer distances from the metal than those in the Ca compound, although the other Cd–F(As) distances are shorter (average 229.5 pm) than those in Ca–F(As) (average 237.6 pm). This indicates that Cd2+ cations bear a lower positive charge than Ca2+ cations, which is probably a consequence of the higher covalency of the Cd–F bonds. 2.1.4. Synthesis and structures of [Mg(HF)2](SbF6)2 and [Ca(HF)2](SbF6)2 The coordination compounds [Mg(HF)2](SbF6)2 and [Ca(HF)2](SbF6)2 [14], were prepared by the reaction of MgF2 or CaF2 with excessive SbF5 in aHF. The stability of these compounds depends among others also upon effective nuclear charge of the central atoms. The interactions are stronger when the central atom has a high effective nuclear charge and a small radius. In our case the compound [Mg(HF)2](SbF6)2 is more stable than [Ca(HF)2](SbF6)2 because the effective nuclear charge on Mg2+ is higher than on Ca2+. This was evident from X-ray powder diffraction patterns of the products after prolonged pumping under dynamic vacuum. In the case of Mg compound some lines which could be attributed to HF compound were consistently found while in the case of Ca only the lines which could be attributed to Ca(SbF6)2 were found. In Mg compounds metal atom is octahedrally coordinated to six fluorine atoms (Fig. 4). Two cis equatorial fluorine atoms are from two HF molecules, the other two cis equatorial fluorine atoms are from two trans bridging SbF6 units, while two axial fluorine atoms are from two cis bridging SbF6 units. The Mg–F distances in the equatorial plane are identical within experimental error (Mg–F(HF) is 197.1(3) pm and Mg–F(Sb) is 197.3(3) pm), while the axial Mg–F(Sb) distances are 194.9(4) and 194.3(4) pm. The Mg–F distances in rutile MgF2 are 4 198.4 and 2 197.9 pm [15].
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there is only one short Ca–F(Sb) distance while the others are longer. It appears that the negative charge is localized mostly on one of the interacting F ligands while the other terminal and bridging F atoms bear lower negative charges. The Ca–F(HF) distances are 230.4(3) and 231.5(3) pm. The Ca–F(Sb) distances range from 232.8(3) to 243.0(3) pm, indicating that the interactions with F ligands from the HF molecules are stronger than the interactions with F ligands from SbF6 units.
Fig. 4. Coordination sphere of Mg in the [Mg(HF)2](SbF6)2 [14].
Although the compounds [Mg(HF)2](SbF6)2 and [Ca(HF)2](SbF6)2 have analogous composition, the structures of both compounds are different due to the different sizes of the central atoms and consequently different coordination numbers. Ionic radius of Mg2+ (CN = 6) is 86.0 pm and of Ca2+ (CN = 8) is 126 pm [16]. The coordination sphere around the calcium atom in [Ca(HF)2](SbF6)2 consists of eight fluorine atoms in the shape of an Archimedian antiprism (Fig. 5). One square face of the Archimedian antiprism is formed by three F ligands from SbF6 and one F ligand from HF molecule. The second square face of the Archimedian antiprism is similar except that the HF molecule is crystallographically different. There are three independent SbF6 units in the structure. Sb(1)F6 and Sb(2)F6 units provide four bridging F ligands while the Sb(3)F6 groups provide two bridging F ligands to the Ca centers. It is interesting that regardless of the number of contacts of the SbF6 groups with Ca
