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 N2: Correlations to Electronic Structure and Reactivity

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Abstract
The vibrational properties and the electronic structure of the side-on end-on N2-bridged Ta complex ([NPN]Ta(μ-H))2(μ-η1:η2-N2) (1) (where [NPN] = (PhNSiMe2CH2)2PPh) are analyzed. Vibrational characterization of the Ta2(μ-N2)(μ-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 15N2 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 Å-1. The Ta−N force constants are calculated to be 2.517 mdyn Å-1 for the Ta−η1-N2 bond and 1.291 and 0.917 mdyn Å-1 for the Ta−η2-N2 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 π orbitals and much higher contributions to the π* orbitals of the N2 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.

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