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Main group heteroatoms in transition metal clusters
0277-5387/U S3.M)+ .OO Pergamon Pms Ltd
Pdy/w&m Vol. 3, No. 12, PP. 1307~1319,1984 Printed in Great Britain
MAIN
GROUP HETEROATOMS IN TRANSITION CLUSTERS*
METAL
J. NICOLA NICHOLLS University Chemical Laboratory, Lensfield Road, Cambridge, U.K. (Received 25 March 1983)
Abstract-The class of transition metal cluster compounds which contain individual main group heteroatoms is surveyed. Hydrido-clusters and clusters containing group IV, V, VI and VII atoms are dealt with in turn with reference to their synthesis, structure and reactivity.
As a result of the anticipation of similarities between transition metal clusters’ and fragments of bulk metals, particular attention has been paid during the development ofmetal cluster chemistry to complexes containing isolated main group atoms. Such clusters have been compared with simple metallic interstitial compounds and atoms bound to metal surfaces and have been postulated as models for intermediates in catalytic processes. ‘*’ This may or may not prove to be a useful analogy, but interest in these molecules has resulted in the synthesis and characterisation of a fascinating and extensive class of transition metal clusters containing main group atoms. These range from truly interstitial compounds with high metal: main group atom ratios, as illustrated by [PtRh,,N(C0),,]33 and [OS,&(CO)~~]~-,~ through “semi-interstitials” like [Ru,C(CO), 31 5 and [Fe,N(CO),,] -,6 to highly exposed surface species such as ~os6(~30)(c~h17 and
CNi3W%)3(~3%18 be Fig. 1). Theheteroatoms display a wide range of bonding
modes and coordination numbers, from eight, in [Rh9As(C0)21]2-,g to three, in [Fe3(C0)gAs2],10 for arsenic and six in [HRu,(CO),,]I1 to one in [H~OS~(CO)~~] I2 for hydrogen. They have been found to stab&e unusual metal cluster geometries like square prismatic or butterfly arrangements and to increase resistance to cluster degradation. Interstitial heteroatoms also allow the synthesis of high nuclearity clusters of metals like rhenium, which require more electrons than could conveniently be provided by external ligands, by furnishing electrons from within the cluster. This * In this article the term “cluster” is used to describe any discrete molecule containing three or more metal atoms held together, although not necessarily in a closopolygonal or polyhedral array, by metal-metal bonds.
principle is illustrated by the recent report of the synthesis of the high nuclearity rhenium cluster [Re7C(CO),,13- l3 which contains an interstitial carbon atom. In addition when the heteroatom is exposed it provides a possible alternative site of attack for incoming ligands. It is often difficult to draw the line between which heteroatoms in transition metal clusters should be described as “interstitial” and which “surface” species. The extremes are well defined : for example, in [Ru6C(CO),,] I4 the carbide-atom is completely enclosed by an octahedron of ruthenium atoms, while the oxygen atom in [Os60(CO)1g] ’ sits above a triangle of osmium atoms. The carbide in the former is therefore described as interstitial while the oxygen in the latter is clearly a surface species. There is, however, a “grey” area in between consisting of clusters containing heteroatoms displaying a wide range of bonding modes which cannot easily be classified. For example, in [Rt13C(C0)~3] 5 the carbon atom lies slightly below the plane of metal atoms making up the case of the square pyramid and could therefore be described as a surface species. However, since it is also bound to the fifth ruthenium atom and is partially enclosed by the metal cage it could be described as occupying an interstitial site, the carbon being in the centre of a nido-octahedral structure and a “semi-interstitial” carbide. For the purposes of this article, since the whole spectrum of bonding modes and coordination numbers of heteroatoms will be covered, I do not intend to dwell on classification, but rather to illustrate the wide variety of structures adopted by transition metalheteroatom clusters. At present the range of main group atoms found in transition metal clusters consists of H, C, Si, Ge, Sn, P, N, As, Sb, 0, S, Se and the halogens. In the following pages is summa&d what is known to date
1307
J. N. NICHOLLS
1308
RRhlON
o"lOC
*
\v*.:
...._..... *_ ......._.. Y--&&y Fe4N
Fig.
1.
:-
.! :
OS60
.’ : \.. i ..
i
l. ,’
: ,;‘ : : : : : :
Ni3S2
[OS,&!(CO),,]~-, The transition metal-heteroatom cores of [PtRh,0N(CO),,]3-, CRudWOM, CFdWO)121-, CW4W%J and CNi3GH5)3S21.
in the field of coordinated heteroatoms in transition metal cluster chemistry. The systems that will be dealt with are those in which the heteroatom is bound exclusively to metal atoms, excluding, for example, “carbide”-species in which the “carbide”atoms are actually present as C,-units and can therefore be alternatively described as organic fragments. Firstly the hydrido-clusters, which include interstitial, terminal and bridging hydrides will be discussed. Clusters involving group IV heteroatoms will be taken next followed by groups V, VI and VII. It will become apparent that in moving from group IV to VII there is an increased tendency for the heteroatom to be found coordinated to the surface of the cluster rather than in an interstitial site. THE HYDRIDO-CLUSTERS
The interstitial nature of hydrogen in binary metallic hydrides is well established but it was not until 1967 that the first genuine example of a transition metal cluster containing an interstitial hydride ligand was reported. That compound was the halide cluster [HNb,I, i] ls and the hydride was located directly using neutron diffraction methods. In the following year the report of another interstitial hydride, this time in a metal carbonyl as opposed to a metal halide cluster, was the subject of some controversy. It was suggestedi that despite the highly symmetrical packing of carbonyl ligands and the lack of 13C-‘H coupling in [HRu,(CO),s] -, which the authors had used as evidence for the interstitial nature of the hydride,l’ the fact that the
‘H NMR showed a large downfield shift indicated a hydride associated with a carbonyl, i.e. a formyl group. The authors’ original formulation was later vindicated after a neutron study resulted in the direct location of the hydride at the centre of the octahedron of ruthenium atoms” (see Fig. 2). Since then, several examples of hydrogen occupying regular or distorted octahedral cavities in clusters have been seen. It has also been shown that a wide range of ‘H NMR shifts exists for interstitial hydrides, from -23.2l’ to 29.36.” Another series of hydrido-clusters is now known in which hydrogen occupies a semi- or nidooctahedral cavity. That is to say the hydride is located in the square plane of a square pyramid of metal atoms and may therefore be considered to be five-coordinate (see Fig. 2). The series is made up of the cluster [HRh14(CO),,]3- ‘* and the family CRh13H5-.(COhl”- (n = 2,3,4).’ g In each case the hydrides have not been directly located but their positions within the clusters may be inferred from solid state bond length considerations. In solution, however, ‘Hand 13C NMR studies have shown that the hydrides migrate freely within their cluster frameworks.20 Mobility of a slightly different nature has been demonstrated for the octahedrally coordinated hydride in the cluster [HRu,(CO),,,] -, described previously. The preparation of this cluster, as for many hydrido-clusters, involves protonation of an dianion anion, in this case the octahedral [RUDE *] 2 -. The hydride immediately occupies a position at the centre of the octahedron : however,
1309
Main group heteroatoms in transition metal clusters
Fig. 2. The ruthenium-hydrogen core of [HRu,(CO),,]and the proposed locations of the hydride ligands in [HRhis(CO),J4and [HOs,,C(CO),,]-.
on further protonation the interstitial hydride leaves the central cluster cavity and adopts a bridging site on the surface of the octahedron
in
CHzRu,Wh1.21 In 1982 an interstitial hydride having an even lower coordination number was reported.22 Since the ligand distribution in the cluster [HOs,,C(CO),,] - was found to be remarkably similar to that of [OS&(CO),,]~-, from which it was prepared, and since the ‘H NMR signal at 15.456 indicated that the hydride ligand was directly bonded to the metal core, it was supposed that the hydride was in an interstitial site. The octahedral cavity was already occupied by a carbide-atom, so it seemed reasonable to suggest that the hydride was sited within a tetrahedral cap (see Fig. 2), and from bond length considerations it appeared to be disordered over two sites. Observation of 1870~-1H coupling in a high resolution ‘H NMR study of [HOS,,,C(CO)~J - has recently allowed confirmation of this proposal in that the 1870~-1H couplings and satellite intensities have proved to be entirely as expected for a tetrahedrally coordinated hydride. 23 A similar situation is observed in the case of the non-carbido cluster [H,0s,,(CO),J2for which it is proposed that three hydrides are to be found in tetrahedral capping sites while the fourth occupies the octahedral cavity filled by carbon in the isoelectronic [OS,,C!(CO),,]~-.~~ The majority ofcluster bound hydrogen atoms are found on the surfaces ofclusters in p3- or p2-bonding situations, as illustrated by [H,Re,(CO),,] 25 and [H4R~4(C0)12] 26 respectively (see Fig. 3) and in
many such clusters the hydride ligands are found to fluxional. For example the hydrides in [H4R~4(C0)12] were found to be mobile, even in the solid state, and although for the analogous osmium cluster they were static in the solid state at temperatures of up to 300 K, the hydrides in ~,OS,(CO),~] are dynamic in solution.23 Although well known in mononuclear chemistry, terminal hydride ligands are rarely observed on clusters : in one example, [H20s3(CO), 1],12 both terminal and p2-bridging hydrides are seen. However, this molecule is only moderately stable and reverts back to [OS~~~H)~(CO),J in the absence of CO (see Fig. 4). Hydrido-clusters may be prepared by the protonation of anions, reactions involving mol-
H4R”4
H4R=4
Fig. 3. The structures of [H,Ru4(CO),,] [H4Re4(CO),,] (carbonyl ligands omitted).
