Density functional methods indicate that the global minimum for Cr-2(NO)(2)(CO)(8) is a staggered D-4d structure in accord with experiment and analogous to the isoelectronic Mn-2(CO)(10). For the unsaturated Cr-2(NO)(2)(CO)(n) derivatives the lowest energy structures are very different from the lowest energy structures for the isoelectronic Mn-2(CO)(n+2) derivatives. Thus the global minimum for Cr-2(NO)(2)(CO)(7) is an unbridged structure with a Cr(NO)(CO)(4) fragment linked to a Cr(NO)(CO)(3) fragment through a CrCr double bond. For Cr-2(NO)(2)(CO)(6) the global minimum is a structure with two bridging CO groups, whereas the global minimum for Mn-2(CO)(8) is an unbridged structure. For Cr-2(NO)(2)(CO)(5) both NO groups are bridging NO groups with one of them having a short enough Cr-O distance to be considered a formal five-electron donor eta(2)-mu-NO group. Thus the isoelectronic substitution of NO for CO with a necessary adjustment in the central metal atom can lead to significant shifts in the relative energies of various structural types of metal carbonyl nitrosyls, particularly for unsaturated molecules. For the mononuclear Cr(NO)(2)(CO)(3) the theoretical structure differs from that deduced from matrix isolation experiments. Moreover, the nu(CO) and nu(NO) vibrational frequencies predicted here for Cr(NO)(2)(CO)(3) correspond more closely with the unassigned species labeled "Cr(NO)(CO)(x)" in the experiments rather than the species claimed to be Cr(NO)(2)(CO)(3).