Complexes with sulphur and selenium donor ligands. Part 6. Kinetics and mechanism of the reaction between 1,2-bis(diphenylphosphino)-ethane and tris(OO′-dimethyl phosphorodithioato)cobalt(III)

Abstract

The kinetics and mechanism of the reaction between [Co{S₂P(OMe)₂)₃] and Ph₂P[CH₂]₂PPh₂(dppe) in CH₂CI₂have been investigated by spectrophotometric and conductance methods. The reaction proceeds in two steps. The first step is (i), deduced from the formation of ions during the reaction and on spectroscopic grounds. The [Co{S₂P(OMe)₂}₃]+ dppe →[Co{S₂P(OMe)₂}₂(dppe)]⁺+[S₂P(OMe)₂]^–(i) activation parameters are ΔH‡= 14.3 ± 1.0 kcal mol^–1 and ΔS+=–17 ± 3 cal K^–1 mol^–1. The low ΔS‡ value and linear dependence of the rate on dppe concentration suggests rate-determining nucleophilic participation of the incoming ligand. A one-ended reversible dissociation of the dithioate ligand with the dppe competing for the resulting five-co-ordinate intermediate or direct nucleophilic attack of the phosphine on [Co{S₂P(OMe)₂}₃] or on the dithioate phosphorus are mechanisms consistent with the data. The second step of the reaction is (ii). [Co{S₂P(oMe)₂}₂(dppe)]⁺+[S₂P(OMe)₂]^–→[Co{S₂P(OMe)₂}{S₂PO(OMe)}(dppe)]+ P(OMe)₂S(SMe)(ii) Although a reliable estimate of the rate constants could not be obtained because of ion-producing side reactions after ca. 90% completion of reaction, it is probable that this step occurs via nucleophilic attack on the P–O–C carbon atom of the co-ordinated [S₂P(OMe)₂]^–.