and
1310
J. N. NICHOLLS
[H,oa~q,
1
bpp\J
Fig. 4. The structures of [H,Os,(CO),,]
ecular hydrogen, or by the addition of molecules containing hydrogen atoms, examples of which are given below :
[RusC(CO),,]
H, --, CH40s4(W121~2g
+HBr + [HRu&(C0)15Br].30
The main reaction of the coordinated hydrogen atoms in these clusters is their removal in deprotonation experiments. Interstitial hydrides such as [HOs,,C(CO),,] - and [HCo,(CO), J - are moderately stable in non-coordinating solvents but in the presence of solvents like THF, CH,OH and (CH3)2CO are readily deprotonated.22 Other interstitial hydrides, like [HRu,(CO),,] - and [H40s10(C0)24]2-, however, resist deprotonation, even under severe conditions. Deprotonation has also been observed for surface bound p2- and p3hydrides, for example [H,OS,(CO),~] loses a proton to give [HOs,(CO),,] -,31 in basic solvents and a proton is readily removed from [HRu,C(CO), 5Br]
1].
by treatment with proton (dimethylamino)-naphthalene].32 GROUP
[CO~(CO)~J~- +H+ --* [HCO~(CO)J-,~~ COWO),2l+
and [H,Os,(CO),
sponge
[1,8-bis-
IV HETEROATOMS
Simple interstitial binary carbides have been known for some time, but it was not until 1962 that the first carbide-cluster was reported in the literature.33 An X-ray structure revealed that the carbonyl cluster [Fe,C(CO), J contained a carbon atom bound to five iron atoms. The carbide-atom was found to be only partially shielded by the Fe, square pyramidal cage and therefore open to attack by incoming ligands, although no such attack was actually observed. Since then the number of carbide-carbonyl clusters has increased enormously and carbides have become the largest and most widely studied group of interstitial clusters. The maximum number of metal atoms to which carbon has been found to be coordinated is eight : [CO,C(CO),,]~provides a rare example of carbon occupying a tetragonal antiprismatic cluster cavity.34 More commonly,
n-5
n.4
nz6
n-8
_ _.._. _*.__ ..__ ./ . w,:. [cogco@-
Fig. 5. The M,C core geometries displayed by transition metal carbide clusters.
1311
Main group heteroatoms in transition metal clusters 0
*. * ‘..H,.” [HFe4WKO)1~]
(i) MeOH (ii) NE13
>
[Fe4 C( CO\31
[FeL(CO),2
C CO2 Me]-
Fig. 6. Reactions occurring at the carbide-carbon atoms of Fe& clusters.
carbon is bound to six metal atoms, in octahedral or trigonal prismatic cages, or five metals, in square pyramidal (nido-octahedral) or bridged butterfly (urachno-pentagonal bipyramidal) sites. Recent developments have also included the discovery of carbon bound to four metals in an open butterfly arrangement. The various M,C skeletons described above are illustrated in Fig. 5. As yet no carbide-clusters have been observed in which carbon is less than four coordinate, although there is evidence for low coordinate carbide-cations in the mass spectra of carbonyl clusters.35 It has now been established by 13C-labelling that the carbide-atom in carbide clusters can be derived either from a carbonyl group, via disproportionation of CO to give CO, and C or loss of the elements of water from a hydrido-cluster, or from an external source : COs3(W,
lPY1 + COs&(CO)2412 - + CO29
CRu3(W,,l+
[H,Ru,(CO),,]
CRw,WO),,l
+C%‘4
--f [Ru,C(CO),,]
[Rh(CO),] - + CC1, + [Rh,C(CO),,] [Co,(CO),]
+ “H20”,36 - + Cl -,37
+ Cg, -+ [C0~5C(C0)4,].~~
New carbide-clusters
may also be prepared via the
*This observation is in accordance with recent MO calculations on iron carbide clusters which predict that the metal-carbide orbitals are not su!lkiently high lying for the carbon atom to be involved in reactions until the number of metal atoms is reduced to four.90
controlled species :
degradation
CRu,C(W,,~
of performed
carbido-
CRu&(CO),,l+ CRu(CO),l,’
Alternatively, preformed carbides may be used as building blocks in the synthesis of larger clusters : CFe4C(CO),212- + CW(CO)3(NCCHs)l + [Fe4WC(C0)r5]2-,40 CFe5C(COh412- + CW(CO),(NCCH,)I + Fe,WC(CO),,]2-.40 Most of the studies on the classes of heteroatomcontaining complexes which follow have concentrated on the synthesis and structural analysis of the molecules in question. The study of carbidoclusters, however, has also involved an investigation of their reactivity towards a variety of reagents. Direct attack at the carbide-carbon atom has been observed for the tetranuclear iron carbide-clusters : reaction of [HFe,C(CO),,]with H+ leads to the formation of a methylidyne (CH) group bound to the tetranuclear cluster41 and treatment of [Fe,C(CO),,] with methanol and then base yields a butterfly (carbomethoxy) methylidyne complex42 (see Fig. 6). No such direct attack at the carbon atom has been seen for the higher nuclearity iron clusters or any of the Re, Ru, OS, Rh or Co carbides.* However, the carbon atom does confer on the cluster considerable stability towards degradation and in addition allows the metal framework to be relatively
1312
J. N. NICHOLLS
Lospq,l
c~~c(q~l
Fig. 7. The reversible carbonylation of [Os,C(CO),,].
flexible. Facile structural transformations therefore result from addition or removal of electrons, giving rise to a wide range of M,C framework geometries. For example, addition of CO to [Os,C(CO),,] results in the opening up of the square based pyramid to give a bridged butterfly arrangement of metals, while further heating of the product [Os,C(CO),,], even under CO, results in the ejection of CO and regeneration of [Os,C(CO), .J (see Fig. 7).43 Similarly, reaction of [Ru&!(CO),,] with HSEt results in the opening up of an octahedron of ruthenium atoms to give another wing tip bridged butterfly structure, but in this case a sixth metal atom is found bridging an edge ofthe basic five atom unit44 (see Fig. 8). It is hoped that we shall soon see the synthesis of other low nuclearity carbide-clusters and it will be interesting to compare their reactivity, in particular that of the carbide-carbon atom with that of the tetranuclear iron carbides. As far as other group IV heteroatom clusters are concerned, a much less extensive series of compounds is known. Complexes of the type [Co,(CO),-ECo(CO),J (E = Si, Ge) have been reported in which the silicon or germanium atom is bound to four cobalt atoms, of which three form a cluster and the fourth is held in place by the tetrahedrally coordinated heteroatom (see Fig. 9) :45,46 a similar situation is observed in the tetrahedrally coordinated tin complex {[(C,H,)Fe
(W212Sn2Fe3WM. 47 In this molecule both faces of the triangular Fe3(C0)9 cluster unit are capped by tin atoms. From the limited number of examples known it would appear that although these atoms are from the same periodic group as carbon they show a much reduced tendency for the formation of complexes with transition metal clusters.
GROUP
V HETEROATOMS
Many features of carbide-clusters are paralleled by nitrido-clusters. Nitrogen, like carbon, appears to stabilise the less frequently observed cluster geometries. Indeed, [Rh,N(CO), 5] - 5o and [Rh& (CO),,]* - 51have the same trigonal prismatic structures, [Fe,C(CO),,] 33 and [HFe,N(CO),,] 52 are both square prismatic and [Os4N(CO),J53 and [Fe,C(CO),,]*6 contain the same open butterfly ofmetal atoms (see Fig. 10). In clusters having high transition metal : heteroatom ratios, however, it has been said that nitrides tend towards lower coordination than carbides and lie in less regular cavities.3 Thus while the carbon atoms in [OsroC(CO)24]*- and [O~,lC(C0)27]*OCCUPY octahedral and trigonal prismatic sites respectively,54 the nitrogen atom in [PtRh,oN(C0)2,]33 is found in a distorted trigonal prismatic cavity. Outside cluster chemistry, nitrogen atoms have
[HRu 6 cCcC+p,]
Fig. 8. The reaction of [Ru&(CO)~~] with ethane-thiol.
