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, k_f, are in the range (1- 8)×10⁵ l. mol^-1 s^-1, ΔH_f^‡ is 16±2 kJ mol^-1 and there is a significant substituent effect. For Ni(dtp)₂, k_f is (1-3)×10⁴ l. mol^-1 s^-1, ΔH_f^‡ is 24±2 kJmol^-1 and there is a minor substituent effect. For Ni(sacac)₂, k_f is about 5‡10² l. mol^-1 s^-1, ΔH_f^‡ 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.