Five-coordinate complexes substitution reactions

The five-coordinate complexes Ir(CO)(PPh3)2L, where HL = /3-diketone, A-benzoyl-A-phenyl-hydroxylamine, salicylaldehyde, 8-hydroxyquinoline, 2-hydroxybenzophenone, 2-hydroxy-8-methoxybenzophenone, were prepared from [Ir(CO)(PPh3)2Cl].632 The resulting compounds all underwent oxidative addition reactions with Br2. Reaction of [(cod)2IrCl]2 with N-substituted 3-hydroxy-2-methyl-4-pyridine gives the bichelated complex (389). 33... 

Evidence has been obtained for the occurrence of five-coordinate complexes in the bis-DMG (158), BAE (61), salen (60), and corrin complexes (70, 71) when one of the axial ligands is an alkyl group (see Section III, B). The detection of a five-coordinate complex under certain conditions is, of course, no proof that ligand substitution reactions occur by an aSjjI mechanism under other conditions, but is nevertheless suggestive. 

The fact that the five-coordinate complex [Ni(CN)s] can be detected does indeed explain why substitution reactions of the four-coordinate complex [Ni(CN)4l are fast. The reason is that, for a detectable amount of [Ni(CN)5] to build up in solution, the forward rate constant kf must be numerically dose to or greater than the reverse rate constant... 

The Associative Mechanism The A mechanism can be expected to predominate in complexes that can expand their coordination number relatively easily. The obvious candidates are the square planar complexes of Pt11, Pdn, and Aum, for which many comparable five-coordinate complexes are also known. The salient feature of ligand substitution kinetics in these square planar complexes, however, is not the detectability of five-coordinate intermediates—the reaction rates are usually accurately second order overall—but rather the wide-ranging dependence of the second-order rate constants on the nature of the incoming ligand. This implies that in the scheme... 

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