This is a quantum chemical survey of a broad class of high-valent transition metal intermediates relevant to the catalytic cycles of the cytochromes P-450 and related heme proteins, viz. iron(IV) and manganese(IV) porphyrins with two univalent pi-bonding axial ligands. The axial ligands in these intermediates invariably exhibit a bent geometry, reflecting a competitive interplay of sigma- and pi-bonding effects. Compared to an oxo ligand, the univalent ligands are distinctly less able to delocalize electronic spin density away from the metal center, which might have significant implications for the chemical reactivity of these species.