Spin State Change in Organometallic Reactions. Experimental and MP2 Theoretical Studies of the Thermodynamics and Kinetics of the CO and N₂ Addition to Spin Triplet Cp*MoCl(PMe₃)₂

The first comparative kinetic study of the addition of the isolobal and isosteric CO and N2 ligands to a spin triplet organometallic compound, i.e. Cp*MoCl(PMe3)2, is reported. A fast and quantitative addition process occurred when interacting Cp*MoCl(PMe3)2 with CO, which is followed by a subsequent slower process involving PMe3 replacement and formation of Cp*MoCl(CO)2(PMe3). The N2 addition, on the other hand, is much slower and proceeds incompletely to an equilibrium position. The temperature dependence of this equilibrium gives the parameters for the reaction ΔH = −22.8 ± 2.1 kcal/mol and ΔS = −67 ± 7 cal·mol-1·K-1. The activation parameters for the CO addition are ΔH⧧ = 5.0 ± 0.3 kcal/mol and ΔS⧧ = −35 ± 4 cal·mol-1·K-1, while the activation parameters for the N2 addition are ΔH⧧ = 14.0 ± 1.0 kcal/mol and ΔS⧧ = −20 ± 3 kcal/mol. Extrapolation of the rates to 25 °C indicates a difference of more than three orders of magnitude:  kCO = 29 ± 3 M-1 s-1 and kN2 = 0.014 ± 0.001 M-1 s-1. Theoretical calcula...