Phosphorus-31 NMR investigation of the comparative hydrolytic breakdown of nickel(II) and cadmium(II) versus zinc(II) bis(O,O-diethyl dithiophosphates) in an aqueous medium

The hydrolytic breakdown of nickel(II) and cadmium(II) bis(O,O-diethyl dithiophosphates) 1b and c, respectively in aqueous dimethoxyethane at 85°C have been studied using ³¹P NMR spectroscopy. The complexes are hydrolysed at different rates although in each case the process leads to identical end-products, viz. phosphoric acid 7, O-ethyl O,O-dihydrogen phosphorothioate 8, O,O-diethyl O-hydrogen phosphorothioate 9, and ethyl dihydrogen phosphate 10. Comparison of the kinetic results obtained with those for the previously studied zinc(II) analogue 1a show that there is ca. one hundred-fold decrease in the rate of hydrolysis for the nickel(II) complex, this being ascribed to the intervention of two hitherto unobserved hydrated species 11 and 12, both of which are sufficiently stable to be observed by ³¹P NMR spectroscopy. Similar intermediates are not observed with the cadmium(II) complex 1c which hydrolyses at a comparable rate to its zinc counterpart. These results are explained in terms of the respective metal-sulfur bond strengths of the dithiophosphate complexes and further support the contention that hydrolysis occurs by attack of water at the metal centre followed by the loss of O,O-diethyl S-hydrogen phosphorodithioate 3, the primary hydrolysis product.