Chelate ring closure kinetics in the reaction of a platinum(II) complex with diphosphines

The reactions of PPh2(CH2)(n)PPh2 (n = 1-4, P-P) and PPh3 with cis-[PtPh2(CO)(SEt2)] have been studied in chloroform-d by H-1- and P-31{H-1}-n.m.r. When n = 2 or 3 the first product observed is [PtPh2(P-P)], where the diphosphine is acting as a chelate, and ring closure is fast compared to the rate of entry of the phosphine into the complex. When n = 1 or 4 the first observed product is [PtPh2(CO)(P-P)], with P-P acting as monodentate, and the second observable stage of reaction is ring closure. The rate constants and activation parameters k(c) at 298 K (s(-1)), Delta H double dagger (kJ mol(-1)), Delta S double dagger (J K-1 mol(-1)) for the dppm and dppb complexes are 0.0198, 88 +/- 1, +17 +/- 3; and 0.00273, 38 +/- 2, -169 +/- 6, respectively. The formation of the large seven-membered ring is a ``strainless'' process, comparable to the intermolecular process. The increase in the enthalpy of activation as ring size decreases is due to ring strain and inter-atomic repulsions associated to the conformation of the four-membered chelate ring.