Kinetics and mechanism of ligand substitution reactions of (2,6-diaminopyridine)[M(CO)5]2 (M = Cr, Mo, W)

The bimetallic complexes (μ-2,6-dap)[M(CO)5]2 [2,6-dap = 2,6-diaminopyridine, M = Cr (1), Mo (2), W (3)] were synthesized from M(CO)6 and 2,6-diaminopyridine. The complexes were characterized on the basis of their carbonyl stretching frequencies and elemental analysis. A thermal kinetic study of the reactions of complexes 1–3 with P(OiPr)3 under pseudo-first-order reaction conditions in chlorobenzene has been conducted. These reactions proceed by a two-step mechanism, giving a M(CO)5 intermediate which rapidly reacts with P(OiPr)3 to give M(CO)5(P(OiPr)3). The first step, which involves initial dissociation of one M–N bond to give the monometallic (2,6-dap)M(CO)5 and M(CO)5, is much slower than the dissociation of the second M–N bond and hence is rate-determining. The rate constants and activation parameters for these reactions have been evaluated and discussed, and a general mechanism is proposed.