EXPERIMENTAL AND THEORETICAL COMPARISON BETWEEN M(CP)CL3LN SYSTEMS OFNB-IV AND MO-IV (CP = ETA-C5H5)

The controlled sodium reduction of Nb(cp)Cl4L (cp = eta-C5H5; L = PMe3 , PMe2Ph or PMePh2) or Nb(eta-C5Me5)Cl-4 the presence of PMe3 afforded the mononuclear 15-electron complexes Nb(cp)Cl3L and Nb(eta-C5Me5)Cl- 3(PMe3), respectively. Reduction of Nb(cp)Cl-4 in the presence of an e xcess of L for PMe2Ph and PMePh2 afforded solids that contain mainly t he 17-etectron Nb(cp)Cl3L2 species but are contaminated by the mono-L derivatives. A UV/VIS investigation of the solution equilibrium betwee n Nb(cp)Cl-3(PMe2Ph)(2) and Nb(cp)Cl-3(PMe2Ph) plus free PMe2Ph afford ed 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 st udy of the equilibrium between Mo(cp)Cl-3(PMe2Ph)(2) and Mo(cp)Cl-3(PM e2Ph) 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 se cond-order Moller-Plesset perturbation (MP2) level on the M(cp)Cl-3(PH 3)(n) (M = Nb or Mo, II = 1 or 2) model systems afforded geometries in good agreement with experimental examples. The calculated PH, dissoci ation 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 3 2.8 kcal mol(-1) relative to the excited (1)A' state and by 23.5 kcal mol(-1) relative to the ground (3)A'' state. Therefore, the regain of pairing energy upon PH3 dissociation from Mo(cp)Cl-3(PH3)(2) provides a calculated stabilization for the 16-electron monophosphine complex o f 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 g reater extent of both M-P sigma bonding and pi back bonding for the d( 2) metal relative to the d(1) metal.