A series of dihalobis(tertiary phosphine)nickel(II) complexes, NiL2X2, has been synthesized. The tertiary phosphine ligands include PR3, PR2Ph3 and PRPh2 types where R is cyclopropyl or cyclohexyl and Ph is phenyl. These tertiary phosphines and several others Were selected to test the relative importance of steric and electronic effects on the planar-tetrahedral structural equilibrium exhibited by NiL2X2 type complexes. The thermodynamics and kinetics of this interconversion Were determined by proton magnetic resonance and magnetic suscepttbility measurements in dichloro-methane solution. Results show that steric factors are relatively unimportant in affecting the thermodynamics of the structural equilibrium. Electronic effects, however, are extremely important and are interpreted via a metal-phosphorus π-bonding scheme. Kinetic parameters were measured by standard pmr line shape techniques and are reported for trialkyl- and dialkylphenylphosphine complexes for the first time. The activation parameters are similar to other NiL2X2 complexes where L = alkyldiphenylphosphine.