Ligand exchange reaction between NiMe 2L 2 (L = [formula omitted] bpy, PEt 3) and ditertiary phosphines Ph 2P(CH 2) nPPH 2 ( n = 1−4) and effect of ligand on ease of reductive elimination of C 2H 6 from NiMe 2(Ph 2P(CH 2) nPPh 2)

Abstract

Reactions of the dimethylnickel(II) complexes NiMe 2L 2 (L = 1 2 bpy, PEt 3 (bpy = 2,2′-bipyridine)) with diphosphines Ph 2P(CH 2) n PPh 2 ( n = 1−4) give NiMe 2(Ph 2P(CH 2 n PPh 2) ( n = 2,3) or produce ethane and Ni o-diphosphine complexes ( n = 1, 4), depending on the chain length of (CH 2) n of the diphosphine ligand employed. The ligand exchange reaction between NiMe 2 (bpy) and Ph 2P(CH 2) 3PPh 2 (dpp) proceeds through an S n 2 process with the activation parameters δ H ≠ = 8.3 kcal/mol and δ S ≠ = −35 e.u. A mechanism involving coordination of dpp via one phosphorus atom and ensuing chelation of dpp accompanied by dissociation of bpy is proposed for the ligand exchange reaction. Kinetic studies on the thermal decomposition of NiMe 2(Ph 2P(CH 2) n PPh 2) ( n = 2, 3) show that the reductive elimination of C 2H 6 from NiMe 2(Ph 2P(CH 2 n PPh 2) ( n = 2, 3) proceeds by a unimolecular process with the activation parameters δ H ≠ = 26.8 kcal/mol and δ S ≠ = 1.9 e.u. for NiMe 2(Ph(CH 2) 2PPh 2) and δ H ≠ = 25.1 kcal/mol and δ S ≠ = 4.8 e.u. for NiMe 2(Ph 2P(CH 2) 3PPh 2). The rate of reductive elimination of NiMe 2(Ph 2P(CH 2) 3PPh 2) is 46 times faster than that of NiMe 2(Ph 2(CH 2) 2PPh 2) at 64.3°C.