A density functional study of the structures, vibrations and bond energiesof dinitrogen phosphine complexes of the first transition series

The structures and vibrational properties of trans-[V(N-2)(2)(PH3)(4)](-), trans-[Cr(N-2)(2)(PH3)(4)], [Mn(H)(N-2)(PH3)(4)], [Fe(N-2)(PH3)(4)] [Fe(H)( N-2)(PH3)(4)](+) and [FeCl(N-2)(PH3)(4)](+) have been computed using densit y functional theory. Good reproduction of metal ligand bond lengths and the trend in N-N stretching frequencies ,(N-N) is obtained showing that simple PH, is a good model for the more complicated phosphine ligands employed ex perimentally. Analysis of the theoretical M-N binding energies shows a good correlation between increasing bond strength and decreasing v(N-N). trans- [V(N-2)(2)(PH3)(4)](-) has the lowest value of v(N-N) (similar to 1740 cm(- 1)) and the largest calculated M-N-2 bond energy (223 kJ mol (1)) while [Fe (H)(N-2)(PH3)(4)](+) has the highest value of v(N-N) (similar to 2100 cm(-1 )) and the lowest computed M-N, bond energy (126 U mol(-1)). The biggest di screpancy between theory and experiment is for trans -[V(N-2)(2)(PH3)(4)](- ). The error is removed by explicitly modelling solvation effects and the i on-pair interactions with alkali metals which are vital for stabilising din itrogenvanadates(- 1). The strong V-N-2 bond is apparently at odds with the reported lability of dinitrogenvanadate(- 1) complexes. However, this assu mes that the lability is reversible. The modelling suggests that N-2 loss i s accompanied by decomposition. (C) 2001 Elsevier Science B.V. All rights r eserved.