Tertiary phosphine ligand-exchange reactions involving the M-quadruple-bond-M quadruply bonded complexes M2Cl4L4, where L=PMe3, PEt3, PBu3n or PMe2Ph

The reactions between M2Cl4L4 complexes and an excess of L or L' (PMe3, PEt3, PBu3n, PMe2Ph or PMePh2) have been studied in [H-2(8)]toluene by P-31-{H-l} NMR spectroscopy. The substitutions proceed ina stepwise manner wherein L' displaces L, except for L' = Me2PCH2CH2PMe2 (dmpe) which yields Mo2Cl4(dmpe-P)(4). No tertiary phosphine in this series is capable of displacing PMe3 from a M2Cl4(PMe3)(4) complex but by spin magnetization transfer the degenerate exchange involving Mo2Cl4(PMe3)(4) and PMe3 (added in excess) can be detected. The complexes Mo2Cl4(PMe3)(4) and Mo2Cl4(PEt3)(4) in benzene showed no PMe3 for PEt3 scrambling at + 50 degrees C over several days despite the fact that Mo2Cl4(PMe3)(4-n)(PEt3)(n), where n = 2 or 3, are kinetically inert to ligand redistribution. In the presence of [H-2(5)]pyridine Mo2Cl4(PMe3)(4) and Mo2Cl4(PEt3)(4) underwent tertiary phosphine scrambling at 25 degrees C and in neat [H-2(5)]pyridine Mo2Cl4(PMe3)(4) revealed the formation of an equilibrium concentration of Mo2Cl4(PMe3)(3)(py) (py = pyridine) and free PMe3. Under similar conditions Mo2Cl4(PEt3)(4) yielded an equilibrium mixture of Mo2Cl4(PEt3)(3)(py) and Mo2Cl4(PEt3)(2)(py)(2) and free PEt3. From kinetics the Delta H-double dagger values are positive in the range +24 to +34 kcal mol(-1) and the Delta S-double dagger values range from +12 to +28 cal K-1 mol(-1). Collectively the data reported are consistent with an interchange dissociative mechanism, I-d, wherein M-P bond breaking contributes significantly to the rate-determining step with related values of Delta H-double dagger being larger for M = W than M = Mo. The rate dependence on the entering ligand is clearly evident from temperature-dependent studies and leads to varying Delta S-double dagger values. The I-d mechanism is proposed to involve pre-equilibria between M2Cl4L4 and the entering L' in an axial site followed by rate-determining M-L displacement. In neat [H-2(5)] pyridine this may be viewed as a solvent-assisted;displacement. The present results are discussed in terms of earlier studies from which researchers inferred a simple dissociative process, D, involving M-PR3 bond rupture as the first and rate-determining step.