Ligand substitution in square planar complexes

A completely empirical LFER can also be constructed with recourse only to kinetic data. This has been the case in the setting up of a scale of nucleophilic power for ligands substituting in square-planar complexes based on the Swain-Scott approach. The second-order rate constants Ay for reactions in MeOH of nucleophiles Y with tra 5-Pt(py)2Cl2, chosen as the standard substrate... 

Initially a solvent molecule attacks a vacant coordination site on Pd in (77-C3H5)Pd(77-C5H5) to give a 20-electron intermediate (XIV) having a distorted square-planar configuration. Probably the latter then becomes stabilized by the Tr-CjHjPd bond changing to a localized <7-bond. Subsequent reactions proceed as ordinary ligand substitution in square-planar complexes (1). 

The detection of a reaction intermediate is usually not possible in coordination chemistry because lifetimes of intermediates are commonly extremely short. The simple mechanisms of reaction are commonly designated as an associative mechanism (A, with an intermediate of expanded coordination number formed) or a dissociative mechanism (D, with an intermediate of reduced coordination number formed). Intermediates of expanded coordination number are important in ligand substitution in square-planar complexes and in a few cases can actually be detected. For example, NifCNls " is known from exchange reaction of Ni(CN)4 with CN (288). Even in octahedral complexes, some evidence for associative processes exists indirectly. The [RulNHsle] " ion reacts with NO in acid to form [RuINHslsNO] and NH4 much more rapidly than can be explained by aquation of the hexaamine as the initial step, and a bimolecular mechanism with a 7-coordinate intermediate has been proposed (11, 226). 

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