The crystal and molecular structures of the complexes [Ni(mstien),(NO2)(H2O)] ClO4 and [Ni(mstien)2(O2N)] Cl are reported (mstien = meso-stilbenediamine, 1, 2-diphenylethane-l, 2-diamine). Both compounds contain nickel(II) in an essentially octahedral ligand environment. In the latter complex, the nitrite ion chelates, while in the former it is present as a nitro group situated trans to a coordinated water molecule. In both complexes the conformations observed for the amine chelate rings can be rationalized on the basis that they minimize intramolecular ligand repulsions. The complex [Ni(mstien)2(O2N)] NO2 is isomorphous with the analogous chloride compound, and spectral evidence suggests that the complex [Ni(mstien)2(O2N)] ClO4 also contains a chelating nitrite group. The electronic spectra of single crystals of [Ni(mstien),(O2N)] X (X = Cl- and NO2-) show bands centred at c. 25000 cm-1 which exhibit VIbrational fine structure. The energies of the progressional spacings are c. 600 cm-1, which suggests that the bands are probably due to n → π* internal nitrite transitions. The band centred at c. 20500 cm-1 in [Ni(mstien)2(NO2)(H2O)] ClO4 also shows well resolved VIbrational fine structure with a progressional spacing of c. 630 cm-1. This implies that the band is not due to a ‘d-d’ transition, but must be caused by the excitation of an electron into the nitrite π* orbital, either from a non-bonding nitrite orbital, or more probably from the nickel ion.