Exchange coupling through diamagnetic [Fe(CO)4]2− bridging ligands in a xenophilic cluster

Abstract

The electronic structure of so-called ‘xenophilic’ clusters, which contain both organometallic fragments and Werner-type paramagnetic transition metal centres, presents a challenge to simple theories of bonding. Density functional theory shows clearly that the cluster Mn₂(thf)₄(Fe(CO)₄)₂ is best described as an exchange-coupled Mn^II₂ dimer, the closed-shell organometallic [Fe(CO)₄]²⁻ fragments acting simply as bridging ligands. The high-spin configuration of the Mn^II ions leads to single occupation of the Mn–Fe σ* orbitals and therefore substantially weaker metal–metal bonding than in conventional low-valent organometallic clusters. The transition metal fragments are effective mediators of superexchange (J_calc = −44 cm⁻¹), leading to the measured effective magnetic moment of ∼5 μ_B at 300 K, considerably lower than the limiting value of 8.37 μ_B for two uncoupled S = 5/2 Mn^II centres.