2.1.5. Synthesis and structures of other coordination compounds with HF as a ligand to metal ion Among metal hexafluoroantimonates coordinated with HF the compound [Hg(HF)](SbF6)2 should be mentioned [17]. The compound was obtained when colorless material [HgXe]2+[SbF6][Sb2F11] came into contact with HF which replaced Xe atoms. Hg atoms are coordinated by seven fluorine atoms from SbF6 units and one F ligand from HF molecule. In the compounds [OsO3F(HF)2]AsF6, [OsO3F(HF)]SbF6 [18] and [Au(HF)2](SbF6)22HF [19], HF acts as a ligand to a metal center. In these cases two HF molecules connected via hydrogen bond bridge the metal center with AF6 unit. The distance Os–F(HF) in [OsO3F(HF)2]AsF6 is 228.2(5) pm [18]. The distance Au–F(HF) is 213.5(3) pm [19] and is comparable with Au–F(Sb) distance of 212.6(3) pm. Another example of the metal cation coordinated with HF is Mg(HF)AuF4AuF6 [20]. The attempt to prepare single crystals of Mg(AuF6)2 resulted in the synthesis of single crystals of Mg(HF)AuF4AuF6 instead. The compound represents the first example of mixed valence Au(III)/Au(V) ternary fluoride. The structure is built up of MgF6 octahedra sharing four equatorial F ligands with four AuF4 units (Mg–F distances range from 196.4(8) to 199.5(8) pm). All four fluorine atoms in AuF4 units are bridging and connect Mg atoms. Coordination sphere of Mg is completed with one F ligand from AuF6 unit (Mg–F distance is 200.3(10) pm and one F ligand from HF molecule Mg–F(HF) is 197.9(10) pm). 2.2. Coordination compounds with AsF3 as a ligand to metal cation Arsenic trifluoride can coordinate to metal cations in two different ways: (a) either via its fluorine ligands or (b) via its electron lone-pair:
Fig. 5. Coordination sphere of Ca in the [Ca(HF)2](SbF6)2 [14].
(a) Examples of AsF3 coordinated to a metal center via one fluorine atom are as follows: [M(AsF3)3](AsF6)3 (M = Ce and Pr) [21,22] [M(AsF3)2](AsF6)2 (M = Fe, Co and Ni) [23] and (H3O)4La2F(AsF3)2(AsF6)9 [24]. The coordination of AsF3 via more than one fluorine atom is exemplified by [Sn(AsF3)2](SbF6)2 [25]. (b) The only coordination compound known so far in which AsF3 is coordinated to the metal cation via its electron lone-pair is F3As–Au+SbF6 [26]. During the reduction of AuF3 to Au2+ and further to Au+ under strong acidic conditions (SbF5/HF) and addition of AsF3 as a mild reducing agent, F3As– Au+SbF6 was obtained. The distances As–F are from 167.0(7) to 168.5(7) pm. The distance As–Au is 226.8(1) pm.
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2.2.1. Synthesis and structure of [M(AsF3)3](AsF6)3 (M = Ce and Pr) The single crystals of the compounds [M(AsF3)3](AsF6)3 (M = Ce and Pr) were prepared in a nickel reaction vessel with a Teflon liner by the reaction between MF3 and HF in supercritical AsF5 during 7 months. It seems that during the reaction some reduction of AsF5 to AsF3 occurred [21], which was the reason for the formation of the coordination compounds with AsF3 as a ligand. The coordination number of Ce and Pr in these compounds is nine (Fig. 6). Six fluorine atoms, from six AsF6 groups form a regular trigonal prism. All three rectangular faces of the trigonal prism are capped by fluorine atoms from three fluorine-bridged AsF3 molecules. Each AsF6 is trans connected to the other lanthanide centers forming tridimensional network. In the case of compounds [Ln(XeF2)n](AF6)3 (Ln = La and Nd; A = As and Sb) the cis connection of AF6 groups with other lanthanide centers resulted in the formation of chains ([Nd(XeF2)3](SbF6)3) or double chains ([Nd(XeF2)2.5](AsF6)3) [27,28]. Both compounds (Ce and Pr) are isomorphous. Due to lanthanide contraction the unit cell in the case of praseodymium is smaller than in the case of the cerium compound. The distance As–F(b) is from 178.9(Ce) to 180.4(Pr) and the distance As–F(t) is from 161.3(Pr) to 164.2(Ce). This is consistent with the stronger attraction of a fluoride ligand of AsF3 in the case of praseodymium than in the case of cerium.