1313
Main group heteroatoms in tradition metal clusters
Fig. 9. The core geometries adopted by M,E clusters (E = Si, Ge, Sn).
been found bound to a single metal centre, as in [Re(N)Cl,(PPh,),],55 linking two metal atoms, as in [Ru,NC~,(H,O)~]~- 56 or three metals as in [Ir3(N)(g04)6(H20)3] “in whichnitrogen sitsin the centre of a plane of three iridium atoms which are not bound to each other. Within cluster chemistry, however, such low metal : heteroatom ratios are rare. The lowest coordination ObSeNed for atomic nitrogen in a cluster is three: in [MO,(N) ‘* the nitrogen atom occupies an KwoLcP3l unusual site in the plane of the three molybdenum atoms. In this case two metal-metal bonds hold the MO, unit in an open triangular arrangement, as shown in Fig. 10 and result in a very exposed nitrogen atom. i-i=&
. EJ
Just as with the c~bido-clusters the source of nitrogen in nitrido-clusters may be either external or internal. For example, addition of NO+ to [HRu,(CO),,]and [Co6(CO)15]zgives [HRu,N(CO),,] 5gand [Co,N(CO),,]6o respectively while treatment of [Rh7(CO)&j3with CO/NO mixtures yields [Rh,N(cO), 5] -,6’ all three reactions involving an external source of nitrogen. An example of an internal source of nitrogen would be a coordinated nitrosyl group: pyrolysis of Peru,,,] yields the nitride [FeRu,N (CO),,]- 61 and the reaction of [M~~O)~~O) (C,H,)] with [MO~(co)6(c~H~)~] is used to prepare the trinuclear cluster [Mo30(0)(CO), (C5H5)3].62 In the latter case it has been proved n=3
tks
.’
‘...
.. A
,’
.’
:
,:’
..
-...
I.
.
_
n=4
IR”sN’CO\J
[Os$-J( COl,2]-
Fig. 10. The various M,N core geometries found in nitride-clusters. [Ru,N(CO),,]characterised spectroscopkally but not crystaIlographically.62
has been
1314
J. N. NICHOLLS nd2
n-3
.,,’
..~~... j .....
v
n=6
[co3(co)9 ASI
n=8
[ccpqd
-
E= P,As
Fig. 11. The transition metal-heteroatom core geometries displayed by clusters involving atomic phosphorus, arsenic or antimony as ligands.
conclusively by I80 labelling that on conversion of the nitrosyl ligand to a nitride the nitrosyl oxygen is lost as CO*. The final method available for the synthesis of new nitrido-clusters is the breakdown or build up of preformed nitridoclusters. For example, reaction of [Rh,N(CO), s] with [PtRh,(CO)1,]2results in the formation of [PtRh,,N(C0),,]3-.3 In contrast with clusters involving group IV heteroatoms which are as yet mainly limited to carbides, there are several examples of the coordination of the other group V elements, P, As and Sb to transition metal clusters. The first organometallic complex containing naked phosphorus as a ligand was reported in 1973:63 in [Co,Cp,P,] each phosphorus atom bridges three transition metals, however this molecule only contains two metal-metal bonds and so does not qualify for inclusion as a cluster compond. Arsenic and antimony have been found in ,u,-sites in metal cluster compounds and in [Fe,(CO),As,] an arsenic atom caps either side of the Fe, triangle,” but the main interest in the group V heteroatom-transition metal clusters is in those containing interstitial P, As or Sb atoms (see Fig. 11). It is interesting to compare such species with the nitride clusters to see just what effect these larger heteroatoms will have on the metal geometries : they will certainly be unable to adopt the same structures as the nitrides since even the
smallest, phosphorus, is too big to fit into an octahedral or trigonal prismatic cavity. Clusters involving interstitial phosphorus and arsenic atoms show a preference for the square antiprism65*66 as the primary building block (see Fig. 11) although when fewer metals are available, as in [Co6P(CO)r6]-, a more open StrUCtUre IUUSt be adopted in order to accommodate the large phosphorus atom. 67 When antimony, which is even larger takes up a five electron donor interstitial coordination mode it requires an even larger cavity andin[Rh,,Sb(C0),,]368wesee thefirstexample of an icosahedral cluster geometry, made possible by the stabilising effect of the central antimony atom. In terms of synthesis the phosphorus, arsenic and antimony clusters all result from pyrolysis reactions in the presence of an external source of the heteroatom: PCl,, white phosphorus or PPh3, AsPh, and SbCl, or SbPh, respectively. It is worth noting that while pyrolysis of phosphite derivatives of osmium clusters results in the formation of carbido-clusters,6g pyrolysis of [Rh(CO),(acac)] with triphenyl phosphine gives a phosphidocluster.66 This difference may simply be due to the P-O bonds in the phosphite being stronger than the P-C bonds in the phosphine or it may reflect differences between the metals, in which case it could have wider implications for the possible synthesis of
1315
Main group heteroatoms in transition metal clusters
MojNHO)
%O4
Fig. 12. Essential features of the structures of [Mo,o(O~CO),Cp,]
phosphido-clusters from other than the cobalt subgroup metals. Once again the presence of the heteroatoms in the group V atom-metal clusters gives added stability towards cluster breakdown : rhodium clusters are usually transformed to [Rh,(CO), J - and [Rh(CO),]- under CO/H2 but [Rh9P(CO),J2is stable to 600-800 atm CO/I& at 23OYY and [RhlzSb(CO),,]3is stable under 500 atm CO/H2 at 150”C6s There has been only one report of the reactivity of an exposed group V heteroatom in a metal cluster. Carbonylation of [Ru,N(CO),,]results in the formation of [Ru,N(CO),,]which reacts further with CO to give [Ru,(NCO)(CO), 3] in which process the nitrido-nitrogen atom is converted to an isocyanato-ligand.62 No reactivity has been reported for the cluster-bound nitrogen atom in [Fe,N(CO),,] - or the phosphorus atom in [Co6P(CO)16]-, although they, too, might be expected to undergo reaction. GROUP
and m,O,F,]
- 3H,O.
oxygen atom 73 but in most other cases the oxygen source has not been identified. In no case has an interstitial oxygen atom in a metal cluster been reported, a feature of oxide chemistry which is in contrast with that of sulphides since two examples of interstitial sulphur containing clusters are known. Atomic sulphur displays a wide variety of bonding modes in its coordination to transition metal clusters. In [Mo~(C~H~)~SJ+ the p2- and ,u3modes are demonstrated74 and in [0s6(c0)17s2] two ,u4S ligands have been identified as a result of a crystallographic investigation” (see Fig. 13).
VI HETEROATOMS
Examples of naked oxygen atoms as ligands are well known in mononuclear transition metal complexes like OsO,, where the metal is in a high formal oxidation state and, unlike the nitrides, terminal oxides are also occasionally found in transition metal clusters. Thus [Mo30(0)(CO),Cp3] contains an (Mo=O) unit, the oxygen atom of which derives from an external and as yet unknown source of oxygen61 (see Fig. 12). Atomic oxygen is also found in W,” Mn,‘l Ti,72 Ru’~ and OS’ clusters, as a p2- or p,-bridgingligand, as illustrated by the complexes [W304F9] - 3H20 ” and [Os60(CO)19] ’ whose structures are shown in Figs. 1 and 12. In the syntheses of [Os60(CO)19] and [Ru3~30)(0)(CO)6(02AsCH2As02)] molecular oxygen has been found to be the source of the
Fig. 13. The molecular structures of the complexes CMo,GH,G%I and[os6(cO),,s,1.
1316
J. N. NICHOLLS
sulphur in the form of, say, an SCN ligand may be used as the starting material. An example of the use of such a route is the preparation of [Rh10S(C0),J2from [Rhs(CO),gSCN),Jz-.78 GROUP
VII HETEROATOMS
A series of transition metal clusters exists in which halide atoms are the only ligands present. They RhlOS %7 =2 display a range of halide bonding modes from p3- to Fig. 14. The Rh-S core geometries of the clusters pLt-to terminal and are characterised by two main ERh,S(Wd2- andERGW%,13-. structural types, illustrated in Fig. 15, containing either octahedral M,(p3X), or M&L~X),, units, which are very stable and only broken up under Selenium, too, has been found to act as a p&and vigorous conditions. Thus, the salt [Mo,C~,C~,]~capping a triangle of metal atoms in, for example [Co3(CO)9Se].76 The exposed sulphur and selenium contains eight face capping and six terminal chlorines” while JNb,Br,2Cl,J4has 12 edge atoms in these molecules appear to be characterised by their reluctance to undergo reactions. There is, bridging bromine and six terminal chlorine however, at least one example of reaction at a p&S ligands81 Such clusters are not limited to the early centre. In 1978 reaction of, [(CH,),OJ’ with transition metals and, in an example from the other end of the transition series, [Pt&!l,,] displays an [HOs,(CO),S]was found to give [HOs,(CO), @$Me)], although the product was never fully octahedral structure with 12 @,I ligandsE2 very similar to that of the core of [Nb6X,J4-. characterised. ” Generally speaking, however, clusters of the later Clusters containing interstitial sulphur atoms involve much higher coordination numbers: in transition metals containing halide atoms are mainly [RhIOS(CO),,Jz- sulphur is found to stab&e an mixed halide~rbonyl species having terminal or p2unusual square prismatic cavity,‘* while in bridging halogens. For example in [Os,C(CO), sIl [Rh17S,(CO),,J3each sulphur atom is bound to the iodide atom is found in a terminal positionE3 on the cluster while in [Ru4(CO)r3Cl]chlorine nine rhodium atoms,7g as shown in Fig. 14. As far as the source of the sulphur atom is bridges the wing tips of an open butterfly of concerned, H2S, SO,, S andC$ have all been used to ruthenium atomsE4 (see Fig. 16). Synthesis of the introduce atomic sulphur into metal clusters. early transition metal halide clusters normally involves the reduction of the higher mononuclear or Alternatively, a cluster which already contains
M=Nb,Ta
Fig. 15. The structures of the metal-halide clusters. The terminally bound halide ligands are omitted for clarity.
