The Lewis basicity of nitrido complexes. Theoretical investigation of the structure and bonding of Cl 2 (PH 3 ) 3 ReN-X (X = BH 3 , BCl 3 , BBr 3 , AlH 3 , AlCl 3 , AlBr 3 , GaH 3 , GaCl 3 , GaBr 3 , O, S, Se, Te)