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by six fluorine atoms. Four fluorine atoms in the equatorial plane are from AsF6 species, which bridge transition metal atoms via trans fluorine ligands. The consequence of such arrangement is the formation of a layer composed of a square network. Apical fluorine atoms are from AsF3 molecules. Distance As(III)–F(b) is 180.1(3) pm in the case of Co and 179.5(3) pm in the case of Fe while As(III)–F(t) are 168.8(4) and 169.1(4) pm in the case of Co and 169.3(4) pm (2) in the case of iron. The lone-pairs of the As(III) ions are directed towards the rather open ring centers in the neighboring layers. The X-ray powder diffraction pattern of the compound [Ni(AsF3)2](AsF6)2 is similar to the X-ray powder diffraction patterns of the iron and cobalt compounds.
The compounds [Co(AsF3)2](AsF6)2 and [Fe(AsF3)2](AsF6)2 are isostructural (Fig. 7). The metal is octahedrally coordinated
2.2.3. Synthesis and structure of (H3O)4La2F(AsF3)2(AsF6)9 The system Ln2O3–AsF5–aHF yields solutions of H3O+, solvated Ln3+, AsF6 ions and under special reaction conditions also AsF3 as the side product of the reduction of AsF5. This system is rather complex and many compounds could be formed. In this paper we would like to mention briefly the compound with AsF3 as a ligand to lanthanum metal. Colorless crystals of (H3O)4La2F(AsF3)2(AsF6)9 were prepared from La2O3 and aHF under solvothermal conditions in AsF5 above its critical temperature (Tc = 337 K) [4,24]. Although the reaction was carried out in a Teflon-lined nickel vessel, AsF3 was formed by the reaction of AsF5 with the metal walls in the reactor with a Teflon liner due to severe reaction conditions. The structure of (H3O)4La2F(AsF3)2(AsF6)9 is very closely related to the structure of (H3O)8La2F(AsF6)13 [24]. The difference is that two AsF6 units that are coordinated only on the La are exchanged for two AsF3. Accordingly there are also less cations (H3O+) needed. Non-bridging distances in AsF3 unit are 168.2(6) and 168.3(7) pm, which is close to distances in AsF3 (169.9–172.1 pm) [29]. The dramatic change of the bridging distance in AsF3 (177.6(5) pm) shows that the bonding to the metal center is rather strong. In accordance with this is
Fig. 6. Coordination of the metal atom (Ln = Ce and Pr) in the compound [Ln(AsF3)3](AsF6)3 [22].
Fig. 7. Coordination sphere of M (M = Fe and Co) in the [M(AsF3)2](AsF6)2 [23].
2.2.2. Synthesis and structures of [M(AsF3)2](AsF6)2 (M = Fe, Co and Ni) The compounds [M(AsF3)2](AsF6)2 (M = Fe, Co and Ni) were prepared by the oxidation of corresponding metal with AsF5 in AsF3 as a solvent at room temperature [23]: AsF3
M þ nAsF5 ! ½MðAsF3 Þ2 ðAsF6 Þ2 þ ðn 3ÞAsF5 ; 295 K
M ¼ Fe; Co and Ni; n > 4
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also the distance La–F(AsF3) = 249.8(5) pm, being in the range of distances La–F(AsF6). This proves that the bonding of AsF3 units in this compound is not weaker than the bonding of AsF6 units. 2.2.4. Synthesis and structure of [Sn(AsF3)2](SbF6)2 There is also coordination of AsF3 via more than one fluorine atom exemplified by [Sn(AsF3)2](SbF6)2 [25]. The compound was prepared by dissolving Sn(SbF6)2 in AsF3. Sn(SbF6)2 was prepared by direct interaction of SnF2 and SbF5. The structure shows two crystallographically different AsF3 groups. One is coordinated to the metal cation by only one fluorine atom (Sn–F(AsF3) = 246 pm), while the second one is coordinated to two metal centers by two fluorine atoms (the shorter Sn–F(AsF3) distance is at 267 pm and the longer Sn– F(AsF3) contact is at 302 pm). 2.2.5. Synthesis and structure of [Ca(AsF3)](AsF6)2 The only compound known up to now in which AsF3 acts as a bridging agent is [Ca(AsF3)](AsF6)2 [30]. The compound was prepared by the reaction of CaF2 with excess of AsF5 in AsF3 as a solvent: AsF3
