Relative Strengths of Early Transition Metal M-H and M-C Bonds in Substituted Niobocenes and Tantalocenes. Thermodynamic Trends and Electronic Factors of Olefin Insertion into a Metal-Hydride Bond

Principles of M-C and M-H bonding interactions are examined experimentally by high-resolution valence photoelectron spectroscopy. Gas-phase He I and He II photoelectron spectra are reported for the following series of d° and d² bent metallocenes: Cp₂MH₃ (M = Nb, Ta), Cp₂M(CO)L (M = Nb, Ta; L = H and M = Nb; L = CH₃), and Cp₂M(C₂H₄)L (M = Nb, Ta; L = H and M = Ta; L = C₂H₅) (Cp = η⁵-C₅H₅). The ligand replacements represented by this series of complexes allow stepwise comparison and assignment of each individual low-energy valence ionization band. The He I/He II spectral comparisons show that the lowest ionization energy band of the carbonyl complexes is higher in metal character than the corresponding ionization of the ethylene complexes. This trend indicates extensive electron delocalization in the metal-ethylene bonding that corresponds more accurately to a metallacyclopropane description with relatively strong metal-carbon bonds. The photoelectron data for the early transition metal hydride and alkyl species show that the metal-hydrogen bonds are slightly more stable than the metal-carbon bonds. Correlation of the ionization information for these substituted early metallocenes reveals that the stabilization of the metallacyclopropane-hydride structures, M(C₂H₄)H, relative to the olefin-inserted metal-alkyl structure, M(C₂H₅), is partly due to stabilization of the metal electron density by back-bonding to the olefin as indicated by earlier theoretical studies. However, this factor alone is not sufficient to offset the energy required to convert a carbon-hydrogen bond in M(C₂H₅) to a metal-hydrogen bond in M(C₂H₄)H. The additional important factor is the stabilization derived from conversion of the metal-alkyl σ bonding to the more favorable delocalized M-C₂ σ-bonding situation in the metallacyclopropane hydride.