Fluoroborylene ligands in binuclear ruthenium carbonyls: Comparison with their iron analogues

Polyhedron 38 (2012) 44–49

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Fluoroborylene ligands in binuclear ruthenium carbonyls: Comparison with their iron analogues Liancai Xu b, Qian-shu Li a,c,⇑, R. Bruce King a,d,⇑ a

Center for Computational Quantum Chemistry, South China Normal University, Guangzhou 510631, PR China Department of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450002, PR China c Institute of Chemical Physics, Beijing Institute of Technology, Beijing 100081, PR China d Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, GA 30602, USA b

a r t i c l e

i n f o

Article history: Received 14 November 2011 Accepted 3 February 2012 Available online 18 February 2012 Keywords: Ruthenium Fluoroborylene Metal carbonyls Metal–metal bonding Density functional theory

a b s t r a c t The qualitative aspects of the chemistry of the fluoroborylene iron and ruthenium carbonyls M(BF)(CO)m (m = 4, 3) and M2(BF)2(CO)n (n = 7, 6) (M = Fe, Ru) are predicted to be very similar. For Ru(BF)(CO)4 the trigonal bipyramidal structures with the BF group in an equatorial position and in an axial position are both found with the equatorially substituted isomer lying 2 kcal/mol below the axially substituted isomer. For the coordinately unsaturated Ru(BF)(CO)3 both singlet and triplet structures are found derived from the equatorially substituted Ru(BF)(CO)4 structure by removal of an equatorial CO group. The structures for the binuclear derivatives Ru2(BF)2(CO)n (n = 7, 6) provide examples of lower energy structures with bridging BF groups relative to similar structures with bridging CO groups. The lowest energy Ru2(BF)2(CO)7 structure has two bridging BF groups and one CO group and is qualitatively similar to the well-known Fe2(CO)9 structure with three bridging CO groups. All of the structures of the unsaturated Ru2(BF)2(CO)6 within 30 kcal/mol of the global minimum have two bridging groups. The structure with one bridging CO group and one bridging BF group lies 16 kcal/mol above the global minimum with two bridging BF groups again showing the preference for bridging BF groups over bridging CO groups. A triplet Ru2(BF)2(CO)6 structure with two bridging BF groups was also found but at the high energy of 26 kcal/mol above that of the corresponding singlet global minimum. This singlet–triplet splitting is much larger than the singlet–triplet splitting of the corresponding Fe2(BF)2(CO)6. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction The fluoroborylene ligand, BF, is of interest since it is isoelectronic with the ubiquitous carbonyl ligand, CO. However, the instability of BF (boron monofluoride) [1] has limited the development of the chemistry of metal fluoroborylene complexes analogous to metal carbonyls. Fragmentary reports of the fluoroborylene iron derivatives Fe(BF)L4 (L = CO and PF3) [2] have appeared in conference proceedings [3] but these compounds have not been properly characterized. The first fluoroborylene metal derivative that has been fully characterized is the ruthenium complex Cp2Ru2(CO)4(l-BF), which was synthesized only in 2009 and has been structurally characterized by X-ray diffraction [4]. This synthesis is related to the somewhat earlier synthesis (2007) [5] of the haloborylene manga-

⇑ Corresponding authors. Addresses: Center for Computational Quantum Chemistry, South China Normal University, Guangzhou 510631, PR China, Institute of Chemical Physics, Beijing Institute of Technology, Beijing 100081, PR China (Q.-S. Li). Department of Chemistry and Center for Computational Chemistry, University of Georgia, Athens, GA 30602, USA. Tel.: +1 706 542 1901; fax: +1 706 542 9454 (R.B. King). E-mail addresses: [email protected] (Q.-S. Li), [email protected] (R.B. King). 0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2012.02.003