CaF2 þ nAsF5 ! ½CaðAsF3 ÞðAsF6 Þ2 þ ðn 2ÞAsF5 295 K
Although AsF3 is a weak ligand, its interaction with practically naked Ca2+ cation is strong enough to form the coordination compound [Ca(AsF3)](AsF6)2, which does not decompose even when AsF3 solvent and excess of AsF5 are removed. The compound has negligible dissociation vapor pressure at room temperature. Calcium is coordinated to eight fluorine atoms (Fig. 8). Six fluorine atoms originating from six AsF6 units are located at the corners of a regular trigonal prism. Two rectangular faces of the trigonal prism are capped by fluorine atoms from two fluorine bridged AsF3 molecules (Ca–F(AsF3) = 241.1 and 243.2 pm). It was found for the first time that the bridging As–F distances in AsF3 are shorter (172.4 and 173.1 pm) than the terminal As– F distance (184.5 pm). As the AsF3 molecule can be easily polarized [31] this results in significant elongation of the terminal bond in comparison with the bridging ones. On the other hand, the terminal fluorine atom is disordered, which by itself should result in the shortening of the terminal bond. 2.3. Coordination compounds with XeF2 as a ligand to metal cation 2.3.1. Syntheses The first isolated compound with XeF2 as a ligand to metal ion was [Ag(XeF2)2](AsF6) [32]. It was formed during the oxidation of xenon with a solution of AgFAsF6 in aHF. It may also be made from AgAsF6 and excess XeF2 in aHF. A review of the coordination compounds with XeF2 as a ligand to metal cations of the type [Mn+(XeF2)p](AF6)n was published recently [33]. In this paper a general review of this subject is given with a special emphasis on the parameters determining the structural diversity of these compounds.
Fig. 8. Coordination sphere of Ca in the [Ca(AsF3)](AsF6)2 [30].
Mn+(AF6)n dissolve in aHF as a solvent yielding solvated cations Mn+(HF)m and AF6 (A = P, As, Sb and Bi) anions. When XeF2 is added to this solution it can, as a stronger Lewis base, replace HF molecules in the coordination sphere of the cation: aHF
Mnþ ðAF6 Þn þ mXeF2 ! ½Mnþ ðXeF2 Þ p ðAF6 Þn 295 K
þ ðm pÞXeF2 XeF2 as a linear molecule with its semi ionic character and its ˚ 3) competes with AF6 relatively small formula volume (65 A in the coordination of the central metal atom. Anions AF6 have different negative charge on their fluorine ligands. In the row P, As, Sb, Bi and F ligands in PF6 have the highest negative charge while the negative charge on F ligands of SbF6 and BiF6 is the lowest. In the compound [Cd(XeF2)](BF4)2 [2] the negative charge on F ligands of BF4 anion is higher than in the case of AF6 anions. XeF2 with its negative charge of 0.5e on each of its F ligands is less competitive with F ligands from BF4 in the coordination of the metal cation. This could be one of the reasons why attempts to prepare other [M(XeF2)m](BF4)2 compounds have failed so far. At the preparation of the coordination compounds with XeF2 as a ligand to metal cation two postulations should be implemented: (a) metal cations should be sufficiently strong Lewis bases so that they will not be able to withdraw F from XeF2 and to generate Xe2F3+AF6 or XeF+AF6 salts and (b) metal cations should not be oxidized by XeF2, which dissolved in aHF is a relatively strong oxidizing agent. In the case when metal cations are oxidized by XeF2, they become weaker Lewis bases and therefore the possibility that they will withdraw F from XeF2 molecule is greater. Different synthetic routes could be used for the preparation of the coordination compounds of the type [Mn+(XeF2)p] (AF6)n:
