Experimental and theoretical comparison between M(cp)Cl₃L_n systems of Nb^IV and Mo^IV (cp = η-C₅H₅)

Abstract

The controlled sodium reduction of Nb(cp)Cl₄L (cp = η-C₅H₅; L = PMe₃, PMe₂Ph or PMePh₂) or Nb(η-C₅Me₅)Cl₄ in the presence of PMe₃ afforded the mononuclear 15-electron complexes Nb(cp)Cl₃L and Nb(η-C₅Me₅)Cl₃(PMe₃), respectively. Reduction of Nb(cp)Cl₄ in the presence of an excess of L for PMe₂Ph and PMePh₂ afforded solids that contain mainly the 17-electron Nb(cp)Cl₃L₂ species but are contaminated by the mono-L derivatives. A UV/VIS investigation of the solution equilibrium between Nb(cp)Cl₃(PMe₂Ph)₂ and Nb(cp)Cl₃(PMe₂Ph) plus free PMe₂Ph afforded an enthalpy of 19.0 ± 1.6 kcal mol^-1 and an entropy of 45 ± 5 cal K^-1 mol^-1 for the ligand dissociation process. A comparative study of the equilibrium between Mo(cp)Cl₃(PMe₂Ph)₂ and Mo(cp)Cl₃(PMe₂Ph) plus free PMe₂Ph cannot be carried out because the equilibration is too slow at room temperature and because of thermal decomposition with ring loss at high temperature. Theoretical calculations at the second-order Møller-Plesset perturbation (MP2) level on the M(cp)Cl₃(PH₃)_n (M = Nb or Mo, n = 1 or 2) model systems afforded geometries in good agreement with experimental examples. The calculated PH₃ dissociation energy for M = Nb of 21.3 kcal mol^-1 is in good agreement with experiment. For M = Mo, the more saturated complex is stabilized by 32.8 kcal mol^-1 relative to the excited ¹A′ state and by 23.5 kcal mol^-1 relative to the ground ³A″ state. Therefore, the regain of pairing energy upon PH₃ dissociation from Mo(cp)Cl₃(PH₃)₂ provides a calculated stabilization for the 16-electron monophosphine complex of 9.3 kcal mol^-1. The observed variations of bonding parameters upon metal change from Nb to Mo and a natural population analysis suggest that the main reason for a greater Mo–PH₃ bonding interaction is the greater extent of both M–P σ bonding and π back bonding for the d² metal relative to the d¹ metal.

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