nese carbonyls (l-BX)Mn2(CO)10 (Fig. 1: X = Cl, Br) by the reaction of NaMn(CO)5 with the corresponding boron trihalide, BX3. These experimental studies show that at least some fluoroborylene transition metal complexes can be synthesized. However, such syntheses are difficult and generally require indirect methods because of the instability of free BF. Theoretical methods are therefore useful in predicting promising areas of future development of fluoroborylene transition metal complexes. Our theoretical studies first focused on fluoroborylene iron carbonyls of various types including the mononuclear derivatives Fe(BF)(CO)n (n = 4, 3) [6], the binuclear derivatives Fe2(BF)2(CO)n (n = 7, 6) [6], the trinuclear derivatives Fe3(BF)3(CO)n (n = 9 [7], 8 [8]), and the cyclopentadienyliron derivatives (g5-C5H5)2Fe2(BF)2(CO)n (n = 2, 1, 0). These studies have shown that structures with BF bridges are preferred energetically over analogous structures with CO bridges. Furthermore, the lowest energy Fe3(BF)3(CO)9 structure is neither a doubly edge-bridged structure analogous to the experimental Fe3(CO)12 structure [9,10] or an unbridged structure analogous to the experimental M3(CO)12 (M = Ru, Os) structures [11–13]. Instead, the lowest energy Fe3(BF)3(CO)9 structure by 19 kcal/mol has an Fe3 triangle bridged at the top and the bottom by two of the three BF ligands leading to a central Fe3B2 trigonal bipyramid. The

L. Xu et al. / Polyhedron 38 (2012) 44–49

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performed with the GAUSSIAN03 program package [35]. The fine grid (75, 302) was the default for the numerical evaluation of the integrals, while the large grid (120, 974) was only used to evaluate the small imaginary vibrational frequencies. All of the predicted triplet structures in the present study are found to have negligible spin contamination, with hS2i values close to the ideal outcome of 2.0. A given Rua(BF)a(CO)b structure is designated ab-cA where a is the number of ruthenium atoms, b is the number of CO groups, and c orders the structures according to their relative energies. The capital letter A indicates whether the structure is a singlet (S) or triplet (T). Thus the lowest energy singlet structure Ru2(BF)2(CO)7 is designated 27-1S. The total energies (E, in Hartree), relative energies (DE, in kcal/mol), and numbers of imaginary vibrational frequencies (Nimag) of the structures within 30 kcal/mol of the global minima are listed in Tables S1 and S2. Figs. 1–3 depict the optimized structures, indicating in parentheses their energies (DE, in kcal/mol) relative to the lowest-lying structures using the BP86 and MPW1PW91 methods, respectively. 3. Results and discussion 3.1. Mononuclear derivatives Ru(BF)(CO)n (n = 4, 3) Fig. 1. Optimized singlet Ru(BF)(CO)n (n = 4 and 3) structures. Distances, relative energies, and imaginary vibrational frequencies in Figs. 1–3 are those obtained by BP86 and MPW1PW91, respectively.

chemistry of the (g5-C5H5)2Fe2(BF)2(CO)n is also predicted to be unusual since in the lowest energy (g5-C5H5)2Fe2(BF)2(CO) structure, the two BF ligands have coupled to form a difluorodiborene (B2F2) unit. This paper describes an initial study on fluoroborylene ruthenium carbonyl derivatives, which are of interest in view of the report of Cp2Ru2(CO)4(l-BF) as the first fluoroborylene derivative structurally characterized by X-ray crystallography [4]. However, the focus on the current work is on mononuclear and binuclear ruthenium species containing only fluoroborylene and carbonyl ligands. This provides an opportunity to compare the preferred structures of such ruthenium derivatives with the previously studied [6] iron carbonyl fluoroborylene derivatives. 2. Theoretical methods Electron correlation effects were considered using density functional theory (DFT) methods, which have evolved as a practical and effective computational tool, especially for organometallic compounds [14–25]. Two DFT methods were used in this study. The BP86 method combines Becke’s 1988 exchange functional (B) with Perdew’s 1986 gradient corrected correlation functional (P86) [26,27]. The MPW1PW91 method is based on the generalized gradient approximation (GGA) [28]. The double-f plus polarization (DZP) basis sets were adopted in the present study. Thus one set of pure spherical harmonic d functions with orbital exponents ad(B) = 0.7, ad(C) = 0.75, ad(O) = 0.85, and ad(F) = 1.0 for boron, carbon, oxygen, and fluorine, respectively, was added to the standard Huzinaga–Dunning contracted DZ sets [29–31], which are thus designated as (9s5p1d/4s2p1d). The SDD (Stuttgart–Dresden ECP plus DZ) [32] was used for ruthenium complexes. Previous research has shown that BP86 and MPW1PW91 methods along with SDD ECP basis sets are reliable to predict geometric parameters and vibrational spectra for second row transition metal carbonyl complexes [33,34]. The geometries of all structures were fully optimized using the two DFT methods. Harmonic vibrational frequencies were determined by evaluating analytically the second derivatives of the energy with respect to the nuclear coordinates. All calculations were