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Table 1 Compounds with XeF2 as a ligand to metal ions M+
M2+ (alkaline earth)
M2+
M3+
[Li(XeF2)3](AsF6) [Ag(XeF2)2](AsF6) [32] [Ag(XeF2)2](PF6) [34]
[Mg(XeF2)4](AsF6)2 [35] [Mg(XeF2)2](AsF6)2 [35] [Mg(XeF2)2](SbF6)2 [28] [Ca(XeF2)5](PF6)2 [36] [Ca2(XeF2)9](AsF6)4 [37] [Ca(XeF2)4](AsF6)2 [38] [Ca(XeF2)2.5](AsF6)2 [38] [Sr(XeF2)3](PF6)2 [Sr(XeF2)3](AsF6)2 [39] [Ba(XeF2)4](PF6)2 [Ba(XeF2)5](AsF6)2 [Ba(XeF2)5](SbF6)2 [40]
[Cu(XeF2)6](SbF6)2 [41] [Zn(XeF2)6](SbF6)2 [41] [Cd(XeF2)](BF4)2 [2] [Cd(XeF2)5](PF6)2 [36] [Cd(XeF2)4](AsF6)2 [11] [Cd2(XeF2)10](SbF6)4 [33] [Cd2(XeF2)6](SbF6)4 [Cd3(XeF2)4](SbF6)6 [Pb3(XeF2)11](PF6)6 [Pb(XeF2)3](PF6)2 [Pb(XeF2)3](AsF6)2 [39]
[La(XeF2)2.5](AsF6)3 [22] [Nd(XeF2)2.5](AsF6)3 [27] [Nd(XeF2)3](SbF6)3 [28]
(a) The reaction of Mn+(AF6)n with excess XeF2 in aHF. The coordination compounds with different p-values could be isolated, after the removal of the solvent, by pumping away excess XeF2 at different temperatures. (b) The reaction of stoichiometric amounts of XeF2 and Mn+(AF6)n in aHF. This synthetic route is convenient when the compound with determined p-value has still some decomposition vapor pressure of XeF2 at the temperature of isolation. (c) The direct synthesis from metal fluoride, XeF2 and appropriate Lewis acid. This synthetic route could be used in the cases when XeF2 and used Lewis acid do not form stable Xe2F3+ or XeF+ salts (e.g. PF5 and BF3). (d) The reaction between Mn+(AF6)n and liquid XeF2 at temperatures around 100 8C. The starting compounds Mn+(AF6)n should be thermally stable at these temperatures.
effective nuclear charge, effective volume, Lewis acidity of the cation, character of the M–F bond, coordination number, etc. The effective nuclear charge experienced by the upper-level electrons of a cation depends on the extent to which these electrons are screened from the nucleus. The screening effect of the neighboring electrons in the same energy level is low, only lower-level electrons act as a shield. The effective volume depends upon the effective nuclear charge. Higher is the effective nuclear charge, smaller is the effective volume of the cation. The coordination number of the central atom is
All coordination compounds with XeF2 as a ligand and with known structures are listed in Table 1. 2.3.2. Structural features in the coordination compounds of the type [Mn+(XeF2)p](AF6)n The coordination compounds of the type [Mn+(XeF2)p] (AF6)n exhibit a great variety of different structures from a molecular structure (e.g. [Mg(XeF2)4](AsF6)2 [35], Fig. 9) over a dimeric structure (e.g. [Cd2(XeF2)10](SbF6)4, Fig. 10) to different polymeric structures such as chain (e.g. [Ca(XeF2)5](PF6)2 [36], Fig. 11) layer (e.g. [Ca(XeF2)4](AsF6)2 [38], Fig. 12) and 3D-structure (e.g. [Ca(XeF2)2.5](AsF6)2 [38], Fig. 13). In all these structures (except in the molecular structure), the metal ions are connected in three different ways: only by XeF2 molecules, only by AF6 anions and by the combination of XeF2 and AF6. The number of XeF2 groups coordinated to the metal atom could vary from one XeF2 up to nine XeF2 molecules. Only the coordination compound with seven XeF2 molecules is not known. The properties of the central metal and the properties of the anion determine the structures of these coordination compounds. The impact of the central atom on the structural diversity of these coordination compounds is determined by the simultaneous influence of cations properties such as: electron affinity,
Fig. 9. Molecular structure of [Mg(XeF2)4](AsF6)2 [35].
