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

摘要

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 MnII 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 μB at 300 K, considerably lower than the limiting value of 8.37 μB for two uncoupled S = 5/2 Mn II centres.