The global minimum Ru(BF)(CO)4 structure 14-1S is a C2v trigonal bipyramidal structure with the BF ligand in an equatorial position (Fig. 1 and Table S1). The C3v trigonal bipyramidal Ru(BF)(CO)4 structure 14-2S with the BF ligand in an axial position lies 2.2 kcal/ mol (BP86) or 2.8 kcal/mol (MPW1PW91) in energy above 14-1S. These two structures are very similar to the corresponding Fe(BF) (CO)4 structures [6]. However, the Fe(BF)(CO)4 structures corresponding to 14-1S and 14-2S are essentially equal in energy. The global minimum Ru(BF)(CO)3 structure 13-1S is a singlet Cs structure. The triplet structure 13-1T is very similar to 13-1S, but its energy is 13.8 kcal/mol (BP86) or 11.8 kcal/mol (MPW1PW91) higher than that of 13-1S. These Ru(BF)(CO)3 structures can be derived from the lowest energy Ru(BF)(CO)4 structure 14-1S by removal of an equatorial carbonyl group. For Fe(BF)(CO)3 the corresponding singlet and triplet structures are predicted to be essentially equal in energy [6]. This difference may lie in the lower ligand field splittings in the first row transition metals as compared with the second and third row transition metals. The m(CO) frequencies of Ru(BF)(CO)n (n = 4, 3) by the BP86 method, ranging from 2073 to 1949 cm1, are lower than the 2091 cm–1 frequency of free CO (Table 1). However, the m(BF) frequencies of Ru(BF)(CO)n (n = 4, 3), ranging from 1488 to 1443 cm1, are higher than the 1314 cm–1 m(BF) frequency of free BF (Table 1). 3.2. Binuclear derivates Ru2(BF)2(CO)n (n = 7, 6)

3.2.1. Ru2(BF)2(CO)7 Five structures with one or three bridging groups were found for Ru2(BF)2(CO)7 (Fig. 2 and Table S2). The global minimum Ru2(BF)2(CO)7 structure 27-1S is a C2v triply bridged structure with all real vibrational frequencies and thus a genuine minimum. Two of the bridges in 27-1S are BF groups whereas the third bridge is a CO group. The Ru–Ru distance of 2.793 Å (BP86) or 2.759 Å (MPW1PW91) is 0.06 Å shorter than the experimental Ru–Ru single bond length of 2.85 Å in Ru3(CO)12, determined by X-ray crystallography [36]. This corresponds to a formal single Ru–Ru bond in 27-1S, thereby giving both ruthenium atoms the favored 18-electron configuration. Structure 27-1S is very similar to the Fe2(CO)6(l-CO)(l-BF)2 structure found in the previous theoretical study to be the global minimum for Fe2(BF)2(CO)7 [6].

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Fig. 2. Optimized singlet Ru2(BF)2(CO)7 structures.

Fig. 3. Optimized Ru2(BF)2(CO)6 structures.

