Molecular orbital calculations are applied to the study of the structure and bonding of nitrosyl metalloporphyrin complexes. The Fenske-Hall approximate molecular orbital method and the Amsterdam Density Functional (ADF) method are used. The calculations provide qualitative and quantitative explanations for the observed structural differences between the nitrosyl porphyrin complexes ofiron, manganese, and cobalt. It is proposed that the energy of the highest occupied molecular orbital (HOMO) of these complexes is primarily responsible for the observed structural differences. The interaction between the nitrosyl ligand and metal dz2 orbital results in an antibonding orbital that is occupied in the complexes of Fe and Co, but unoccupied in the Mn complex. Bending of the metal-N-O linkage in complexes of Fe and Co results in stabilization of this orbital and consequently a more stable configuration for the complex. In addition, the binding affinity of these complexes for a sixth ligand is influenced by the energy and occupation of this largely metal-based orbital. The conclusions drawn from these calculations may provide evidence for the mechanism of activation of the enzyme soluble guanylyl cyclase, which is activated upon binding NO at metal porphyrin site.

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