Theoretical versus experimental charge and spin-density distributions in trans-[Ni(NH3)4(NO2)2]

Abstract

The experimental charge and spin-density distributions for [Ni(NH₃)₄(NO₂)₂] have been compared with theory at various levels. Ab initio unrestricted Hartree–Fock (UHF) and discrete variational X_α(DVX_α) Hartree–Fock–Slater molecular orbital (m.o.) calculations are reported together with cellular ligand field (c.l.f.) results. The UHF and DVX_α approaches yield closely similar descriptions of the charge and spin densities, and qualitatively reproduce the main features of both types of experimental data, namely the Ni–N covalence is strong, the NO₂^– ion is a better σ donor than the NH₃ molecule, and the Ni–N π-bonding is small. Both theories indicate quite appreciable O(NO₂) participation in the bonding and antibonding m.o.s involving nickel. C.l.f. calculations which include only the Ni–N interactions reproduce the experimental d–d spectra and the signs of the single-crystal paramagnetic anisotropies quite well, but assign a weaker σ-donor role to the nitrite ligand relative to NH₃. An extension of the model to include explicit Ni–O interactions is more satisfactory and places the NO₂^– ion as the stronger σ donor consistent with the other theoretical and experimental data.