Four- and five-co-ordination in complexes of palladium(II) and platinum(II) with NN′-disubstituted thio- and seleno-ureas

Abstract

Complexes of NN′-disubstituted thio- or seleno-ureas (L) with bivalent palladium or platinum (Pd, Pt = M) belong mostly to the following types: (i) M^IIL₄X₂(X = halogen), behaving typically as 1 : 1 electrolytes containing the five-co-ordinated cation [M^IIL₄X]⁺; (ii)[M^IIL₄X]ClO₄, resulting from replacement of the outer halide ion of M^IIL₄X₂ through ClO₄^–; (iii) M^IIL₄(ClO₄)₂, where one perchlorate tends to be co-ordinated to M, thus again achieving five-co-ordination. True four-co-ordinate species [M^IIL₂X₂] are formed in solution equilibria from [M^IIL₄X]⁺ in the presence of an excess of X^–, but are not stable in the solid state, except in a few instances with platinum(II). The spectrochemical parameters of the involved chromophores are discussed on the assumption of a distorted square-pyramidal structure of the five-co-ordinated forms; S-bonded thioureas have a position quite close to water in the spectrochemical series of quadratic chromophores, and their optical electronegativity (x_opt) is about 2·65, while seleno-ureas have slightly lower values of both the ligand-field strength and x_opt.