Five coordinate carbonyl species

Evidence in support of this mechanism was provided by changes in the C=0 stretching frequency as the monodentate ester (1735 cm-1) was first chelated (1610 cm-1) and then hydrolyzed to [Co(en)2(Gly)]2+ (1640 cm-1). The reaction presumably proceeds via the five-coordinate species (31), and the ester carbonyl oxygen competes so effectively with solvent water for the vacant coordination site that the chelate ester is formed exclusively. 

Carbonylation of solutions of K3[CoH(CN)s] yields [Co(CN)3(CO)2]2, however attempts to isolate pure salts of this species have been unsuccessful owing to its tendency to decompose and disproportionate.1 On treatment of the product solutions with phosphines moderately stable five-coordinate Co1 complexes of the type [Co(CN)(CO)2(PR3)2], [Co(CN)2(CO)(PR3)2] and [Co(CN)2(CO)2(PR3)]- are obtained (Scheme l).16 An alternative route to [Co(CN)(CO)2(PR3)2] complexes (PR3 = PEt2Ph, PEtPh2, PPh3) is provide by direct carbonylation of [Co(CN)2(PR3)2]. In C2H4C12 solution addition of CO gives [Co(CN)2(CO)(PR3)2] initially, which is then reduced by CO to the cobalt(I) product.17... 

In the solid state, the Sn atom in the itaconic species 335 adopts a distorted octahedral arrangement with the five- and six-membered fused rings via the two carbonyl oxygen-to-tin coordinations . On the basis of IR and multinuclear NMR spectroscopy data it was suggested that one carbonyl group presumably involved in the six-membered chelate is released from the tin atom, which results in a five-coordinate tin atom in solution in both coordinating and noncoordinating solvents. 

Studies of the base hydrolysis of cis-[Co(en)2(GlyOR)X] ions led to interesting developments. Base hydrolysis of halopentamine complexes of cobalt(III) occurs by an SnICB mechanism leading to a five-coordinate intermediate. In the N-bonded ester complexes of the [Co(en)2(GlyOR)Cl] type, the ester carbonyl group and solvent water can compete for the vacant site in the five-coordinate intermediate (Scheme 5) giving rise to the chelated glycine ester species and the hydroxypentamine respectively. 

On the contrary, reactions of nickel complexes often provide five-coordinate species [76], ft has been clarified that five-coordinate insertion pathway for carbonylation of cafionic monomethylnickel derivatives is energetically favorable under reaction conditions of copolymerizing CO and ethylene (Scheme 7.11) [77]. 

Involvement of a five-coordinate species during CO insertion was suggested experimentally and theoretically in carbonylation of monoalkylpalladium com-... 

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