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

The structures and vibrational properties of trans-[V(N2)2(PH3)4]−, trans-[Cr(N2)2(PH3)4], [Mn(H)(N2)(PH3)4], [Fe(N2)(PH3)4], [Fe(H)(N2)(PH3)4]+ and [FeCl(N2)(PH3)4]+ have been computed using density functional theory. Good reproduction of metal ligand bond lengths and the trend in N–N stretching frequencies ν(N–N) is obtained showing that simple PH3 is a good model for the more complicated phosphine ligands employed experimentally. Analysis of the theoretical MN binding energies shows a good correlation between increasing bond strength and decreasing ν(N–N). trans-[V(N2)2(PH3)4]− has the lowest value of ν(N–N) (∼1740 cm−1) and the largest calculated MN2 bond energy (223 kJ mol−1) while [Fe(H)(N2)(PH3)4]+ has the highest value of ν(N–N) (∼2100 cm−1) and the lowest computed M–N2 bond energy (126 kJ mol−1). The biggest discrepancy between theory and experiment is for trans-[V(N2)2(PH3)4]−. The error is removed by explicitly modelling solvation effects and the ion-pair interactions with alkali metals which are vital for stabilising dinitrogenvanadates(−1). The strong V–N2 bond is apparently at odds with the reported lability of dinitrogenvanadate(−1) complexes. However, this assumes that the lability is reversible. The modelling suggests that N2 loss is accompanied by decomposition.