The Lewis-acidity of square-planar nickel(II) complexes. II. A kinetic investigation of the addition of substituted phenanthrolines to Bis(O,O-diethylphosphorodithioato-S,S')nickel(II), Bis(O-ethylxanthato)nickel(II) and Bis(4-mercaptopent-3-en-2-onato)nickel(II) in toluene

Abstract

The rates of the addition reaction between substituted phenanthrolines and the square-planar nickel complexes bis(O,O-diethyl phosphorodithioato-S,S')nickel(II), Ni(dtp)2, bis(O-ethylxanthato)-nickel(II), Ni(xan)2, and bis(4-mercaptopent-3-en-2-onato)nickel(II), Ni(sacac)2, have been measured in toluene by the stopped-flow technique. For Ni(xan)2, the second-order rate constants, kf, are in the range (1- 8)×105 l. mol-1 s-1, ΔHf‡ is 16�2 kJ mol-1 and there is a significant substituent effect. For Ni(dtp)2, kf is (1-3)×104 l. mol-1 s-1, ΔHf‡ is 24�2 kJmol-1 and there is a minor substituent effect. For Ni(sacac)2, kf is about 5‡102 l. mol-1 s-1, ΔHf‡ is 50 kJ mol-1 and there is no substituent effect. The results are discussed in terms of the relative importance of Ni-N bond formation and displacement of the in-plane ligands in the transition state.