| Abstract
| - The first two series of Co−NO bond dissociation enthalpies in benzonitrile solution weredetermined for 12 cobalt(II) nitrosyl porphyrins and for 12 cobalt(III) nitrosyl porphyrins by titration calorimetrywith suitable thermodynamic cycles. The results display that the energy scales of the heterolytic CoIII−NObond dissociation, the homolytic CoIII−NO bond dissociation, and the homolytic CoII−NO bond dissociationare 14.7−23.2, 15.1−17.5, and 20.8−24.6 kcal/mol in benzonitrile solution, respectively, which not onlyindicates that the thermodynamic stability of cobalt(II) nitrosyl porphyrins is larger than that of thecorresponding cobalt(III) nitrosyl porphyrins for homolysis in benzonitrile solution but also suggests thatboth cobalt(III) nitrosyl porphyrins and cobalt(II) nitrosyl porphyrins are excellent NO donors, and in addition,cobalt(III) nitrosyl porphyrins are also excellent NO+ contributors. Hammett-type linear free energy analysessuggest that the nitrosyl group carries negative charges of 0.49 ± 0.06 and 0.27 ± 0.04 in T(G)PPCoIINOand in T(G)PPCoIIINO, respectively, which indicates that nitric oxide is an electron-withdrawing group bothin T(G)PPCoIINO and in T(G)PPCoIIINO, behaving in a manner similar to Lewis acids rather than to Lewisbases. The energetic and structural information disclosed in the present work is believed to furnish hintsto the understanding of cobalt nitrosyl porphyrins' biological functions in vivo.
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