Carbonyl ligands bridging

The structures are shown in Fig. 26.8c and d and differ in that, whereas the Ir compound consists of a tetrahedron of metal atoms held together solely by M-M bonds, the Rh and Co compounds each incorporate 3 bridging carbonyls. A similar difference was noted in the case of the trinuclear carbonyls of Fe, Ru and Os (p. 1104) and can be explained in a similar way. The M4 tetrahedra of Co and Rh are small enough to be accommodated in an icosahedral array of CO ligands whereas the larger Ir4 tetrahedron forces the adoption of the less dense cube octahedral array of ligands. 

Kiindig EP, Dupre C, Bourdin B, Cunningham A Jr, Pons D (1994) New C2-chiral bidentate ligands bridging the gap between donor phosphine and acceptor carbonyl ligands. Helv Chim Acta 77 421 28... 

While essentially all the metal carbonyl complexes for group 4B contain terminal CO ligands, only recently have some bonafide doubly bridging carbonyl complexes been reported. However, these complexes are hetero-nuclear, since the carbonyl ligand bridges a zirconium atom with the metal center of a late transition metal. 

The cluster anion, [Os3Ir(CO)i3], was prepared in 50% yield by reaction of Os3(CO)i2 with [Ir(CO)4]. The single-crystal X-ray structure analysis shows it to consist of a tetrahedral metal core with one of the 13 carbonyl ligands bridging. The catalytic activity for carbonylation of... 

However, these reactions jure associated with terminal rather than bridging carbonyl groups. In metal "ketonic compounds, the center for attack is normally the metal. This may be rationalized in terms of additional delocalization of the positive charge from the carbon to the metal center and implies a "carbyne contribution to the carbonyl bridge, of the form discussed above for the methylated products, again emphasizing the possibility that in systems of this type reaction may occur at either the metal or ligand center... 

In the cluster Ni4( 2 -CO)6 [P(CH2 CH2 CN)3 ]4, the presence of just 10 ligands gives rise to a most symmetrical tetrahedral structure, which has all edges occupied by bridging carbonyl groups, (Fig. 12)17. ... 

Comparison with the corresponding dianion reveals important structural differences. In fact, the triply bridging apical carbonyl is no longer present and one of the bonds of the bridging carbonyl (namely, Fe3-Cl) weakens (becoming half-bonding). This asymmetric disposition of the ligand leads to an inequivalence of all the three Fe-Fe distances. 

Ru3(CO)i2 is an orange air-stable crystalline solid, soluble in a wide variety of organic solvents, insoluble in water, and volatile. Its IR spectrum, in hexane, shows vco bands at 2061 (vs), 2031 (s), and 2011 (m) cm . No band assignable to a bridging carbonyl ligand is observed. ... 

Fig. 13. Solution structure of Rh5(CO)15] (139). x represents a bridging carbonyl ligand.

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