The mechanism of oxidative addition of iodine to a dimethylplatinum(II) complex

The oxidative addition of iodine to [PtMe2(bipy)], 1, occurs easily with trans stereochemistry to give trans-[PtI2Me2(bipy)], 2a, which then equilibrates slowly with cis-[PtI2Me2(bipy)], 2b. The structures of 2a and 2b were determined in a crystal containing both isomers and an additional iodine molecule. Low temperature NMR studies show that an intermediate charge transfer complex [PtMe2(I2)(bipy)], 3, or solvated iodine complex [PtMe2(I2)(S)(bipy)], S = solvent, may be formed in dichloromethane or acetone solution before formation of 2a. In dilute benzene solution, as monitored by UV–visible spectroscopy, the rearrangement of the intermediate complex to 2a followed first order kinetics, and exhibited a large negative entropy of activation, as expected if cleavage of the I–I bond to give an ionic intermediate [PtIMe2(bipy)]+I−, is rate determining. The overall reaction is slower in more polar solvents because, although the I–I bond is more easily cleaved to give [PtIMe2(bipy)(S)]+I−, the subsequent displacement of solvent, S, by iodide to give 2a is slow.