Experimental and computational investigations of phosphine exchange in 15-electron [CrCpCl2(PR3)] systems by stopped-flow and density functional calculations:: a single-state SN2 mechanism

The exchange of the phosphine ligand on the half-sandwich 15-electron, spin quartet [CrCpCl2L] system has been investigated experimentally by stopped-few kinetics with visible detection and theoretically by calculations with DFT methods on the PH3 self-exchange model system. The exchange of PMePh2 with PMe3 follows clean second-order kinetics with the activation parameters Delta H double dagger = 7.0(2) kcal mol(-1) and Delta S double dagger = - 24.3(8) cal K-1 mol(-1) consistent with an associative exchange. The rate constant for the exchange of L with PMe3 in [CrCpCl2L] at room temperature varies only within a factor of 8 for the series of complexes with L = PPh3, PMePh2, PMe2Ph, PEt3, or eta(1)-dppe. The computational work showed that the PH3 self-exchange process occurs via a symmetric transition state along the spin quartet hypersurface, without crossover to the spin doublet state. The optimized transition stale corresponds to a first-order saddle point with Cr-P distances of 3.190 and 3.174 Angstrom, located 7.6 kcal mol(-1) above the [CrCpCl2(PH3)] (spin quartet) + PH3 combination, or 13.6 kcal mol(-1) below the [CrCpCl2(PH3)(2)] doublet minimum. Thus? the phosphine exchange reaction can be classified as a classical S(N)2 process.