Theoretical and spectroscopic investigations of the bonding and reactivity of (RO)₃M≡N molecules, where M = Cr, Mo, and W

The electronic structures of the molecules (^tBuO) ₃M≡N (M = Cr, Mo, W) have been investigated with gas phase photoelectron spectroscopy and density functional calculations. It is found that the alkoxide orbitais mix strongly with the M≡N triple bond orbitais and contribute substantially to the valence electronic structure. The first ionization of (^tBuO)₃Cr≡N is from an orbital of a₂(C_3v) symmetry that is oxygen based and contains no metal or nitrogen character by symmetry. In contrast, the first ionizations of the molybdenum and tungsten analogues are from orbitais of a₁ and e symmetry that derive from the highest occupied M≡N σ and π orbitais mixed with the appropriate symmetry combinations of the oxygen p orbitais. In this a₁ orbital, the oxygen p orbitais mix with the highest occupied M≡N orbital of σ symmetry. This mixing reduces the metal character, consequently reducing the metal-nitrogen overlap interaction in this orbital. From computational modeling, the polarity of the M≡N bond increases down the group such that W≡N has the highest charge separation. In addition to investigation of the effects of the metals, the electronic influences of substitution at the alkoxide ligands have been examined for the molecules (RO)₃Mo≡N (R = C(CH₃)₂H, C(CH₃)₃, and C(CH₃)₂CF₃). The introduction of CF₃ groups stabilizes the molecular orbital energies and increases the measured ionization energies, but does not alter the overall electronic structure. The bonding characteristics of the ( ^tBuO)₃M≡N series are compared with those of organic nitriles.