1317
Main group heteroatoms in transition metal clusters
[ossc’co~5 I I-
[RuJCOI,~
Cl ] -
Fig. 16. The structures of [Os,C(CO),,I]CRuACO)zdX - .
and
dinuclear halides in the presence of an excess of the metal in question at elevated temperatures, while in the case of the carbonyl halides addition of X2, HX or [Au(PPh,)X] to a neutral carbonyl cluster or reaction of an anion with I2 is the usual route. As has been mentioned above the M,X, and M6XlZ cores of the early transition metal halides are very stable to degradation as these central units contain the bridging halides in each case. The terminally bound halogen atoms may however be removed, as the halide ions X-, and the cations CMJ,14+ and CM& A *+ have been studied as solutions in neutral monodentate ligand solvents like amines, alcohols or water.s5 It has also proved possible to remove the halogen atoms from some of the carbonyl halide clusters, or to persuade them to change from being terminal one electron donors to bridging three electron donor ligands and vice versa.
Thus the two bridging iodine ligands in [OS,&(CO)24IJ are removed stepwise on treatment with I-, pyridine or phosphites6 giving first [Os,,C(CO),,I]and then the dianion [OS,,C(CO),~]*- (see Fig. 17) ; the iodine atoms are removed as I+ to give I2 or IL2 (L = py, P(OR),) respectively. The halideligand in [HRu,C(CO),,Cl] is removed in a different way : simple thermolysis of the pentanuclear chloro-cluster in hydrocarbon solvents results in the ejection of molecular HCl, leaving the carbonyl cluster [Ru&(CO),,] in solution.32 On heating [HRuSC(CO),,Cl] in the presence of an excess of HCI, however, this reaction is suppressed in favour of loss of CO and formation of the p2C1 species [HRu,C(CO)~,C~], a step which may be reversed by addition of CO to this or any of the formally analogous series [ARu,C(CO),,X] (A = Au(PR,), H ; X = Cl, Br), to give the [ARu,C(CO)~,X] molecules” (see Fig. 18). This example clearly demonstrates the ease with which halide heteroatoms may be converted from two to four electron donors and back in transition metal carbonyl clusters. CONCLUSION
A large number of clusters are known which contain main group heteroatoms bound exclusively to transition metals. The heteroatoms display a wide range of bonding modes and their coordination
Fig. 17. The stepwise removal of iodine ligands from [Os,,C(CO),,I,].
[Ru5C(CO),d
bRugc(CO)15 Xl ( A=AuFPh3,H
; X=CI , Br ,I
h5
x]
C(COl,4
)
Fig. l&The conversion ofhalideligands between terminal and bridging bonding modes in Ru,C clusters.
1318
J. N. NICHOLLS
numbers range from 12, for antimony, to one, for hydrogen, oxygen and the halogens. When found in interstitial sites they often impart considerable stability towards degradation to the metal cluster, as seen by comparison with the stability of simple metal carbonyl clusters. They are often associated with the less commonly observed metal geometries such as icosahedral M, 2, square antiprismatic EM,, square pyramidal M, and open butterfly M, structures and, when exposed, provide possible alternative sites for attack by incoming reagents. Hydride and halide ligands can often be removed from their parent clusters. The group IV, V and VI heteroatoms, however, seem to be more firmly bound and direct attack at these cluster-bound heteroatoms has as yet been limited to the carbon atom in Fe, clusters, the sulphur atom in CHOMW~SI - and the nitrogen atom in [Ru,N(CO),,] - which are converted to (CH) or (CCO,Me), SCH3) and (NCO) groups respectively. While many carbide-, hydrido- and nitridoclusters are known few clusters have been prepared which contain heteroatoms like phosphorus, arsenic and sulphur. The study of these species has also largely been directed towards synthesis and there is therefore insufficient information to allow a real comparison of the reactivity of the cluster-bound heteroatoms. Clearly this is an area which will receive more attention as the study of transition metal clusters containing main group heteroatoms progresses. Acknowledgement-I wish to thank Gonville and Caius College, Cambridge, for the award of a Research Fellowship.
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5.
6. 7.
8.
M. Tachikawa and E. L. Muetterties, Prog. Znorg. Chem. 1981,28,203. E. L. Muetterties, T. N. Rhodin, E. Bard, C. F. Brucker and W. R. Pretzer, Chem. Rev. 1979,79,91. S. Martinengo, G. Ciani and A. Sironi, J. Am. Chem. Sot. 1982,104,328. P. F. Jackson, B. F. G. Johnson, J. Lewis, W. J. H. Nelson and M. McPartlin, J. Chem. Sot., Dalton Trans. 1982,2099. D. H.Farrar,P. F. Jackson,B. F.G. Johnson, J. Lewis, J. N. Nicholls and M. McPartlin, J. Chem. Sot., Chem. Commun. 1981,415. D. E. Fjare and W. L. Gladfelter, J. Am. Chem. Sot. 1981,103,1572. R. J. Goudsmit, B. F. G. Johnson, J. Lewis, P. R. Raithby and K. H. Whitmire, J. Chem. Sot., Chem. Commun. 1983,246. H. Varenkamp, V. A. Uchtman and L. F. Dahl, J. Am. Chem. Sot. 1968,90,3272.
9. J. L. Vidal, Znorg. Chem. 1981,20,243. 10. L. F. J. Delbaere, L. J. Kruczynski and D. W. McBride, J. Chem. Sot., Dalton Trans. 1973,307. 11. C. R. Eady, B. F. G. Johnson, J. Lewis, M. C. Malatesta, P. Machin and M. McPartlin, J. Chem. Sot., Chem. Commun. 1976,945. 12. J. R. Shapley, J. B. Keister, M. R. Churchill and B. G. de Boer, J. Am. Chem. Sot. 1975,4145. 13. G. Ciani, G. D’Alfonso, M. Freni, P. Romiti and A. Sironi, J. Chem. Sot., Chem. Commun. 1982,339. 14. A. Sirugi, M. Bianchi and E. Benedetti, J. Chem. Sot., Chem. Commun. 1969,596. 15. A. Simon, 2. Anorg. Chem. 1967,355,311. 16. P. Chini, G. Longoni, S. Martinengo and A. Ceriotti, Adv. Chem. 1978,167,l. 17. P.F. Jackson,B.F. G. Johnson, J. Lewis,P. R. Raithby, M. McPartlin, K. D. Rouse, J. Allibon and S. A. Mason, J. Chem. Sot., Chem. Commun. 1980,295. 18. G. Ciani, A. Sironi and S. Martinengo, J. Organomet. Chem. 1980,192, C42. 19. G. Ciani, A. Sironi and S. Martinengo, J. Chem. Sot., Dalton Trans. 1981. 519. 20. B. T. Heaton, S. Martinengo, R. J. Goodfellow and P. Chini, J. Chem. Sot., Chem. Commun. 1977,39. 21. M. R. Churchill and J. Wormald, J. Am. Chem. Sot. 1971,93,5670. 22. P. F. Jackson, B. F. G. Johnson, J. Lewis, M. McPartlin and W. J. H. Nelson, J. Chem. Sot., Chem. Commun. 1982,48. 23. E. C. Constable, B. F. G. Johnson, J. Lewis,G. N. Pain and M. J. Taylor, J. Chem. Sot., Chem. Commun. 1982, 754. 24. D. Braga, J. Lewis, B. F.G. Johnson,M. McPartlin, W. J. H. Nelson and M. D. Vargas, J. Chem. SOL, Chem. Commun. 1983,241. 25. R. Bau, S. W. Kirtley, T. N. Sorrel1 and S. Winarko, J. Am. Chem. Sot. 1974,96,998. 26. R. D. Wilson, S. M. Wu, R. A. Love and R. Bau, Znorg. Chem. 1978,17,1271. 27. A. T. Nicol and R. W. Vaughan, Adv. Chem. Ser. 1978, 167, 245. 28. D. W. Hart, R. G. Teller, C. Y. Wei, R. Bau, G. Longoni, S. Campanella, P. Chini and T. F. Koetzle, Angew. Chem. Znt. Ed. Engl. 1979,18,80. 29. K. Wong, Ph.D. thesis, Cambridge University, 1978. 30. I. A. Oxton, D. B. Powell, D. H. Farrar, B. F. G. Johnson, J. Lewis and J. N. Nicholls, Znorg. Chem. 1981,20,4302. 31. C. R. Eady, B. F. G. Johnson and J. Lewis, J. Chem. Sot., Chem. Commun. 1976,302. 32. B. F. G. Johnson, J. Lewis, J. N. Nicholls, J. Puga, P. R. Raithby, M. J. Rosales, M. McPartlin and W. Clegg, J. Chem. Sot., Dalton Trans. 1983,277. 33. E. H. Braye, L. F. Dahl, W. Hiibel and D. L. Wampler, J. Am, Chem. Sot. 1962,84,4633. 34. V. G. Albano, P. Chini, G. Ciani, S. Martinengo, D. Strumolo and M. Sansoni, J. Chem. Sot., Dalton Trans. 1978,463. 35. B. F. G. Johnson and J. Lewis, Act. Chem. Res. 1968, 245. 36. J. N. Nicholls,D. H. Farrar,B. F. G. Johnson, J. Lewis
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61. N. D. Feasey and S. A. R. Knox, J. Chem. Sot., Chem.