Table 1 The m(CO) and m(BF) stretching frequencies (cm1) and the infrared intensities in km/ mol (in parentheses) for the mononuclear Ru(BF)(CO)n (n = 4, 3) derivatives predicted by the BP86 method. BP86

14-1S (C2v) 14-2S (C3v) 13-1S (Cs) 13-1T (Cs)

m(CO)

m(BF)

2073(50),2003(1320),2001(280),1983(1144) 2061(277),2003(302),1976(1174),1976(1180) 2049(42),1976(1495),1965(852) 2013(130),1965(671),1949(1502)

1443(659) 1488(583) 1448(653) 1451(569)

The singly bridged Ru2(l-BF)(BF)(CO)7 structure 27-2S, with a bridging BF group, is predicted to lie 10.2 kcal/mol (BP86) or

9.9 kcal/mol (MPW1PW91) in energy above 27-1S (Fig. 2 and Table S2). The Ru–Ru distance of 2.960 Å (BP86) or 2.912 Å (MPW1PW91) in 27-2S is 0.15 Å longer than in 27-1S but still can correspond to a formal single bond. This gives both ruthenium atoms in 27-2S the favored 18-electron configuration. The longer Ru–Ru bond in 27-2S relative to that in 27-1S relates to the presence of three bridging groups in 27-1S but only one bridging group in 27-2S. Structures 27-1S and 27-2S are very similar to the two lowest energy structures of Ru2(CO)9 [37]. However, these two structures of Ru2(CO)9 are almost degenerate and the global minimum structure of Ru2(CO)9 is singly CO bridged whereas for Ru2(BF)2(CO)7, the corresponding structure 27-2S lies 10 kcal/mol above 27-1S.

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The next higher Ru2(BF)2(CO)7 structure 27-3S has a single bridging BF group and lies 14.6 kcal/mol (BP86) or 14.9 kcal/mol (MPW1PW91) in energy above 27-1S (Fig. 2 and Table S2). Structure 27-3S differs from 27-2S in the position of the terminal BF group relative to the bridging BF group. The Ru–Ru distance of 2.956 Å (BP86) or 2.918 Å (MPW1PW91) in 27-3S is similar to that in 27-2S and indicates the single bond required to give both Ru atoms the favored 18-electron configuration. The triply bridged Ru2(BF)2(CO)7 structure 27-4S can be obtained from structure 27-1S by swapping one bridging BF group with one terminal CO group (Fig. 2 and Table S2). The higher energy of 27-4S relative to 27-1S by 20.1 kcal/mol (BP86) or 23.3 kcal/mol (MPW1PW91) indicates that BF is favored over CO as a bridging group as was found for Fe2(BF)2(CO)7 [6]. Structure 27-4S is not a genuine minimum structure but has an imaginary vibrational frequency at 54i cm–1 (BP86) or 58i cm–1 (MPW1PW91). Following the corresponding normal mode converts the two bridging CO groups in 27-4S to terminal CO groups leading to 27-3S. The Ru–Ru distance of 2.784 Å (BP86) or 2.745 Å (MPW1PW91) in 27-4S is very close to that in the likewise triply bridged 27-1S. This indicates the single bond required to give both ruthenium atoms the favored 18-electron configuration in 27-4S. The C2v singly CO bridged Ru2(BF)2(CO)7 structure 27-5S is a high-energy structure lying 29.7 kcal/mol (BP86) or 32.4 kcal/mol (MPW1PW91) above the global minimum structure 27-1S. The high energy of structure 27-5S is another indication that BF bridging is favored over CO bridging in these binuclear ruthenium derivatives.

