Mechanism of the decomposition reaction of trans-[Mo(N2)2(PPh2Me)4] and of its reaction with pyridine

Abstract

The complex trans-[Mo(N₂)₂(PPh₂Me)₄]1 decomposes in benzene solution to give molybdenum, phosphine and dinitrogen. The kinetics of this process has been studied from absorbance vs. time curves which reveal two consecutive steps occurring at comparable rates. Relevant kinetic parameters for each step are k₁= 0.52 × 10^–2 s^–1 at 10.0 °C, ΔH₁^‡= 75 kJ mol^–1, ΔS₁^‡=–29 J K^–1 mol^–1, k₂= 2.13 × 10^–3 s^–1 at 10.0 °C, ΔH₂^†= 38 kJ mol^–1, ΔS₂^‡=–163 J K^–1 mol^–1. A mechanism is proposed in which there is an initial dissociation of PPh₂Me to form a five-co-ordinate intermediate [Mo(N₂)₂(PPh₂Me)₃]2. The presence of 2 in benzene solutions of 1 has been detected by ¹H NMR spectroscopy. In the next step associative attack on 2 occurs to form an unstable species 3 which probably contains co-ordinated solvent and decomposes rapidly. From kinetic data for solutions containing an excess of PPh₂Me, the equilibrium constant for the phosphine dissociation step has been determined to be K₁= 1.64 × 10^–3 mol dm^–3 at 10.0 °C. The formation of trans, mer-[Mo(N₂)₂(PPh₂Me)₃(py)] from 1 and pyridine (py) is kinetically controlled by phosphine dissociation.