Commun. 1982,1062. 62. M. L. Blohm, D. E. Fjare and W. L. Gladfelter, Inorg. Chem. 1983,22,1004.
1319
63. G. L. Siion and L. F. Dahl, J. Am. Chem. Sec. 1973, %, 2175. 64. A. Vizi-Orosz, V. Galamb, G. Palyi, L. Marko, G. Bor and G. Natile, J. Organomet. Chem. 1976,107,235. 65. J. L. Vidal, W. E. Walker and R. C. Schoening, fnorg. Chem. 1981,20,238 ; J. L. Vidal, Znorg. Chem. 1981,20, 243. 66. J. L. Vidal~ W. E. Walker, R. L. Pruett and R. C. Schoening, Znorg. Chem. 1979,18,129. 67. G. Ciani and A. Sironi, J. Organomet. Chem. 1983,241, 385. Gem.1981, 68. J. L. Vidal and J. M. Troup, J. Org~~t. 213,351. 69. J. M. Fernandez, B. F. G. Johnson, J. Lewis and P. R. Raithby, J. Chem. Sot., Dalton Trans. 1981,225O. 70. K. Mennemann and R. Mattes, Angew. Chem. Znt. Ed. Engl. 1976,15, 118. 71. A. Bino, F. A. Cotton, Z. Dori and B. W. S. Kolthammer, J. Am. Chem. Sot. 1981,103,5779. 72. J. C. Huffman, J. G. Stone, W. C. Krusell and K. G. Caulton, J. Am. Chem. Sot. 1977,99,5830. 73. G. Lavigne, N. Lugan and J. J. Bonnet, Nouu. J. Chim. 1981,5,423. 74. P. J. Vergamini, H. Varenkamp and L. F. DabI, J. Am. Gem. Sot. 1971,93,6327. 75. R. D. Adams, I. T. Horvath and L. W. Yang, J. Am. Chem. Sot. 1983,105,1533. 76. C. E. Strouse and L. F. Dal& J. Am. Chem. Sot. 1971, 93, 6032. 77. B. F. G. Johnson, J. Lewis, D. Pippard and P. R. Raithby, J. Chem. Sot., Chem. Corn. 1978,551, 78 G. Ciani, L. Garlaschelli, A. Sironi and S. Martinengo, J. Chem. Sot., Chem. Com_meE.1981,563. 79. J. L. Vidal, R. A. Fiato, L. A. Cosby and R. L. Pruett, Znorg. Chem. 1978,17,2574. 80. F. A. Cotton, R. M. Wing and R. A. Zimmerman, Inorg. Chem. 1967,6,12. 81. B. Spreckelmeyer and H. G. v. Schnering, 2. Anorg. Alfg. Chem. 1971,3%,27. 82. K. Brodersen, G. Thiele and H. G. v. Scbnering, 2. Anorg. Allg. Chem. 1965,337, 120. 83. P. F. Jackson,B. F. G. Johnson, J. Lewis, J. N. Nicholls, M. McPartlin and W. J. H. Nelson, J. Chem. Sot., Chem. Co~un. 1980,564. 84. G. R. Steinmetz, A. D. Harley and G. L. Geoffroy, lnorg. Chem. 1980,298s. 85. J. C. Sheldon, P. A. Vaughan, J. H. Sturtevant and L. Pauling, J. Am. Chem. Sot. 1950,72,5477. 86. D. H. Farrar, P. F. Jackson,B. F.G. Johnson, J. Lewis, W. J. H. Nelson, M. D. Vargas and M. McPartlin, J. Chem. Sot., Chem. Commun. 1981,1009. 87. B. F. G. Johnson, J. Lewis, J. N. Nicholls, J. Puga and K. H. Whitmire, J. Chem. Sot., Dalton Trans. 1983, 787. 88. J. A. Bertrand, F. A. Cotton and W. A. Doll&, Znorg. Chem. 1963,2,1166. 89. K. Jodden, H. G. V. Scbnering and H. Schafer, Angew. Chem. int. Ed. Engl. 1975,14,570. 90. J. Kolis, F. Basolo and D. F. Schriver, J. Am. Chem. Sot. 1982,104,5626.
Pdy/w&m Vol. 3, No. 12, PP. 1307~1319,1984 Printed in Great Britain
MAIN
GROUP HETEROATOMS IN TRANSITION CLUSTERS*
METAL
J. NICOLA NICHOLLS University Chemical Laboratory, Lensfield Road, Cambridge, U.K. (Received 25 March 1983)
Abstract-The class of transition metal cluster compounds which contain individual main group heteroatoms is surveyed. Hydrido-clusters and clusters containing group IV, V, VI and VII atoms are dealt with in turn with reference to their synthesis, structure and reactivity.
As a result of the anticipation of similarities between transition metal clusters’ and fragments of bulk metals, particular attention has been paid during the development ofmetal cluster chemistry to complexes containing isolated main group atoms. Such clusters have been compared with simple metallic interstitial compounds and atoms bound to metal surfaces and have been postulated as models for intermediates in catalytic processes. ‘*’ This may or may not prove to be a useful analogy, but interest in these molecules has resulted in the synthesis and characterisation of a fascinating and extensive class of transition metal clusters containing main group atoms. These range from truly interstitial compounds with high metal: main group atom ratios, as illustrated by [PtRh,,N(C0),,]33 and [OS,&(CO)~~]~-,~ through “semi-interstitials” like [Ru,C(CO), 31 5 and [Fe,N(CO),,] -,6 to highly exposed surface species such as ~os6(~30)(c~h17 and
CNi3W%)3(~3%18 be Fig. 1). Theheteroatoms display a wide range of bonding
modes and coordination numbers, from eight, in [Rh9As(C0)21]2-,g to three, in [Fe3(C0)gAs2],10 for arsenic and six in [HRu,(CO),,]I1 to one in [H~OS~(CO)~~] I2 for hydrogen. They have been found to stab&e unusual metal cluster geometries like square prismatic or butterfly arrangements and to increase resistance to cluster degradation. Interstitial heteroatoms also allow the synthesis of high nuclearity clusters of metals like rhenium, which require more electrons than could conveniently be provided by external ligands, by furnishing electrons from within the cluster. This * In this article the term “cluster” is used to describe any discrete molecule containing three or more metal atoms held together, although not necessarily in a closopolygonal or polyhedral array, by metal-metal bonds.
principle is illustrated by the recent report of the synthesis of the high nuclearity rhenium cluster [Re7C(CO),,13- l3 which contains an interstitial carbon atom. In addition when the heteroatom is exposed it provides a possible alternative site of attack for incoming ligands. It is often difficult to draw the line between which heteroatoms in transition metal clusters should be described as “interstitial” and which “surface” species. The extremes are well defined : for example, in [Ru6C(CO),,] I4 the carbide-atom is completely enclosed by an octahedron of ruthenium atoms, while the oxygen atom in [Os60(CO)1g] ’ sits above a triangle of osmium atoms. The carbide in the former is therefore described as interstitial while the oxygen in the latter is clearly a surface species. There is, however, a “grey” area in between consisting of clusters containing heteroatoms displaying a wide range of bonding modes which cannot easily be classified. For example, in [Rt13C(C0)~3] 5 the carbon atom lies slightly below the plane of metal atoms making up the case of the square pyramid and could therefore be described as a surface species. However, since it is also bound to the fifth ruthenium atom and is partially enclosed by the metal cage it could be described as occupying an interstitial site, the carbon being in the centre of a nido-octahedral structure and a “semi-interstitial” carbide. For the purposes of this article, since the whole spectrum of bonding modes and coordination numbers of heteroatoms will be covered, I do not intend to dwell on classification, but rather to illustrate the wide variety of structures adopted by transition metalheteroatom clusters. At present the range of main group atoms found in transition metal clusters consists of H, C, Si, Ge, Sn, P, N, As, Sb, 0, S, Se and the halogens. In the following pages is summa&d what is known to date
1307
J. N. NICHOLLS
1308
RRhlON
o"lOC
*
\v*.:
...._..... *_ ......._.. Y--&&y Fe4N
Fig.
1.