3.2.2. Ru2(BF)2(CO)6 The global minimum structure 26-1S of Ru2(BF)2(CO)6 has two bridging BF group and C2h symmetry with all real vibrational frequencies (Fig. 3 and Table S3). The Ru@Ru distance of 2.733 Å (BP86) or 2.702 Å (MPW1PW91) in 26-1S is shorter by 0.12Å than the experimental Ru–Ru single bond distance in Ru3(CO)12 [36]. This suggests the formal double bond required to give both Ru atoms in 26-1S the favored 18-electron configuration. The doubly bridged Ru2(BF)2(CO)6 structure 26-2S, with one bridging BF group and one bridging CO group, lies 15.4 kcal/mol (BP86) or 17.3 kcal/mol (MPW1PW91) in energy above 26-1S (Fig. 3 and Table S3). The Ru@Ru distance of 2.677 Å (BP86) or 2.643 Å (MPW1PW91) in 26-2S is 0.06 Å shorter than that in 26-1S and may likewise correspond to the formal double bond needed to give both Ru atoms in 26-2S the favored 18-electron configuration. The considerably higher energy of 26-2S relative to 26-1S is again an indication that bridging BF groups are more favorable than bridging CO groups, at least in ruthenium carbonyl chemistry. The Ru2(BF)2(CO)6 structure 26-3S is similar to structure 26-2S, lying 16.4 kcal/mol (BP86) or 18.5 kcal/mol (MPW1PW91) above

26-1S (Fig. 3 and Table S3). Structure 26-3S differs from 26-2S in the location of the terminal CO groups relative to the bridging CO group. However, 26-3S is not a genuine minimum structure but has an imaginary vibrational frequency at 14i cm1 (BP86) or 13i cm1 (MPW1PW91). Following the corresponding normal mode leads to 26-2S. Structures with two bridging BF groups similar to 26-1S, 26-2S, and 26-3S were found for the analogous Fe2(BF)2(CO)6 [6]. The doubly bridged triplet Ru2(BF)2(CO)6 structure 26-1T is a relatively high energy structure lying 28.6 kcal/mol (BP86) or 24.4 kcal/mol (MPW1PW91) above 26-1S (Fig. 3 and Table S3). Structure 26-1T is similar to the singlet structure 26-1S except for a significantly longer Ru–Ru distance of 2.844 Å (BP86) or 2.824 Å (MPW1PW91). This Ru–Ru distance in 26-1T is similar to the experimental single bond distance of 2.85 Å in Ru3(CO)12 suggesting a similar single bond. This gives each Ru atom the 17-electron configuration for a binuclear triplet. The triplet Ru2(BF)2(CO)6 structure 26-1T lies at a much higher energy relative to the singlet global minimum 26-1S than the corresponding triplet Fe2(BF)2(CO)6 structures lie above its singlet global minimum [6]. The v(CO) frequencies of the terminal CO groups range from 2079 cm1 to 1975 cm1 in the binuclear Ru2(BF)2(CO)n (n = 6, 7) derivatives (Table 2). As expected, the m(CO) frequencies of the bridging CO groups are significantly lower, ranging from 1884 to 1830 cm-1. The pattern of the m(BF) frequencies in the Ru2(BF)2(CO)n (n = 6, 7) derivatives is similar to that of the m(CO) frequencies since the bridging m(BF) frequencies are significantly lower than the terminal m(BF) frequencies. Thus the terminal m(BF) frequencies lie in the range 1466–1433 cm1 whereas the bridging m(BF) frequencies range from 1348 to 1237 cm1. The vibrational frequencies of the terminal and bridging CO groups in the Ru2(BF)2(CO)n (n = 6, 7) derivatives are lower than the m(CO) frequency of free CO. However, the terminal and bridging m(BF) frequencies groups are higher than that of free BF. The BP86 method predicts the HOMO (r) at -6.8 eV for free BF to lie above the HOMO (r) at –8.9 eV for free CO group. However, the LUMO (p⁄) of –2.3 eV for free BF is predicted to lie slightly lower than the LUMO(p⁄) of –2.1 eV for free CO. This suggests that BF can act both as a stronger r-donor and a stronger p-acceptor than CO groups when bonded to a transition metal. Their difference on infrared frequencies shown by mononuclear and binuclear ruthenium complexes may relate to these differences in the r-donor and p-acceptor capabilities of the BF and CO ligands. 3.3. Dissociation energies Table 3 reports the dissociation energies for removal of one CO group from the global minima of the mononuclear Ru(BF)(CO)4 and binuclear Ru2(BF)2(CO)7 structures according to the following equations:

