DFT calculations employing the OLYP and B3LYP functionals have been used to map out the low energy states of the metalloporphyrin-nitroxyl adducts "M(Por) + NO-" and "M(Por) + HNO", where M = Fe, Co, and Mn and Pore-is the dianion of unsubstituted porphyrin. For [Fe(Por)(NO)](-), the calculations yield two low-energy solutions, with M-s = 0 and 1. The Ms = 0 solution is thought to represent the experimentally observed diamagnetic ground states of {FeNO}(8) porphyrins, and both functionals yield FeNO geometrical parameters in excellent agreement with a recent crystal structure. For [Co(Por)(NO)](-), the lowest-energy solution for both OLYP and B3LYP is a true {CoNO}(9) state that appears to be best described as a high-spin Co(II) center with a d(xy)(2)d(xz)(2)d(yz)(1)d(z2)(2)d(x2-y2)(1) configuration antiferromagnetically coupled to a NO- diradical. Such an electronic configuration is expected to lead to diagnostic structural features, including long equatorial Co-N distances (similar to 2.1 angstrom), a strong displacement (similar to 0.4 angstrom) of the metal from the mean plane of the equatorial nitrogens, and a relatively short Co-N(O) distance (1.8 angstrom), which should all be experimentally observable. The d(x2-y2)(1) electronic configuration should also lead to characteristic EPR hyperfine parameters. The calculations also indicate a number of other low-energy states for [Co (Por)(NO)](-), including multiple {CoNO}(8) porphyrin anion radical states. For [Mn(Por)(NO)](-), both functionals indicate a rather complex electronic state landscape, including multiple {MnNO}6 porphyrin anion radical states as well as a high-spin S = 3/2 {MnNO}(7) state. Both functionals clearly indicate a low-spin Pe(Il) state for [Fe(Por)(HNO)]. On the other hand, two comparably low-energy states are predicted for both [Co(Por)(HNO)] and [Mn(Por)(HNO)]. For [Co(Por)(HNO)], the two states are a low-spin Co(II) state with a d(xy)(2)d(xz)(2)d(y)(2)d(z2)(1) configuration and a low-spin Co(III)(HNO)(center dot-) state. For [Mn(Por)(HNO)], the two states may be described as low-(S = 1/2) and intermediate spin (S = 3/2) Mn(II). The latter state has a relatively long Mn-N(O) distance of about 2.07 angstrom, which may be indicative of facile HNO dissociation.