Equilibrium and Kinetic Studies of Isomerization of (OC)5M[η1-PPh2CH2CH(PPh2)2] and (OC)5M[η1-PPh2CH(PPh2)CH2PPh2] (M=Cr, Mo, W)

It has been found that (OC)5M[η1-PPh2CH2CH(PPh2)2] and (OC)5M[η1-PPh2CH(PPh2)CH2PPh2] (M=Cr, Mo, W) exist in equilibrium in solution. This linkage isomerization is the first experimental evidence for the exchange of terminal and coordinated phosphorus groups in pentacarbonyl complexes of group 6 metals. (OC)5M[η1-PPh2CH2CH(PPh2)2] ⇌ (OC)5M[η1-PPh2CH(PPh2)CH2PPh2] (M = Cr, Mo, W) In this work, equilibrium constants (K) at several temperatures were measured in order to obtain the values for the change of enthalpy (ΔH°), the change of entropy (ΔS°), and the change of free energy (ΔG°) for the isomerization. The relative stabilities of the two linkage isomers were determined. Rate constants (k) at several temperatures were also measured for the isomerization of W and Cr complexes. From these, the half reaction time (t1/2), activation energy (Ea), enthalpy of activation (ΔH≠), entropy of activation (ΔS≠), and free energy of activation (ΔG≠) were calculated. A mechanism was proposed for the isomerization. Energy diagrams were also plotted from the equilibrium and kinetic data. Phosphorus-31 nuclear magnetic resonance spectroscopy was applied to monitor the change of concentration of the isomers