Table 2 The m(CO) and m(BF) stretching frequencies (cm–1) and the infrared intensities in km/mol (in parentheses) for the binuclear Ru(BF)2(CO)n (n = 7, 6) derivatives predicted by the BP86 method. The bridging m(CO) and m(BF) frequencies are reported in bold type. BP86

27-1S(C2v) 27-2S(C1) 27-3S(C1) 27-4S(Cs) 27-5S(C2v) 26-1S(C2h) 26-2S(Cs) 26-1T(C2h)

m(CO)

m(BF)

2066(2),2032(1751),2008(1537),2006(1385), 2002(0),2001(0),1884(481) 2078(111),2030(733),2008(923),2005(1908), 1996(498),1986(374),1978(35) 2079(73),2036(847),2007(1887),2006(317), 1994(938),1987(489),1979(89) 2059(217),2023(1591),2006(673),2002(1396), 1997(27),1874(285),1852(813) 2068(71),2021(361),2003(1015),2002(2152), 1983(138),1975(0),1830(598) 2059(0),2029(1754),2003(1380),2000(0), 1992(1871),1988(0) 2057(272),2024(1373),2002(672),1995(1569), 1986(59),1865(397) 2041(0),2007(2105),1991(1526),1988(0), 1980(1405),1977(0)

1352(325),1333(587) 1466(727),1297(396) 1433(546),1302(480) 1450(564),1348(551) 1463(270),1457(1129) 1332(0),1310(674) 1461(892),1237(214) 1338(0),1309(699)

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Table 3 Energies (kcal/mol) for successive removal of CO groups from Ru2(BF)2(CO)n (n = 7, 6) and Ru(BF)(CO)m (m = 4, 3) as well as for the dissociation of the binuclear derivatives into mononuclear fragments. Comparison with the corresponding reported [37] predicted energies of the homoleptic Ru2(CO)n (n = 9, 8) and Ru(CO)m (m = 5, 4) is included. All energies are based on the lowest energy structures.

Ru2(BF)2(CO)7 ? Ru2(BF)2(CO)6 + CO Ru(BF)(CO)4 ? Ru(BF)(CO)3 + CO Ru2(BF)2(CO)7 ? Ru(BF)(CO)4 + Ru(BF)(CO)3 Ru2(BF)2(CO)6 ? 2Ru(BF)(CO)3 Ru2(CO)9 ? Ru2(CO)8 + COa Ru2(CO)9 ? Ru(CO)5 + Ru(CO)4a Ru2(CO)8 ? 2 Ru(CO)4a Ru(CO)5 ? Ru(CO)4 + COa a

BP86

MPW1PW91

26.5 39.5 54.7 67.7 27.9 21.1 30.4 37.1

27.8 39.3 59.5 71.0 32.6 23.7 28.5 37.4

The data are from Ref. [33].

RuðBFÞðCOÞm ! RuðBFÞðCOÞm1 þ COðm ¼ 4; 3Þ Ru2 ðBFÞ2 ðCOÞn ! Ru2 ðBFÞ2 ðCOÞn1 þ COðn ¼ 7; 6Þ These predicted CO dissociation energies are comparable to the experimental CO dissociation energies [38] for Ni(CO)4, Fe(CO)5, and Cr(CO)6 of 27, 41, and 37 kcal/mol, respectively. Thus, the CO dissociation energy of 39 kcal/mol for Ru(BF)(CO)4 is close to the experimental CO dissociation energy of 41 kcal/mol for Fe(CO)5, and slightly larger than the CO dissociation energy of 37 kcal/mol of the Ru(CO)5 predicted using the same method [37]. Table 3 also reports the dissociation energies of the binuclear Ru2(BF)2(CO)n into mononuclear fragments by the following reactions:

