Spectroscopic properties and quantum chemistry-based normal coordinate analysis (QCB-NCA) of a dinuclear tantalum complex exhibiting the novel side-on end-on bridging geometry of N-2: Correlations to electronic structure and reactivity

The vibrational properties and the electronic structure of the side-on end-on N-2-bridged Ta complex ([NPN]Ta(mu-H))(2)(mu-eta(1):eta(2)-N-2) (1) (where [NPN] = (PhNSiMe2CH2)(2)PPh) are analyzed. Vibrational characterization of the Ta-2(mu-N-2)(mu-H)(2) core is based on resonance Raman and infrared spectroscopies evaluated with a novel quantum chemistry-based normal coordinate analysis (QCB-NCA). The N-N stretching frequency is found at 1165 cm(-1) exhibiting a N-15(2) isotope shift of -37 cm(-1). Four other modes of the Ta2N2H2 core are observed between 430 and 660 cm(-1). Two vibrations of the bridging hydrido ligands are also identified in the spectra. On the basis of experimental frequencies and the QCB-NCA procedure, the N-N force constant is determined to be 2.430 mdyn -Angstrom(-1). The Ta-N force constants are calculated to be 2.517 mdyn Angstrom(-1) for the Ta-eta(1)-N-2 bond and 1.291 and 0.917 mdyn Angstrom(-1) for the Ta-eta(2)-N-2 bonds, respectively. DFT calculations on 1 suggest that the bridging dinitrogen ligand carries a charge of -1.1, which is equally distributed over the two nitrogen atoms. However, orbital analysis reveals that the terminal nitrogen makes lower contributions to the pi orbitals and much higher contributions to the pi* orbitals of the N-2 ligand than the bridging nitrogen. This suggests that reactions of the dinitrogen ligand with electrophiles should preferentially occur at the terminal N atom, in agreement with experimental results.