The nature of the M≡E bond: Theoretical investigation of the molecules [(RO)₃M≡E] (M = Mo, W; E = N, P, As, Sb, Bi; R = H, Me) and [(Me₃CO)₃Mo≡P]

The electronic and molecular structures of the metal-pnicogen complexes [(RO)₃M≡E] (M = Mo, W; E = N, P, As, Sb, Bi; R = H, Me) and [(Me₃CO)₃M≡P] have been investigated at the DFT level using the exchange correlation functionals B3LYP and BP86. The nature of the M≡E interactions was analyzed with charge and energy decomposition methods. The bonding analyses show that the metal-pnicogen bonds are genuine triple bonds containing a σ- and a degenerate π-bond. The M-E σ-bonds are always polarized towards the pnicogen atom E, particularly for the nitrogen complexes. The M-N π-bonds are also polarized towards nitrogen, while the π-bonds of the heavier pnicogens P-Bi show a small polarization towards the metal atom. The M-E σ-bonds at the pnicogen E are sp ⁿ-hybridized; the p character is always > 80% of the total AO contribution. The hybridization of the metal atoms in the nitrido complexes is approximately sd² and that in the heavier homologues, approximately sd. The ratio of the energy contributions of the σ- and π-orbitals to the covalent bonding in [(HO)₃M≡E] is about 50:50 for all pnicogen atoms E. The metal-nitrido bonds in [(HO)₃M≡N] are more covalent than electrostatic. The covalent character becomes smaller when the pnicogen atom becomes heavier with the trend N ≫P > As > Sb > Bi. The [(HO)₃M≡E] tungsten complexes have stronger bonds that are slightly less covalent than the molybdenum complexes, the π-bonding contribution in the former species is slightly larger than in the latter.