The structures for the binuclear derivatives Ru2(BF)2(CO)n (n = 7, 6) provide additional examples of lower energy structures with bridging BF groups relative to similar structures with bridging CO groups. The lowest energy Ru2(BF)2(CO)7 structure has two bridging BF groups and one CO group and is qualitatively similar to the well-known Fe2(CO)9 structure with three bridging CO groups. The corresponding Ru2(BF)2(CO)7 stationary point with only one bridging BF group and two bridging CO groups is a transition state lying 22 kcal/mol above the global minimum, which leads to a singly BF-bridged structure lying 15 kcal/mol above the global minimum. All unsaturated Ru2(BF)2(CO)6 structures within 30 kcal/mol of the global minimum have two bridging groups. The structure with one bridging CO group and one bridging BF group lies 16 kcal/mol above the global minimum with two bridging BF groups again showing the preference for bridging BF groups over bridging CO groups. A triplet Ru2(BF)2(CO)6 structure with two bridging BF groups was also found but at the high energy of 26 kcal/mol of the corresponding singlet global minimum. This singlet–triplet splitting is much larger than the singlet–triplet splitting of the corresponding Fe2(BF)2(CO)6. Acknowledgments We are indebted to the Research Fund for the Doctoral Program of Higher Education (20104407110007), the National Natural Science Foundation of China (20973066), the Doctor’s Innovation Fund of Zhengzhou University of Light Industry (2011BSJJ012), and the US National Science Foundation (Grant CHE-1057466) for support of this research.

Ru2 ðBFÞ2 ðCOÞn ! RuðBFÞðCOÞx þ RuðBFÞðCOÞy ðn ¼ x þ yÞ

Appendix A. Supplementary data

The dissociation energy of Ru2(BF)2(CO)7 into Ru(BF)(CO)4 + Ru(BF)(CO)3 is approximately twice the dissociation energy of Ru2(BF)2(CO)7 into Ru2(BF)2(CO)6 + CO. This contrasts with the dissociation energy of Ru2(CO)9 into Ru(CO)5 + Ru(CO)4,which is 8 kcal/mol less than the dissociation energy of Ru2(CO)9 into Ru2(CO)8 + CO. This indicates that substitution of bridging CO groups with bridging BF groups strengthens the underlying metal–metal interaction. Thus the chemistry of Ru2(BF)2(CO)7 is predicted to involve maintaining the binuclear Ru2 unit whereas the chemistry of Ru2(CO)9 is predicted to involve fragmentation into mononuclear units. This may relate to the instability of Ru2(CO)9 under ambient conditions.

Table S1 and S2: Total energies (E, in Hartree), relative energies (DE, in kcal/mol), numbers of imaginary vibrational frequencies (Nimag), for Ru(BF)(CO)n (n = 4, 3) and Ru2(BF)2(CO)n (n = 7, 6); Table S3–S5: Theoretical harmonic vibrational frequencies for Ru(BF) (CO)n (n = 4, 3), Ru2(BF)2(CO)7, Ru2(BF)2(CO)6 using the BP86/DZP SDD method; Table S6-S18: Theoretical Cartesian coordinates for Ru(BF)(CO)n (n = 4,3) and Ru2(BF)2(CO)n (n = 7, 6) using the BP86/ DZP SDD method; Complete Gaussian reference (reference 35). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2012.02.003.

4. Summary The qualitative aspects of the chemistry of the fluoroborylene iron and ruthenium carbonyls M(BF)(CO)m (m = 4, 3) and M2(BF)2(CO)n (n = 7, 6) are predicted to be very similar. For Ru(BF) (CO)4 the trigonal bipyramidal structures with the BF group in an equatorial position and in an axial position are both found with the equatorially substituted isomer lying 2 kcal/mol below the axially substituted isomer. For Fe(BF)(CO)4 the predicted energies for the corresponding equatorially and axially substituted isomers are found to be essentially identical. For the coordinately unsaturated Ru(BF)(CO)3 both singlet and triplet structures were found derived from the equatorially substituted Ru(BF)(CO)4 structure by removal of an equatorial CO group. The singlet structure is predicted to lie 10 kcal/mol below the triplet structure. For Fe(BF)(CO)3 the corresponding singlet and triplet structures are essentially equal in energy possibly related to lower field splittings in the first row transition metal iron relative to the corresponding second row transition metal ruthenium.

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