Ligand Design for Securing Ferromagnetic Exchange Coupling in Multimetallic Complexes

An approach is suggested for using ligands to control exchange coupling in multinuclear ions. The idea arose from structural, EPR, and magnetic studies of [PPh₄]3 (Scheme 1). Ferromagnetic coupling has been found between the Co^II and each Co^III in 3 with J = −22 ± 5 cm⁻¹ (JS₁ · S₂). It is suggested that dominant antiferromagnetic superexchange is absent because of the strong σ‐donor capacity of the tetradentate ligand [k⁴‐PAC^*]⁴⁻ (Fig. 1). The ligand interacts at Co^III primarily with a single d orbital; it is thus best able to participate in superexchange. The interaction makes the unique d orbital strongly σ‐antibonding and empty for each d⁶, S = 1, Co^III ion in 3, that is, unavailable for antiferromagnetic coupling, but available for ferromagnetic pathways by a Goodenough‐Kanamori mechanism. By corollary, when any [k⁴‐PAC^*]⁴⁻‐type ligand with any magnetic ion M_a in the tetradentate site binds any magnetic ion M_b in the bidentate site, ferromagnetic coupling should be favored provided M_a is not a d⁹ ion.