Two noninnocent ligands are theoretically studied using wave function based methods to demonstrate their ability to undergo singlet−triplet transition under the effect of an external charge mimicking the electrostatic role of a metal ion. It is shown that the singlet−triplet energy difference is very sensitive to the metal ion charge which tunes the HOMO−LUMO energy difference of these ligands. While the latter is reduced as the charge is enhanced in the glyoxal-bis-(2-mercaptoanil) (gma) ligand, it is increased in the bis(imino)pyridine diradical ligand. This result shows a strong analogy with the crystal field theory, interchanging the roles played by the metal ion and the ligand. As the metal ion is explicitly treated in the Fe(gma)CN complex, this analogy can be pushed further resulting in a "metal field theory" conceptualization.
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