Metal-nitroxyl (M-HNO/M-NO-) coordination units are found in denitrification enzymes of the global nitrogen cycle, and free HNO exhibits pharmacological properties related to cardiovascular physiology that are distinct from nitric oxide (NO). To elucidate the properties that control the binding and release of coordinated nitroxyl or its anion at these biological metal sites, we synthesized {CoNO}(8) (1, 2) and {CoNO}(9) (3, 4) complexes that contain diimine-dipyrrolide supporting ligands. Experimental (NMR, IR, MS, EPR, XAS, XRD) and computational data (DFT) support an oxidation state assignment for 3 and 4 of high spin Co-II (S-Co = 3/2) coordinated to (NO-)-N-3 (S-NO = 1) for S-tot = 1/2. As suggested by DFT, upon protonation, a spin transition occurs to generate a putative low spin Co-II-(HNO)-H-1 (S-Co = S-tot = 1/2); the Co-NO bond is similar to 0.2 angstrom longer, more labile, and facilitates the release of HNO. This property was confirmed experimentally through the detection and quantification of N2O (similar to 70% yield), a byproduct of the established HNO self-reaction (2HNO -> N2O + H2O). Additionally, 3 and 4 function as HNO donors in aqueous media at pH 7.4 and react with known HNO targets, such as a water-soluble Mn-III-porphyrin ([Mn-III(TPPS)](3-); TPPS = meso-tetrakis(4-sulfonatophenyl)porphyrinate) and ferric myoglobin (metMb) to quantitatively yield [Mn(TPPS)(NO)](4-) and MbNO, respectively.