Abstract: The kinetics of nuclcophilic substitution reactions of bis(diphenyl-alkylphosphine) pentachlorophenylnickel (Ⅱ) halides with NO₂^- and SCN^- in MeOH EtOH and DMF was studied by spectrophotometric method. The results indicated that the reactions followed a two-term reaction rate law, that is, r = (k₁ + k₂[L]) [complex], k₁, and k₂ values were determined at different temperatures and the corresponding activated thermodynamic parameters △H^≠ and △S^≠ were evalualed. Reaction mechanisms have been proposed and factors influencing the reaction rate such as solvent effect, nature of the leaving and entering groups, different alkyl groups in the phosphine ligand, etc, have been discussed.