Experimental and theoretical comparison between M(cp)Cl3Ln systems of NbIV and MoIV (cp = η-C5H5)
Résumé
The controlled sodium reduction of Nb(cp)Cl 4 L (cp = η-C 5 H 5 ; L = PMe 3 , PMe 2 Ph or PMePh 2 ) or Nb(η-C 5 Me 5 )Cl 4 in the presence of PMe 3 afforded the mononuclear 15-electron complexes Nb(cp)Cl 3 L and Nb(η-C 5 Me 5 )Cl 3 (PMe 3 ), respectively. Reduction of Nb(cp)Cl 4 in the presence of an excess of L for PMe 2 Ph and PMePh 2 afforded solids that contain mainly the 17-electron Nb(cp)Cl 3 L 2 species but are contaminated by the mono-L derivatives. A UV/VIS investigation of the solution equilibrium between Nb(cp)Cl 3 (PMe 2 Ph) 2 and Nb(cp)Cl 3 (PMe 2 Ph) plus free PMe 2 Ph 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)Cl 3 (PMe 2 Ph) 2 and Mo(cp)Cl 3 (PMe 2 Ph) plus free PMe 2 Ph 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)Cl 3 (PH 3 ) n (M = Nb or Mo, n = 1 or 2) model systems afforded geometries in good agreement with experimental examples. The calculated PH 3 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 1 A′ state and by 23.5 kcal mol -1 relative to the ground 3 A″ state. Therefore, the regain of pairing energy upon PH 3 dissociation from Mo(cp)Cl 3 (PH 3 ) 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–PH 3 bonding interaction is the greater extent of both M–P σ bonding and π back bonding for the d 2 metal relative to the d 1 metal.
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