The co-ordination number of transition-metal ions. Part V. The relative stabilities of some tetrahedral and octahedral cobalt(<scp>II</scp>) complexes of alkyl pyridines, and the effect of substituent position on metal–pyridine π-bonding

Abstract

Solid-state structures for a series of CoLnX2 complexes are assigned on the basis of magnetic and electronic spectra properties (L = 3-methyl-, 3-ethyl-, 4-methyl-, 4-ethyl-, 4-propyl-, 3,4-dimethyl-, or 3,5-dimethyl-pyridine; X = halide or pseudohalide; n= 2 or 4). The spectra and electrical conductances of chloroform and nitromethane solutions are also reported. Two types of solution behaviour were observed for tetrahedral CoL2X2 complexes. Class 2 compounds undergo ligand-replacement reactions with the formation of ionic species in the presence of an excess of amine, whereas Class 1 compounds exist in equilibrium with the neutral octahedral adducts, CoL4X2. Thermodynamic data for the latter equilibria show that, while the length of the alkyl side-chain has little or no effect, the stabilities of the octahedral (relative to the tetrahedral) complexes are substantially less for the 3-than for the 4-substituted pyridines. The results are interpreted in terms of variations in the π-acceptor capacities of the differently substituted pyridines. Supporting evidence is provided by thermodynamic data for configuration equilibria in the solid state, and by the ligand-exchange reactions of some analogous nickel(II) complexes.