Charge effects on oxygen atom transfer

The rhenium(V) complex [(HCpz(3))ReOCl2](+) ([1](+)), the tris(pyrazolyl)me thane analogue of the known tris(pyrazolyl)borate complex (HBpz(3))ReOCl2 ( 2), has been prepared. The two complexes are strikingly similar, as are the phosphine oxide adducts [(HCpz(3))ReCl2(OPPh3)]Cl ([3]Cl) and (HBpz(3))ReC l2(OPPh3) (4), which have been characterized by X-ray crystallography. Comp arison of the bimolecular reduction of [1]BF4 and 2 by triarylphosphines re veals a pronounced charge effect, with the cationic species being reduced b y PPh3 about 1000 times faster than its neutral analogue in CH2Cl2 at room temperature. Ligand substitution of the adducts [3](+) and 4 is dissociativ e, with the cationic complex dissociating phosphine oxide about 56 times mo re slowly than the neutral compound. The relative impact of charge on groun d and transition states in atom transfer reactions is discussed.