Semi-bridging carbonyl

A single CO bridging two M atoms - as in the case of the rather unstable Os2(CO)9 shown above - is uncommon unless the M atoms form additional bonds, apart from the terminal M-C bonds. An example is Co4(CO)12 (Fig. 8.8 in Section 8.5). In a number of cases, semi-bridging carbonyl is encountered two M atoms (directly bonded to each other) are bridged unsymmetrically by one or more CO groups so that one M-C distance is appreciably shorter than the other. Triply-bridging CO is also found, i.e. the C atom is bonded to three M atoms. 

On this basis, formally at least, the series 1-3 contain metal-metal triple bonds. As we shall see, the nature of the metal-metal bonding is quite different from that of carbon-carbon bonding in ethyne. Similarly, it is useful to regard the metal-metal bond order in the series 4-6 as double. However, the nature of the M=M bond is quite different from the C=C bond in ethene. The carbonyls are semi-bridging or bridging in 1-6 and are extensively involved in the metal-metal interactions. This has resulted in some differences of opinion as to whether one should really regard the metal-metal bonds as multiple. 

There is some confusion as to a suitable representation for the actual structure of 7 shown in Fig. 1. The metal-metal bond is 2.462 A long and seems too short for a V—V single bond. Certainly, the reactivity of 7 is consistent with multiple bonding. What is at question is its nature and the role of the semi-bridging carbonyls. 

The two semi-bridging carbonyls are manifest by two low frequency stretches, vco(hexane) 1869 and 1828 cm-1 (9). A single Cp signal in the NMR spectrum shows the molecule is fluxional (9). 

The increased abundance of the C3 isomer with increasing ligand basicity presumably occurs because the semi-bridging carbonyl attached to the substituted metal can become more full-bridging and remove the excess electron density released by the basic phosphine. Consistent with this hypothesis is the... 

The next exchange process to occur at slightly higher temperatures in each of these clusters involves migration of the carbonyls around the Fe-M-M triangle which possesses the bridging carbonyls. It seems reasonable to propose that the intermediates in this cyclic movement are the tautomers which have the semi-bridging carbonyls bound mainly to Ru or Os, instead of Fe, as indicated in Scheme 5. 

One factor which may contribute to the longer MeM bond length in the carbonyl complexes is the presence of semi-bridging carbonyls which interact with orbitals involved in MeM multiple bonding. An "asymmetry parameter", a, may be defined as a = (d2-dj)/ dx (see Table I) where d2 = long M -C0 distance and d short M-C distance. For symmetrically bridged carbonyls, d2 = d and a = 0. For terminal carbonyls, a > ca. 0.6, and the semi-bridging appellation is appropriate if 0.1 < a < 0.5. 

Cotton has proposed that semi-bridging carbonyls serve to remove excess charge on M if M is electron rich relative to M (see I) (14J. In this type of semi-bridging interaction, the carbonyl group accepts electron density from M into the ir -orbitals. We have shown that a plot of the asymmetry parameter (a) vs. the MC0 angle (e) clearly reveals two types of behavior for semi-bridging carbonyl groups (see Fig. 3). The acceptor type of behavior is... 

The bottom curve of Fig. 3 shows that acceptor semi-bridging carbonyls bend back as the carbon approaches the second metal (i.e., as the asymmetry decreases) until 9=135° for symmetrically bridged carbonyls. In the second type of interaction, which we denote as "donor semi-bridging", the M-C-0 angle is nearly invariant with respect to the asymmetry parameter. That the carbonyls remain essentially linear when they act in a donor capacity is reasonable when one considers that the oxygen must also approach the metal, M, in structure II. 

Jemmis et al. have recently reported the results of extended Huckel calculations on Cp2M2(C0)i, compounds (19). Their results indicate that the carbonyls in Cp2Mo2(C0)i, are net electron-withdrawing. However, these authors did not address the question of bent v . linear semi-bridging carbonyls. Clearly, this topic warrants further theoretical scrutiny. 

