The manganese carbonyl nitrosyls Mn(NO)(CO)(4), Mn-2(NO)(2)(CO)(n) (n = 7, 6, 5, 4), and Mn-3(NO)(3)(CO)(9) have been studied by density functional theory (DFT) using the B3LYP and BP86 methods for comparison of their predicted structures with those of isoelectronic iron carbonyl derivatives. DFT predicts a trigonal bipyramidal structure for Mn(NO)(CO)(4) with an equatorial NO group very close to the experimental structure. The predicted lowest energy structure for Mn-2(NO)(2)(CO)(7) has two bridging NO groups in contrast to the known structure of the isoelectronic Fe-2(CO)(9), which has three bridging CO groups. The structures for the unsaturated binuclear Mn-2(NO)(2)(CO)(n) (n = 6, 5, 4) derivatives are similar to those of the corresponding binuclear iron carbonyls Fe-2(CO)(n+2) derivatives but always with a preference of bridging NO groups over bridging CO groups. The trinuclear Mn-3(NO)(3)(CO)(9) is predicted to have a structure analogous to the known structure for Fe-3(CO)(12) but with two bridging NO groups rather than two bridging CO groups across one of the metal-metal edges of the M-3 triangle. The dark red solid photolysis product of Mn(NO)(CO)(4) characterized by its nu(CO) and nu(NO) frequencies similar to 45 years ago is suggested by these DFT studies not to be the originally assumed Mn-2(NO)(2)(CO)(7) analogous to Fe-2(CO)(9). Instead, this photolysis product appears to be Mn-2(NO)(2)(CO)(5) with a Mn equivalent to Mn formal triple bond analogous to (eta 5-C5H5)(2)V-2(CO)(5) obtained from the photolysis of (eta(5)-C5H5)V(CO)(4).