Effect of chain length of aliphatic diamines on the structure and reactivity of dichloro(diamine)(η-ethylene)platinum(II) complexes

Abstract

Bidentate aliphatic diamines react with Zeise's salt, or its dimer, to give complexes of formula [Pt(C₂H₄)Cl₂L][L =NNN′N′-tetramethyl-ethylenediamine (Me₄en), -propane-1,3-diamine (Me₄pn), or -butane-1,4-diamine (Me₄bn)]. In chloroform solution the Me₄en complex has a five-co-ordinate structure up to the temperature (10 °C) where decomposition with release of ethylene readily occurs. The Me₄pn complex (34 °C) exhibits a geometry which is intermediate between four- and five-co-ordination. On decreasing the temperature, the five-co-ordination becomes increasingly important and is attained at –30 °C. The Me₄bn complex at 34 °C has a purely four-coordinate structure involving equivalent ends of the diamine (obtained by rapid head-to-tail rearrangement). On decreasing the temperature to –30 °C the four-co-ordinate η-ethylene species is still present but in equilibrium with a new species, probably formed by nucleophilic attack on ethylene of a free end of the diamine. Decomposition studies in 1,2-dichloroethane have shown that the tendency of the [Pt(C₂H₄)Cl₂L] species to release ethylene, giving [PtCl₂L], increases with increasing preference for five- relative to four-co-ordination.