Five-coordinate species substitution reaction

The rate of axial ligand dissociation increases dramatically with the number of electrons added, being much faster (Eq. 4) for [Re(X)(CO)3(bpy)] than [Re(X)(CO)3(bpy)] X = Cl or Br. Five-coordinated species [Re(CO)3(bpy)] are formed. The same reaction occurs for other polypyridine ligands. Only [Re(Br)(CO)3(abpy ")] reacts via slow substitution of Br by a solvent (THF) molecule instead of dissociation, and full axial ligand labilization requires addition of one more (third) electron [136]. The complexes [Re(S)(CO)3(bpym)] S = PrCN, THF are also stable in solvent S even at room temperature [137]. For other polypyridines, the two-electron reduced solvento species [Re(S)(CO)3(N,N)] S = CH3CN, PrCN were observed [137] only at low temperature, in the solvent S, in equilibrium with [Re(CO)3(N,N)] . 

The mechanism of reductive elimination with C—C bond formation has been studied for [Ni(CN)PhPa], where P=PEta or PCys (tricyclohexylphosphine). The thermal decomposition of [Ni(CN)Ph(PCy3>2] gives very little PhCN, but with an excess of P(OEt)3 this is formed quantitatively by a reaction, first-order in both complex and triethyl phosphite. An associative reaction with reductive elimination from the five-co-ordinate intermediate is most likely since there is no rate retardation by added PCys, and the rate characteristics are very like those of bimolecular substitution, which, of course, requires the formation of a very similar intermediate. For the reaction of P(OEt)a with [Ni(CN)Ph(PEt3)2] competitive substitution of phosphine by phosphite and reductive elimination need to be considered to account for the kinetics in this case added PEts does lead to rate retardation. Nonetheless, reductive elimination from a five-coordinate species still seems to operate. 

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