:-
.! :
OS60
.’ : \.. i ..
i
l. ,’
: ,;‘ : : : : : :
Ni3S2
[OS,&!(CO),,]~-, The transition metal-heteroatom cores of [PtRh,0N(CO),,]3-, CRudWOM, CFdWO)121-, CW4W%J and CNi3GH5)3S21.
in the field of coordinated heteroatoms in transition metal cluster chemistry. The systems that will be dealt with are those in which the heteroatom is bound exclusively to metal atoms, excluding, for example, “carbide”-species in which the “carbide”atoms are actually present as C,-units and can therefore be alternatively described as organic fragments. Firstly the hydrido-clusters, which include interstitial, terminal and bridging hydrides will be discussed. Clusters involving group IV heteroatoms will be taken next followed by groups V, VI and VII. It will become apparent that in moving from group IV to VII there is an increased tendency for the heteroatom to be found coordinated to the surface of the cluster rather than in an interstitial site. THE HYDRIDO-CLUSTERS
The interstitial nature of hydrogen in binary metallic hydrides is well established but it was not until 1967 that the first genuine example of a transition metal cluster containing an interstitial hydride ligand was reported. That compound was the halide cluster [HNb,I, i] ls and the hydride was located directly using neutron diffraction methods. In the following year the report of another interstitial hydride, this time in a metal carbonyl as opposed to a metal halide cluster, was the subject of some controversy. It was suggestedi that despite the highly symmetrical packing of carbonyl ligands and the lack of 13C-‘H coupling in [HRu,(CO),s] -, which the authors had used as evidence for the interstitial nature of the hydride,l’ the fact that the
‘H NMR showed a large downfield shift indicated a hydride associated with a carbonyl, i.e. a formyl group. The authors’ original formulation was later vindicated after a neutron study resulted in the direct location of the hydride at the centre of the octahedron of ruthenium atoms” (see Fig. 2). Since then, several examples of hydrogen occupying regular or distorted octahedral cavities in clusters have been seen. It has also been shown that a wide range of ‘H NMR shifts exists for interstitial hydrides, from -23.2l’ to 29.36.” Another series of hydrido-clusters is now known in which hydrogen occupies a semi- or nidooctahedral cavity. That is to say the hydride is located in the square plane of a square pyramid of metal atoms and may therefore be considered to be five-coordinate (see Fig. 2). The series is made up of the cluster [HRh14(CO),,]3- ‘* and the family CRh13H5-.(COhl”- (n = 2,3,4).’ g In each case the hydrides have not been directly located but their positions within the clusters may be inferred from solid state bond length considerations. In solution, however, ‘Hand 13C NMR studies have shown that the hydrides migrate freely within their cluster frameworks.20 Mobility of a slightly different nature has been demonstrated for the octahedrally coordinated hydride in the cluster [HRu,(CO),,,] -, described previously. The preparation of this cluster, as for many hydrido-clusters, involves protonation of an dianion anion, in this case the octahedral [RUDE *] 2 -. The hydride immediately occupies a position at the centre of the octahedron : however,
1309
Main group heteroatoms in transition metal clusters
Fig. 2. The ruthenium-hydrogen core of [HRu,(CO),,]and the proposed locations of the hydride ligands in [HRhis(CO),J4and [HOs,,C(CO),,]-.
on further protonation the interstitial hydride leaves the central cluster cavity and adopts a bridging site on the surface of the octahedron
in
CHzRu,Wh1.21 In 1982 an interstitial hydride having an even lower coordination number was reported.22 Since the ligand distribution in the cluster [HOs,,C(CO),,] - was found to be remarkably similar to that of [OS&(CO),,]~-, from which it was prepared, and since the ‘H NMR signal at 15.456 indicated that the hydride ligand was directly bonded to the metal core, it was supposed that the hydride was in an interstitial site. The octahedral cavity was already occupied by a carbide-atom, so it seemed reasonable to suggest that the hydride was sited within a tetrahedral cap (see Fig. 2), and from bond length considerations it appeared to be disordered over two sites. Observation of 1870~-1H coupling in a high resolution ‘H NMR study of [HOS,,,C(CO)~J - has recently allowed confirmation of this proposal in that the 1870~-1H couplings and satellite intensities have proved to be entirely as expected for a tetrahedrally coordinated hydride. 23 A similar situation is observed in the case of the non-carbido cluster [H,0s,,(CO),J2for which it is proposed that three hydrides are to be found in tetrahedral capping sites while the fourth occupies the octahedral cavity filled by carbon in the isoelectronic [OS,,C!(CO),,]~-.~~ The majority ofcluster bound hydrogen atoms are found on the surfaces ofclusters in p3- or p2-bonding situations, as illustrated by [H,Re,(CO),,] 25 and [H4R~4(C0)12] 26 respectively (see Fig. 3) and in
many such clusters the hydride ligands are found to fluxional. For example the hydrides in [H4R~4(C0)12] were found to be mobile, even in the solid state, and although for the analogous osmium cluster they were static in the solid state at temperatures of up to 300 K, the hydrides in ~,OS,(CO),~] are dynamic in solution.23 Although well known in mononuclear chemistry, terminal hydride ligands are rarely observed on clusters : in one example, [H20s3(CO), 1],12 both terminal and p2-bridging hydrides are seen. However, this molecule is only moderately stable and reverts back to [OS~~~H)~(CO),J in the absence of CO (see Fig. 4). Hydrido-clusters may be prepared by the protonation of anions, reactions involving mol-
H4R”4
H4R=4
Fig. 3. The structures of [H,Ru4(CO),,] [H4Re4(CO),,] (carbonyl ligands omitted).
and
1310
J. N. NICHOLLS
[H,oa~q,
1
bpp\J
Fig. 4. The structures of [H,Os,(CO),,]
ecular hydrogen, or by the addition of molecules containing hydrogen atoms, examples of which are given below :
[RusC(CO),,]
H, --, CH40s4(W121~2g
+HBr + [HRu&(C0)15Br].30
The main reaction of the coordinated hydrogen atoms in these clusters is their removal in deprotonation experiments. Interstitial hydrides such as [HOs,,C(CO),,] - and [HCo,(CO), J - are moderately stable in non-coordinating solvents but in the presence of solvents like THF, CH,OH and (CH3)2CO are readily deprotonated.22 Other interstitial hydrides, like [HRu,(CO),,] - and [H40s10(C0)24]2-, however, resist deprotonation, even under severe conditions. Deprotonation has also been observed for surface bound p2- and p3hydrides, for example [H,OS,(CO),~] loses a proton to give [HOs,(CO),,] -,31 in basic solvents and a proton is readily removed from [HRu,C(CO), 5Br]
1].
by treatment with proton (dimethylamino)-naphthalene].32 GROUP
[CO~(CO)~J~- +H+ --* [HCO~(CO)J-,~~ COWO),2l+
and [H,Os,(CO),
sponge
[1,8-bis-
IV HETEROATOMS
Simple interstitial binary carbides have been known for some time, but it was not until 1962 that the first carbide-cluster was reported in the literature.33 An X-ray structure revealed that the carbonyl cluster [Fe,C(CO), J contained a carbon atom bound to five iron atoms. The carbide-atom was found to be only partially shielded by the Fe, square pyramidal cage and therefore open to attack by incoming ligands, although no such attack was actually observed. Since then the number of carbide-carbonyl clusters has increased enormously and carbides have become the largest and most widely studied group of interstitial clusters. The maximum number of metal atoms to which carbon has been found to be coordinated is eight : [CO,C(CO),,]~provides a rare example of carbon occupying a tetragonal antiprismatic cluster cavity.34 More commonly,
n-5
n.4
nz6
n-8
_ _.._. _*.__ ..__ ./ . w,:. [cogco@-
Fig. 5. The M,C core geometries displayed by transition metal carbide clusters.
1311
Main group heteroatoms in transition metal clusters 0
*. * ‘..H,.” [HFe4WKO)1~]
(i) MeOH (ii) NE13
>
[Fe4 C( CO\31
[FeL(CO),2
C CO2 Me]-
Fig. 6. Reactions occurring at the carbide-carbon atoms of Fe& clusters.
