Nucleophilic Substitution, Palladium(II), Formazan, 1-(2-hydroxyphenyl)-3,5-diphenylformazan, Solvent Effects, Substitution Reactions Nucleophilic substitution in different non-aqueous solvents at square planar complexes of palladium(II) with 1-(2-hydroxyphenyl)-3,5-diphenylformazan (H2Fo) according to FoPdX + Y → FoPdY + X, with X = ammonia and pyridine and Y = triphenylphos-phine, thiourea and the thiocyanate ion, obeys the rate law: rate = k1[FoPdX] + k2[FoPdX][Y]. Values of k1 and k2 were obtained at 25.0 °C. From measured solubilities and literature values of transfer functions the transfer chemical potentials for initial and transition state were calculated. A comparison of these quantities in different solvents leads to the conclusion that non-specific solvation effects are dominant in these compounds. The mutual dependence of the effects of leaving and entering ligands and solvent on the substitution rate indicates a level of synchronicity of bond-breaking and bond-making that is unusually high for palladium(II) complexes.