The Lewis basicity of nitrido complexes.: Theoretical investigation of the structure and bonding of Cl2 (PH3)3ReN-X (X = BH3, BCl3, BBr3, AlH3, AlCl3, AlBr3, GaH3, CaCl3, GaBr3, O, S, Se, Te)

Quantum chemical calculations using gradient-corrected density functional theory (B3LYP) and ab initio methods at the MP2 level are reported for the geometries and bond energies of the nitrido complexes Cl-2 (PH3)(3)ReN-X(X = BH3, BCl3, BBr3, AlH3, AlCl3, AlBr3, GaH3, GaCl3, GaBr3, O, S, Se, Te). The theoretical geometries are in excellent agreement with experimental values of related complexes which have larger phosphine ligands. The parent nitrido complex Cl-2(PH3)(3)ReN is a very strong Lewis base. The calculated bond dissociation energy of Cl-2(PH3)(3)ReN-AlCl3 is D-e = 43.7 kcal/mol, which is nearly as high as the bond energy of Me3N-AlCl3. The donor-acceptor bonds of the other Cl-2(PH3)(3)ReN-AY(3) complexes are also very strong. Even stronger N-X bonds are predicted for most of the nitrido-chalcogen complexes, which exhibit the trend X = O >> S > Se > Te. Analysis of the electronic structure shows that the parent compound Cl-2(PH3)(3)ReN has a Re-N triple bond. The Re-N sigma bond is clearly polarized towards nitrogen, while the two pi bonds are nearly nonpolar. The Re-N sigma and pi bonds become more polarized toward nitrogen when a Lewis acid or a chalcogen atom is attached. Bonding in AY(3) complexes should be described as Cl-2(PH3)(3)Re equivalent to N-->AY(3), while the chalcogen complexes should be written with double bonds Cl-2(PH3)(3)Re=N=X. The charge-decomposition analysis indicates that the nitrogen-chalcogen bonds of the heavier chalcogen complexes with X = S, Se, Te call also be interpreted as donor-acceptor bonds between the nitrido complex acting as a Lewis base and the chalcogen atom with an empty p(sigma) orbital acting as a Lewis acid. The nitrido oxo complex Cl-2(PH3)(3) Re N=O has a covalent N-O double bond.