Differences in the Nature and Stability of Cadmium Complexes with Group 15/Group 16 Donor Ligands As Determined by Multinuclear (³¹P, ⁷⁷Se, ¹¹³Cd) Magnetic Resonance and Electrochemical Techniques

Multinuclear (³¹P, ⁷⁷Se, ¹¹³Cd) magnetic resonance and electrochemical studies have been carried out to investigate the nature and stability of cadmium complexes with Ph₂PCH₂P(E)Ph2 [E = S (dpmS), Se (dpmSe)] and Ph₂P(E)CH₂P(E)Ph₂[E = S (dpmS₂), Se (dpmSe₂)] in dichloromethane solution. The NMR studies indicate a cadmium preference for selenium or sulfur rather than phosphorus coordination in the dpmE complexes but are limited in scope with the NMR technique to examination of the 1:2 cadmium to ligand stoichiometry due to the labile nature of the systems in the presence of excess ligand. Cadmium-selenium coupling is observed in some cases. The electrochemical reductions of both [Cd(dpmE)₂]²⁺and [Cd(dpmE₂)₂]²⁺at a mercury electrode are reversible two-electron processes. The variations in half-wave potential of these processes as a function of ligand concentration have been measured in order to calculate the number of moles of coordinated ligand and the stability constants of the resultant compounds in excess of 1:2 cadmium to ligand stoichiometry. Ligand coordination numbers of 4 and 6 are attained with the dpmE complexes, but the dpmE₂complexes have a maximum ligand coordination number of 4 even at very high ligand to metal concentration ratios. The magnitudes of the calculated stability constants suggest that all of the dpmE and dpmE₂complexes of cadmium(II) are essentially nondissociated in solution. The use of a Cd(Hg) amalgam electrode allows examination of the voltammetry of these cadmium systems in the presence of substoichiometric ligand concentrations and reemphasizes the high stability of the dpmE and dpmE₂complexes. The preference of cadmium for the group 16 donor atoms rather than phosphorus in the dpmE complexes is confirmed by an investigation of the interactions of cadmium perchlorate with PPh₃and PPh₃Se and by suitable mixing experiments.