The electronic and geometric structures of the title species have been studied computationally using quasi-relativistic gradient-corrected density functional theory. The valence molecular orbital ordering of UO22+ is found to be pi (g) < (u) < (g) much less than sigma (u) (highest occupied orbital), in agreement with previous experimental conclusions. The significant energy gap between the sigma (g) and sigma (u) orbitals is traced to the "pushing from below" mechanism: a filled-filled interaction between the semi-core uranium 6p atomic orbitals and the sigma (u) valence level. The U-N bonding in UON+ and UN2 is Significantly more covalent than the U-O bonding in UON+ and UO22+. UO(NPH3)(3+) and U(NPH3)(2)(4+) are similar to UO22+, UON+ and UN2 in having two valence molecular orbitals of metal-ligand sigma character and two of pi character, although they have additional orbitals not present in the triatomic systems, and the U-N sigma levels are more stable than the U-N pi orbitals. The inversion of U-N sigma/pi orbital ordering is traced to significant N-P (and P-H) a character in the U-N sigma levels. The pushing from below mechanism is found to destabilize the U-N f(sigma) molecular orbital with respect to the U-N d(sigma) level in U(NPH3)(2)(4+). The uranium f atomic orbitals play a greater role in metal-ligand bonding in UO22+, UN2, and U(NPH3)(2)(4+) than do the d atomic orbitals, although, while the relative roles of the uranium d and f atomic orbitals are similar in UO22+ and U(NPH3)(2)(4+), the metal d atomic orbitals have a more important role in the bonding in UN2. The preferred UNP angle In [UCl4{NPR3}(2)] (R = H, Me) and [UOCl4{NP(C6H5)(3)}](-) is found to be close to 180 degrees in all cases. This preference for linearity decreases in the order R = Ph > R = Me > R = H and is traced to steric effects which in all cast:s overcome an electronic preference for bending at the nitrogen atom. Comparison of the present iminato (UNPR3) calculations with previous extended Huckel work on d block imido (MNR) systems reveals that in all cases there is little or no preference for linearity over bending at the nitrogen when R is (a) only a-bound to the nitrogen and (b) sterically unhindered. The U/N bond order in iminato complexes is best described as 3.