Using density functional theory-based geometry optimizations, we have searched for eta (1)-NO, eta (1)-ON (isonitrosyl), and eta (2)-NO (side-on bound NO) linkage isomers of a number of metalloporphyrin-NO complexes, M(Por)(NO)(L), where For = porphinato dianion, M = Mn(II), Fe(II), Fe(III), Ru(II), Ru(III), Co(II), and Rh(II), and L = no ligand, SMe, Ph, and imidazole. The eta (1)-NO isomer had the lowest energy in all cases. and the isonitrosyl isomer was also located as a higher energy potential energy minimum in a number of cases. The eta (2)-NO isomer was only located as a minimum for Mn(II) (L = no ligand), Fe(III) (L = no ligand), and Ru(III)(L = Ph. imidazole, pyrdine), suggesting that an {MNO}(6) electron count is important for stabilization of the eta (2) mode of ligation. However, in the presence of axial ligands L, the side-on isomers of {FeNO}(6) complexes were not stable and opened up to an unusual geometry where the FeN(O) and NO vectors were tilted in opposite directions relative to the heme normal. Exactly such a geometry, as well as a "normal" upright geometry, has been observed in a recent crystallographic determination of nitrophorin 4 (Nature Struct. Biol. 2000, 7, 551), a salivary protein from the blood-sucking insect Rhodnius prolixus. Together, the calculated and experimental result illustrate the extreme softness of the FeNO potential energy surface toward various forms of tilting and bending deformations.