carbon is bound to six metal atoms, in octahedral or trigonal prismatic cages, or five metals, in square pyramidal (nido-octahedral) or bridged butterfly (urachno-pentagonal bipyramidal) sites. Recent developments have also included the discovery of carbon bound to four metals in an open butterfly arrangement. The various M,C skeletons described above are illustrated in Fig. 5. As yet no carbide-clusters have been observed in which carbon is less than four coordinate, although there is evidence for low coordinate carbide-cations in the mass spectra of carbonyl clusters.35 It has now been established by 13C-labelling that the carbide-atom in carbide clusters can be derived either from a carbonyl group, via disproportionation of CO to give CO, and C or loss of the elements of water from a hydrido-cluster, or from an external source : COs3(W,
lPY1 + COs&(CO)2412 - + CO29
CRu3(W,,l+
[H,Ru,(CO),,]
CRw,WO),,l
+C%‘4
--f [Ru,C(CO),,]
[Rh(CO),] - + CC1, + [Rh,C(CO),,] [Co,(CO),]
+ “H20”,36 - + Cl -,37
+ Cg, -+ [C0~5C(C0)4,].~~
New carbide-clusters
may also be prepared via the
*This observation is in accordance with recent MO calculations on iron carbide clusters which predict that the metal-carbide orbitals are not su!lkiently high lying for the carbon atom to be involved in reactions until the number of metal atoms is reduced to four.90
controlled species :
degradation
CRu,C(W,,~
of performed
carbido-
CRu&(CO),,l+ CRu(CO),l,’
Alternatively, preformed carbides may be used as building blocks in the synthesis of larger clusters : CFe4C(CO),212- + CW(CO)3(NCCHs)l + [Fe4WC(C0)r5]2-,40 CFe5C(COh412- + CW(CO),(NCCH,)I + Fe,WC(CO),,]2-.40 Most of the studies on the classes of heteroatomcontaining complexes which follow have concentrated on the synthesis and structural analysis of the molecules in question. The study of carbidoclusters, however, has also involved an investigation of their reactivity towards a variety of reagents. Direct attack at the carbide-carbon atom has been observed for the tetranuclear iron carbide-clusters : reaction of [HFe,C(CO),,]with H+ leads to the formation of a methylidyne (CH) group bound to the tetranuclear cluster41 and treatment of [Fe,C(CO),,] with methanol and then base yields a butterfly (carbomethoxy) methylidyne complex42 (see Fig. 6). No such direct attack at the carbon atom has been seen for the higher nuclearity iron clusters or any of the Re, Ru, OS, Rh or Co carbides.* However, the carbon atom does confer on the cluster considerable stability towards degradation and in addition allows the metal framework to be relatively
1312
J. N. NICHOLLS
Lospq,l
c~~c(q~l
Fig. 7. The reversible carbonylation of [Os,C(CO),,].
flexible. Facile structural transformations therefore result from addition or removal of electrons, giving rise to a wide range of M,C framework geometries. For example, addition of CO to [Os,C(CO),,] results in the opening up of the square based pyramid to give a bridged butterfly arrangement of metals, while further heating of the product [Os,C(CO),,], even under CO, results in the ejection of CO and regeneration of [Os,C(CO), .J (see Fig. 7).43 Similarly, reaction of [Ru&!(CO),,] with HSEt results in the opening up of an octahedron of ruthenium atoms to give another wing tip bridged butterfly structure, but in this case a sixth metal atom is found bridging an edge ofthe basic five atom unit44 (see Fig. 8). It is hoped that we shall soon see the synthesis of other low nuclearity carbide-clusters and it will be interesting to compare their reactivity, in particular that of the carbide-carbon atom with that of the tetranuclear iron carbides. As far as other group IV heteroatom clusters are concerned, a much less extensive series of compounds is known. Complexes of the type [Co,(CO),-ECo(CO),J (E = Si, Ge) have been reported in which the silicon or germanium atom is bound to four cobalt atoms, of which three form a cluster and the fourth is held in place by the tetrahedrally coordinated heteroatom (see Fig. 9) :45,46 a similar situation is observed in the tetrahedrally coordinated tin complex {[(C,H,)Fe
(W212Sn2Fe3WM. 47 In this molecule both faces of the triangular Fe3(C0)9 cluster unit are capped by tin atoms. From the limited number of examples known it would appear that although these atoms are from the same periodic group as carbon they show a much reduced tendency for the formation of complexes with transition metal clusters.
GROUP
V HETEROATOMS
Many features of carbide-clusters are paralleled by nitrido-clusters. Nitrogen, like carbon, appears to stabilise the less frequently observed cluster geometries. Indeed, [Rh,N(CO), 5] - 5o and [Rh& (CO),,]* - 51have the same trigonal prismatic structures, [Fe,C(CO),,] 33 and [HFe,N(CO),,] 52 are both square prismatic and [Os4N(CO),J53 and [Fe,C(CO),,]*6 contain the same open butterfly ofmetal atoms (see Fig. 10). In clusters having high transition metal : heteroatom ratios, however, it has been said that nitrides tend towards lower coordination than carbides and lie in less regular cavities.3 Thus while the carbon atoms in [OsroC(CO)24]*- and [O~,lC(C0)27]*OCCUPY octahedral and trigonal prismatic sites respectively,54 the nitrogen atom in [PtRh,oN(C0)2,]33 is found in a distorted trigonal prismatic cavity. Outside cluster chemistry, nitrogen atoms have
[HRu 6 cCcC+p,]
Fig. 8. The reaction of [Ru&(CO)~~] with ethane-thiol.
1313
Main group heteroatoms in tradition metal clusters
Fig. 9. The core geometries adopted by M,E clusters (E = Si, Ge, Sn).
been found bound to a single metal centre, as in [Re(N)Cl,(PPh,),],55 linking two metal atoms, as in [Ru,NC~,(H,O)~]~- 56 or three metals as in [Ir3(N)(g04)6(H20)3] “in whichnitrogen sitsin the centre of a plane of three iridium atoms which are not bound to each other. Within cluster chemistry, however, such low metal : heteroatom ratios are rare. The lowest coordination ObSeNed for atomic nitrogen in a cluster is three: in [MO,(N) ‘* the nitrogen atom occupies an KwoLcP3l unusual site in the plane of the three molybdenum atoms. In this case two metal-metal bonds hold the MO, unit in an open triangular arrangement, as shown in Fig. 10 and result in a very exposed nitrogen atom. i-i=&
. EJ
Just as with the c~bido-clusters the source of nitrogen in nitrido-clusters may be either external or internal. For example, addition of NO+ to [HRu,(CO),,]and [Co6(CO)15]zgives [HRu,N(CO),,] 5gand [Co,N(CO),,]6o respectively while treatment of [Rh7(CO)&j3with CO/NO mixtures yields [Rh,N(cO), 5] -,6’ all three reactions involving an external source of nitrogen. An example of an internal source of nitrogen would be a coordinated nitrosyl group: pyrolysis of Peru,,,] yields the nitride [FeRu,N (CO),,]- 61 and the reaction of [M~~O)~~O) (C,H,)] with [MO~(co)6(c~H~)~] is used to prepare the trinuclear cluster [Mo30(0)(CO), (C5H5)3].62 In the latter case it has been proved n=3
tks
.’
‘...
.. A
,’
.’
:
,:’
..
-...
I.
.
_
n=4
IR”sN’CO\J
[Os$-J( COl,2]-
Fig. 10. The various M,N core geometries found in nitride-clusters. [Ru,N(CO),,]characterised spectroscopkally but not crystaIlographically.62
has been
1314
J. N. NICHOLLS nd2
n-3
.,,’
..~~... j .....
v
n=6
[co3(co)9 ASI
n=8
[ccpqd
-
E= P,As
Fig. 11. The transition metal-heteroatom core geometries displayed by clusters involving atomic phosphorus, arsenic or antimony as ligands.
conclusively by I80 labelling that on conversion of the nitrosyl ligand to a nitride the nitrosyl oxygen is lost as CO*. The final method available for the synthesis of new nitrido-clusters is the breakdown or build up of preformed nitridoclusters. For example, reaction of [Rh,N(CO), s] with [PtRh,(CO)1,]2results in the formation of [PtRh,,N(C0),,]3-.3 In contrast with clusters involving group IV heteroatoms which are as yet mainly limited to carbides, there are several examples of the coordination of the other group V elements, P, As and Sb to transition metal clusters. The first organometallic complex containing naked phosphorus as a ligand was reported in 1973:63 in [Co,Cp,P,] each phosphorus atom bridges three transition metals, however this molecule only contains two metal-metal bonds and so does not qualify for inclusion as a cluster compond. Arsenic and antimony have been found in ,u,-sites in metal cluster compounds and in [Fe,(CO),As,] an arsenic atom caps either side of the Fe, triangle,” but the main interest in the group V heteroatom-transition metal clusters is in those containing interstitial P, As or Sb atoms (see Fig. 11). It is interesting to compare such species with the nitride clusters to see just what effect these larger heteroatoms will have on the metal geometries : they will certainly be unable to adopt the same structures as the nitrides since even the
smallest, phosphorus, is too big to fit into an octahedral or trigonal prismatic cavity. Clusters involving interstitial phosphorus and arsenic atoms show a preference for the square antiprism65*66 as the primary building block (see Fig. 11) although when fewer metals are available, as in [Co6P(CO)r6]-, a more open StrUCtUre IUUSt be adopted in order to accommodate the large phosphorus atom. 67 When antimony, which is even larger takes up a five electron donor interstitial coordination mode it requires an even larger cavity andin[Rh,,Sb(C0),,]368wesee thefirstexample of an icosahedral cluster geometry, made possible by the stabilising effect of the central antimony atom. In terms of synthesis the phosphorus, arsenic and antimony clusters all result from pyrolysis reactions in the presence of an external source of the heteroatom: PCl,, white phosphorus or PPh3, AsPh, and SbCl, or SbPh, respectively. It is worth noting that while pyrolysis of phosphite derivatives of osmium clusters results in the formation of carbido-clusters,6g pyrolysis of [Rh(CO),(acac)] with triphenyl phosphine gives a phosphidocluster.66 This difference may simply be due to the P-O bonds in the phosphite being stronger than the P-C bonds in the phosphine or it may reflect differences between the metals, in which case it could have wider implications for the possible synthesis of
1315
Main group heteroatoms in transition metal clusters
MojNHO)
%O4
Fig. 12. Essential features of the structures of [Mo,o(O~CO),Cp,]
phosphido-clusters from other than the cobalt subgroup metals. Once again the presence of the heteroatoms in the group V atom-metal clusters gives added stability towards cluster breakdown : rhodium clusters are usually transformed to [Rh,(CO), J - and [Rh(CO),]- under CO/H2 but [Rh9P(CO),J2is stable to 600-800 atm CO/I& at 23OYY and [RhlzSb(CO),,]3is stable under 500 atm CO/H2 at 150”C6s There has been only one report of the reactivity of an exposed group V heteroatom in a metal cluster. Carbonylation of [Ru,N(CO),,]results in the formation of [Ru,N(CO),,]which reacts further with CO to give [Ru,(NCO)(CO), 3] in which process the nitrido-nitrogen atom is converted to an isocyanato-ligand.62 No reactivity has been reported for the cluster-bound nitrogen atom in [Fe,N(CO),,] - or the phosphorus atom in [Co6P(CO)16]-, although they, too, might be expected to undergo reaction. GROUP
and m,O,F,]
- 3H,O.
