Organometallic nucleophiles. A mechanistic study of halide displacement at saturated carbon by 2- and 4-pyridyl complexes of palladium(II) and platinum(II)

The reactions of the 2- and 4-pyridyl complexes [MX(C5H4NCn)(dppe)] and trans-[MX(C5H4NCn)(PPh3)2] (M  Pd, Pt; X  Cl, Br; n = 2, 4)- involving halide displacement from the organic halides XCH2R (X  Cl, Br; R  CN, Ph, CH  CH2) by the pyridyl nitrogen have been studied kinetically by conductivity in acetone or acetonitrile at 25°C. The kinetic data fit the second-order rate law rate k2[XCH2R][complex], in agreement with an SN2 process at saturated carbon. The higher rates are displayed by the 2-pyridylplatinum derivatives with X  Br in both the metal complex and the organic halide. The higher nucleophilic power of the 2-pyridyl complexes compared with their 4-pyridyl analogs is paralleled by a higher basicity, as reflected by a higher pKa value. In any case, the metal-containing substituent enhances considerably the reactivity and basicity of the pyridine nitrogen. The rates are scarcely influenced by the coordination geometry around the metal.