The great majority of {NiNO}(10) complexes are characterized by short Ni-N(O) distances of 1.60-1.65 angstrom and linear NO units. Against this backdrop, the {CuNO}(10) unit in the recently reported [Cu(CH3NO2)(5)(NO)](2+) cation (1) has a CuNO angle of about 120 degrees and a very long 1.96 angstrom Cu-N(O) bond. According to DFT calculations, metal-NO bonding in 1 consists of a single Cu(d(z)(2))-NO(pi*) sigma-interaction and essentially no metal(d(pi))-NO(pi*) pi-bonding, which explains both the bent CuNO geometry and the long, weak Cu-N(O) bond. This sigma-interaction is strongly favored by a ligand trans to the NO; indeed such a trans ligand may be critical for the existence and stability of a {CuNO}(10) unit. By contrast, {NiNO}(10) complexes exhibit a strong avoidance of such trans ligands. Thus, a five-coordinate {NiNO}(10) complex appears to favor a trigonal-bipyramidal structure with the NO in an equatorial position, as in the case of [Ni(bipy)(2)(NO)](+) (6). An unusual set of Ni(d)-NO(pi*) orbital interactions accounts for the strongly bent NiNO geometry for this complex.