Resonance structures metal carbonyl group

The metal-carbonyl group bond may be described by the following resonance structures ... 

The Pauson-Khand reaction gives the same product as the group 4 metal-mediated reductive coupling and carbonylation, and both reactions proceed by essentially the same mechanism formation of an alkyne-metal tt complex, insertion of an alkene, insertion of CO, and reductive elimination. Some details differ, however. When an alkyne is added to Co2(CO)g, CO evolves, and an isolable, chromatographable alkyne-Co2(CO)6 complex is obtained. This butterfly complex contains four Co(II)-C bonds, and the Co-Co bond is retained. The formation of the alky n e-C o2 (C O) 6 complex involves the formation of an ordinary tt complex of the alkyne with one Co(0) center, with displacement of CO. The tt complex can be written in its Co(II) cobaltacyclopropene resonance structure. The tt bond of the cobaltacyclopropene is then used to form a tt complex to the other Co center with displacement of another equivalent of CO. This second tt complex can also be written in its cobaltacyclopropene resonance structure. The alkyne-Co2(CO)6 complex has two 18-electron Co(II) centers. 

The carbonyl oxygens of transition metal carbonyls also give quite sharp lines, and various structural correlations have been identified which supplement those available from the C resonances. In some phosphine substituted molybdenum tetracarbonyls the correlation of ( 0) with 5( C) and 5( P) was good for carbonyl groups in positions trans to the phosphine ligands, but poor for the m. ... 

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