Fig. 10. Dimeric type of structure exemplified with [Cd2(XeF2)10](SbF6)4 [33].
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Fig. 14. Coordination of the metal center (M = Zn and Cu) in the compound [M(XeF2)6](SbF6)2 [41]. Fig. 11. Chain in the structure of [Cd(XeF2)4](AsF6)2 [11].
Fig. 12. Layer in the structure of [Ca(XeF2)4](AsF6)2 [38].
Fig. 13. Three-dimensional structural type in the case of [Ca(XeF2)2.5](AsF6)2 [38].
determined among the others also by the effective volume of the cation. The electron affinity of the cation, which is illustrated by the corresponding ionization potential of the atom, is responsible for the strength of the interaction of the metal cation with the negative fluoride ligands either from XeF2 ligands or from AF6 anions. Higher is the electron affinity stronger is the interaction of the cation with the fluoride ions. Lewis acidity of the cation illustrates the power of the cation to attract F ligands. If the Lewis acidity of the cation is strong enough to withdraw the F from XeF2 molecule, the corresponding binary fluoride is formed besides XeF+ or Xe2F3+ salts instead of the coordination compound. The F ligand either from XeF2 molecule or from AF6 anions donates its electrons to the metal center. The covalency of this M–F bond determines the amount of the negative charge transferred from the ligands to the central atom. XeF2 as a ligand can be bridging—that is connecting two metal centers or non-bridging—that is interacting only with one metal center. The F ligands of the non-bridging XeF2 molecules are being pulled away from their Xe atoms. Each of these XeF2 molecules is on the ionization pathway: F–Xe–F ! F– Xe+ + F [42]. One F ligand moves closely to xenon atom becoming more XeF+ like, while the other F ligand, approaching to the cation, moves away from the xenon atom. In the non-bridging XeF2 molecule the electron charge from the XeF2 is delocalized towards the cation. Therefore, the rest of the molecule is less capable of interacting with another metal center. Clearly, the F ligands approaching the Mn+ must be more negatively charged in non-bridging XeF2 molecules than in the more symmetrical, bridging XeF2 molecules. The impact of the anion AF6 on the structural diversity of these coordination compounds is less important than the impact of the cation. The following properties of the AF6 anion should be considered: Lewis basicity of AF6, the charge on the F-ligands of AF6 and size of the anion. The F donor ability of AF6 is important. The gaseous ionization energies for the process AF6 ! AF5 + F are 4.08 eV (A = P), 4.42 eV (A = As) and 5.07 eV (A = Sb) [43]. This must be a consequence of greater effective nuclear charge at Sb center
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than at the As center or P center. It is therefore to be expected that the F ligands of the SbF6 will bear less charge than those in AsF6 or PF6 anion. The most interesting compounds in this series of the coordination compounds are compounds with homoleptic centers. The first such compound isolated and structurally characterized was [Ca2(XeF2)9](AsF6)4 [37], which has only one from its two crystallographically different Ca atoms homoleptically coordinated by XeF2 molecules. Recently two new compounds [Cu(XeF2)6](SbF6)2 and [Zn(XeF2)6](SbF6)2 [41] (Fig. 14) have been isolated. In these cases we can talk about real homolectic compounds because all metal centers are connected only by non-bridging XeF2 molecules. Here are some requirements that should be fulfilled in order to obtain homoleptic compounds. The electron affinity of the cation should be higher than 15 eV, the basicity of the anion should be low (e.g. SbF6), the coordination number of the cation should be 6. This is the best arrangement regarding the repulsions of the negative F-ligands of the coordinating XeF2 and the repulsions of their positive XeF+ tails. In some coordination compounds where two or more crystallographically different metal centers are available (e.g. [Ca2(XeF2)9](AsF6)4 and [Pb3(XeF2)11](PF6)6) it is possible, that one of such centers is homoleptically coordinated by XeF2 molecules. In these cases the requirements mentioned above are not necessarily to be fulfilled. The coordination number could be higher and also anions with more negative fluorine ligands are possible (e.g. PF6). The differences in the properties of the determined cation and anion, which govern the structural behavior of these coordination compounds, are sometimes very subtle therefore it is difficult or even impossible to predict what kind of the structure new isolated coordination compounds would adopt. 