Reaction with S-Donors. A somewhat similar (o+ir) 4-electron donation is also exhibited by thioketones in their adducts with Cp2Mo2(C0h (22). In this instance, one of the carbonyls trans to the R2CS ligand adopts a semi-bridging bonding mode (eq. 22). 

Reaction of Hydrides. The donor semi-bridging interaction of the carbonyls in 1 might render them more susceptible to nucleophilic attack by such reagents as hydrides. We reasoned that the formation of n2-formyl species, e.g., IV, or alternatively, anionic metal hydrides, e.g., V, might result. The actual reaction of l with Et3BH is consistently more complex than anticipated (-3.31. 

The electron counting" in structure X deserves a brief comment. In the absence of a M-M bond, the Mo in CpMo(C0)2(n3-ally1) has an 18-electron count. The Mo in X corresponds to a 16-elec-tron metal in a CpMo(C0)2(a-alkyl) complex. We therefore propose that the metal-metal bond is formed by a Mo— Mo dative bond which renders Mo negative with respect to Mo in structure X. The semi-bridging carbonyl then removes some of the excess negative charge on Mo by accepting electrons into the CO ir -orbitals. 

Figures 14 and 15 show the 0RTEP plot of the cluster, 21 (58). In the cation, which has virtual C3-symmetry, the carbonyls are arranged in two sets, axial and equatorial. The latter set may be classed as barely semi-bridging (the average Mo-C-0eq angle is 168(1)° ys. 176(2)° for Mo-C-0ax) (1 4). The three Mo-Mo distances in 21 are 3.106(2), 3.085(2), and 3.064(2)fl (ave. 3.085(21)A). The Mo-S distances are all equal and have an average value of 2.360(4) A. Both the Mo-Mo and Mo-S distances are longer than the corresponding bond lengths in the related cluster, Cp3Mo3Si,+ (22) viz. 2.812 (Mo-Mo), 2.293 (Mo-p3S) (59).

The knowledge of the principal components of the chemical shift tensor and of the csa has been exploited to quantify the degree of asymmetry of bonding for semi-bridging CO groups in iron carbonyl complexes. In fact very good... 

The MAS NMR spectrum of Fe3(CO)i2 differs markedly from those of Ru3(CO)i2 and Os3(CO)i2 in many respects. At ambient temperature the isotropic region displays three pairs of resonances of similar integrated intensities whose position is not consistent with bridging or semi-bridging carbonyls. Only at low temperature is the observed spectrum consistent with... 

The 17e metal-based radical Ta 3(CO)4(dppe) was formed via hydrogen atom extraction from TaIH(CO)4(dppe).659 In solution, this radical abstacted halogen atoms from many organic halides RX to give TaIX(CO)4(dppe). Ta°(CO)4(dppe) existed in solution as an equilibrium mixture of monomer and dimer [(dppe)(CO)3Ta](/i2-CO)2. The latter is the form stable in the solid state. While no crystal structures are available, DFT calculations indicated a stable pseudo-octahedral stereochemistry for the monomer and several possible (/i2-CO)2 structures for the dimer.656 Each of the latter featured linear, semi-bridging carbonyls supporting a weak, delocalized Ta—Ta interaction. 

An X-ray diffraction study of MnRe(CO)io has shown the solid to exhibit a reversible structural change at 343 K with a transition enthalpy of 3.23 kcal moP The concept of the semi-bridging carbonyl group has been extended to the Group VIIB carbonyls. MO calculations on M2(CO)iq (M = Mn or Re) suggest that the equatorial carbonyl groups of the former, but not the latter, can be considered as semi-bridging. ... 

Some years ago, Cotton [47] proposed that semi-bridging CO groups in carbonyl complexes (as in types (2) and (4)) may be thought of as intermediates between terminal (1) and symmetrical bridging (3) arrangements. 

Reaction of the thiocarbyne TpW( = CSMe)(CO)2 with the Ni(I) dimer Cp2Ni2(At-CO)2 generates a trinuclear metal complex based on a Ni2W triangle TpCp2WNi2(/t3-CSMe)(CO)2 (Scheme 71). Two infrared stretches at 1875 and 1806 cm are consistent with a semi-bridging role for the tungsten carbonyls. 

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