oxygen atom 73 but in most other cases the oxygen source has not been identified. In no case has an interstitial oxygen atom in a metal cluster been reported, a feature of oxide chemistry which is in contrast with that of sulphides since two examples of interstitial sulphur containing clusters are known. Atomic sulphur displays a wide variety of bonding modes in its coordination to transition metal clusters. In [Mo~(C~H~)~SJ+ the p2- and ,u3modes are demonstrated74 and in [0s6(c0)17s2] two ,u4S ligands have been identified as a result of a crystallographic investigation” (see Fig. 13).
VI HETEROATOMS
Examples of naked oxygen atoms as ligands are well known in mononuclear transition metal complexes like OsO,, where the metal is in a high formal oxidation state and, unlike the nitrides, terminal oxides are also occasionally found in transition metal clusters. Thus [Mo30(0)(CO),Cp3] contains an (Mo=O) unit, the oxygen atom of which derives from an external and as yet unknown source of oxygen61 (see Fig. 12). Atomic oxygen is also found in W,” Mn,‘l Ti,72 Ru’~ and OS’ clusters, as a p2- or p,-bridgingligand, as illustrated by the complexes [W304F9] - 3H20 ” and [Os60(CO)19] ’ whose structures are shown in Figs. 1 and 12. In the syntheses of [Os60(CO)19] and [Ru3~30)(0)(CO)6(02AsCH2As02)] molecular oxygen has been found to be the source of the
Fig. 13. The molecular structures of the complexes CMo,GH,G%I and[os6(cO),,s,1.
1316
J. N. NICHOLLS
sulphur in the form of, say, an SCN ligand may be used as the starting material. An example of the use of such a route is the preparation of [Rh10S(C0),J2from [Rhs(CO),gSCN),Jz-.78 GROUP
VII HETEROATOMS
A series of transition metal clusters exists in which halide atoms are the only ligands present. They RhlOS %7 =2 display a range of halide bonding modes from p3- to Fig. 14. The Rh-S core geometries of the clusters pLt-to terminal and are characterised by two main ERh,S(Wd2- andERGW%,13-. structural types, illustrated in Fig. 15, containing either octahedral M,(p3X), or M&L~X),, units, which are very stable and only broken up under Selenium, too, has been found to act as a p&and vigorous conditions. Thus, the salt [Mo,C~,C~,]~capping a triangle of metal atoms in, for example [Co3(CO)9Se].76 The exposed sulphur and selenium contains eight face capping and six terminal chlorines” while JNb,Br,2Cl,J4has 12 edge atoms in these molecules appear to be characterised by their reluctance to undergo reactions. There is, bridging bromine and six terminal chlorine however, at least one example of reaction at a p&S ligands81 Such clusters are not limited to the early centre. In 1978 reaction of, [(CH,),OJ’ with transition metals and, in an example from the other end of the transition series, [Pt&!l,,] displays an [HOs,(CO),S]was found to give [HOs,(CO), @$Me)], although the product was never fully octahedral structure with 12 @,I ligandsE2 very similar to that of the core of [Nb6X,J4-. characterised. ” Generally speaking, however, clusters of the later Clusters containing interstitial sulphur atoms involve much higher coordination numbers: in transition metals containing halide atoms are mainly [RhIOS(CO),,Jz- sulphur is found to stab&e an mixed halide~rbonyl species having terminal or p2unusual square prismatic cavity,‘* while in bridging halogens. For example in [Os,C(CO), sIl [Rh17S,(CO),,J3each sulphur atom is bound to the iodide atom is found in a terminal positionE3 on the cluster while in [Ru4(CO)r3Cl]chlorine nine rhodium atoms,7g as shown in Fig. 14. As far as the source of the sulphur atom is bridges the wing tips of an open butterfly of concerned, H2S, SO,, S andC$ have all been used to ruthenium atomsE4 (see Fig. 16). Synthesis of the introduce atomic sulphur into metal clusters. early transition metal halide clusters normally involves the reduction of the higher mononuclear or Alternatively, a cluster which already contains
M=Nb,Ta
Fig. 15. The structures of the metal-halide clusters. The terminally bound halide ligands are omitted for clarity.
1317
Main group heteroatoms in transition metal clusters
[ossc’co~5 I I-
[RuJCOI,~
Cl ] -
Fig. 16. The structures of [Os,C(CO),,I]CRuACO)zdX - .
and
dinuclear halides in the presence of an excess of the metal in question at elevated temperatures, while in the case of the carbonyl halides addition of X2, HX or [Au(PPh,)X] to a neutral carbonyl cluster or reaction of an anion with I2 is the usual route. As has been mentioned above the M,X, and M6XlZ cores of the early transition metal halides are very stable to degradation as these central units contain the bridging halides in each case. The terminally bound halogen atoms may however be removed, as the halide ions X-, and the cations CMJ,14+ and CM& A *+ have been studied as solutions in neutral monodentate ligand solvents like amines, alcohols or water.s5 It has also proved possible to remove the halogen atoms from some of the carbonyl halide clusters, or to persuade them to change from being terminal one electron donors to bridging three electron donor ligands and vice versa.
Thus the two bridging iodine ligands in [OS,&(CO)24IJ are removed stepwise on treatment with I-, pyridine or phosphites6 giving first [Os,,C(CO),,I]and then the dianion [OS,,C(CO),~]*- (see Fig. 17) ; the iodine atoms are removed as I+ to give I2 or IL2 (L = py, P(OR),) respectively. The halideligand in [HRu,C(CO),,Cl] is removed in a different way : simple thermolysis of the pentanuclear chloro-cluster in hydrocarbon solvents results in the ejection of molecular HCl, leaving the carbonyl cluster [Ru&(CO),,] in solution.32 On heating [HRuSC(CO),,Cl] in the presence of an excess of HCI, however, this reaction is suppressed in favour of loss of CO and formation of the p2C1 species [HRu,C(CO)~,C~], a step which may be reversed by addition of CO to this or any of the formally analogous series [ARu,C(CO),,X] (A = Au(PR,), H ; X = Cl, Br), to give the [ARu,C(CO)~,X] molecules” (see Fig. 18). This example clearly demonstrates the ease with which halide heteroatoms may be converted from two to four electron donors and back in transition metal carbonyl clusters. CONCLUSION
A large number of clusters are known which contain main group heteroatoms bound exclusively to transition metals. The heteroatoms display a wide range of bonding modes and their coordination
Fig. 17. The stepwise removal of iodine ligands from [Os,,C(CO),,I,].
[Ru5C(CO),d
bRugc(CO)15 Xl ( A=AuFPh3,H
; X=CI , Br ,I
h5
x]
C(COl,4
)
Fig. l&The conversion ofhalideligands between terminal and bridging bonding modes in Ru,C clusters.
1318
J. N. NICHOLLS
numbers range from 12, for antimony, to one, for hydrogen, oxygen and the halogens. When found in interstitial sites they often impart considerable stability towards degradation to the metal cluster, as seen by comparison with the stability of simple metal carbonyl clusters. They are often associated with the less commonly observed metal geometries such as icosahedral M, 2, square antiprismatic EM,, square pyramidal M, and open butterfly M, structures and, when exposed, provide possible alternative sites for attack by incoming reagents. Hydride and halide ligands can often be removed from their parent clusters. The group IV, V and VI heteroatoms, however, seem to be more firmly bound and direct attack at these cluster-bound heteroatoms has as yet been limited to the carbon atom in Fe, clusters, the sulphur atom in CHOMW~SI - and the nitrogen atom in [Ru,N(CO),,] - which are converted to (CH) or (CCO,Me), SCH3) and (NCO) groups respectively. While many carbide-, hydrido- and nitridoclusters are known few clusters have been prepared which contain heteroatoms like phosphorus, arsenic and sulphur. The study of these species has also largely been directed towards synthesis and there is therefore insufficient information to allow a real comparison of the reactivity of the cluster-bound heteroatoms. Clearly this is an area which will receive more attention as the study of transition metal clusters containing main group heteroatoms progresses. Acknowledgement-I wish to thank Gonville and Caius College, Cambridge, for the award of a Research Fellowship.
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