2.3.3. Raman spectroscopy Raman spectroscopy is a very important tool for the characterization of these coordination compounds. The reactions are performed in transparent FEP reaction vessels, which allow identifying the reaction products in the solution and during the isolation. In this way also less stable species, e.g. [Mg(XeF2)6](AsF6)2 could be detected before excess XeF2 and HF are pumped away. Xenon fluorides and their coordination compounds have rather strong Raman active Xe–F stretching modes while A–F and M–F vibrations are usually far less intense. The totally symmetric stretching mode for XeF2 [44] or symmetrical XeF2 in the complexes like XeF2(XeF5AsF6)2 [42] is at or near 497 cm1. When XeF2 is distorted due to the interaction with one or two metal centers, the band at 497 cm1 is replaced by two bands: the band at higher frequency (n(Xe–F)) that represents the vibration of the short Xe–F bond, and the band at lower frequency (n(Xe F)) that represents the vibration of the longer Xe–F bond. The frequency for n(Xe–F)+ should be in the range between 600 and 620 cm1 [45]. The strength of the interaction of nonbridging XeF2 molecules with the metal center could be seen from n(Xe–F) band which is in the range from 544 to 584 cm1. The stretching band of the bridging XeF2 molecules is in the range from 500 to 535 cm1. The higher frequency of this
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stretching mode can be explained by the interaction of XeF2 with metal cations as M F–Xe–F M. 3. Conclusions Mn+(AF6)n, A = P, As, Sb and Bi proved to be excellent starting compounds for the preparation of novel salts of the type [Mn+(L)m](AF6)n with L = HF, AsF3 and XeF2. [La(HF)2](AsF6)3 represents the first coordination compound with the HF ligand coordinated directly to the metal center. [Ca(AsF3)2](AsF6)2 represents the first coordination compound in which AsF3 acts as a bridging ligand between two metal cations. In the case of XeF2 as a ligand a whole series of new coordination compounds of the type [Mn+(XeF2)p](AF6)n was isolated and characterized. The properties of the cation and the anion are determining the structural features of these interesting compounds. 4. Preparation procedure The general experimental procedures about the synthesis and characterization of the coordination compounds mentioned in this review can be found in the corresponding literature cited in this review. Here, only crucial experimental techniques for this chemistry are given. The coordination compounds of the type [Mn+(L)m] (AF6)n with L = HF, AsF3 and XeF2 were prepared by the reaction of Mn+(AF6)n with the corresponding ligand in the appropriate solvent. In the case of HF as a ligand anhydrous HF was used as a solvent. Compounds with coordinated HF seem to be more stable in the form of crystals while during the quick removal of the solvent coordinated HF is usually lost. In the case of AsF3 as a ligand, again the best results were obtained in the system where AsF3 is a ligand and a solvent. The alternative method is the reaction with AsF5 at supercritical conditions. AsF3 is formed in the reaction vessel by the reduction on the walls of the reaction vessel. The solvent aHF is not appropriate because the solvolysis of the compound takes place, at least partially, losing AsF3 which yields an impure product. In the case of L = XeF2 the reactions were performed between Mn+(AF6)n and excess XeF2 in aHF. After the reaction was completed the solvent and excess XeF2 were pumped away. In the case of phosphates and borates also direct reaction of binary fluorides and XeF2 in aHF as a solvent acidified by PF5 or BF3 under pressure was used. Acknowledgements The authors gratefully acknowledge the Slovenian Research Agency for the financial support of the Research Program P10045 Inorganic Chemistry and Technology and Dr. Evgeny Goreshnik for the preparation of the figures of the structures. References [1] H.D.B. Jenkins, H.K. Roobottom, H.K. Passmore, L. Glasser, Inorg. Chem. 38 (1999) 3609–3620.
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