Coordination of N2 ligands to lanthanum: the complexes La (N2)1-8

The structures and bonding properties of La(N-2)(x) (x=1-8) complexes were investigated by density functional theory (DFT) computations using the B3LYP exchange-correlation functional in conjunction with a quasi-relativistic pseudopotential for La. The quality of the DFT electronic structures was confirmed in selected cases by relativistic multireference calculations using CASPT2 theory. From the end-on and side-on dinitrogen coordination modes in general, the structures with end-on coordination were found to be the more stable. The first coordination sphere of the complexes is filled by eight and six N-2 ligands in the end-on and side-on type species, respectively. The main bonding interaction is the donation of La 5d valence electrons to anti-bonding orbitals of N-2 resulting in characteristic elongation of the NN bonds. These directional interactions determine the (from steric point of view in several cases less logic) equilibrium molecular structures. The charge transfer resulted in partial charges up to 1.5 e of the originally neutral components (La, N-2) leading to electrostatic attractive interactions which compose the minor contribution in the bonding.