Kinetics and mechanism of the reaction of bis-μ-diethylphosphidobis(tetracarbonylmanganese) with tri-n-butylphosphine

Article abstract of DOI:10.1016/0022-328X(93)80470-V

The dinuclear phosphido-bridged manganese complex <(OC)4Mn(μ-PEt2)2Mn(CO)4> undergoes thermal CO substitution by PⁿBu3 in decalin to give <(OC)4Mn(μ-PEt2)2Mn(CO)3(PⁿBu3)> and <(ⁿBu3P)(OC)3Mn(μ-PEt2)2Mn(CO)3(PⁿBu3)>.A kinetic study of the first CO/PⁿBu3 exchange indicates that all substitutions for the forward and reverse reactions occur via a dissociative mechanism.The rate of CO dissociation is rather low (k = 6.27 * 10^-5 s^-1 at 130 deg C), mainly as a consequence of a high activation enthalpy (ΔH^* = 165 kJ mol^-1).The discriminating ability ofthe resulting coordinatively unsaturated intermediate <(OC)4Mn(μ-PEt2)2Mn(CO)3> is only slightly in favour of attack of the more basic PⁿBu3 compared to CO, as indicated by the values of the competition rate ratio nBu3) = 0.36-0.47>.This behaviour is compared with that of the isostructural chromium complex and discussed in the light of the electronic and structural features related to the metal-metal distance in the two complexes (Mn-Mn = 3.675 Angstroem, Cr-Cr = 2.905 Angstroem, for the μ-PMe2 derivatives).