Inverted Ligand Field in a Pentanuclear Bow Tie Au/Fe Carbonyl Cluster

Gold chemistry has experienced in the last decades exponential attention for a wide spectrum of chemical applications, but the +3 oxidation state, traditionally assigned to gold, remains somewhat questionable. Herein, we present a detailed analysis of the electronic structure of the pentanuclear bow tie Au/Fe carbonyl cluster [Au{η-Fe(CO)}] together with its two one-electron reversible reductions. A new interpretation of the bonding pattern is provided with the help of inverted ligand field theory. The classical view of a central gold(III) interacting with two [Fe(CO)] units is replaced by Au(I), with a d gold configuration, with two interacting [Fe(CO)] fragments. A d configuration for the gold center in the compound [Au{η-Fe(CO)}] is confirmed by the LUMO orbital composition, which is mainly localized on the iron carbonyl fragments rather than on a d gold orbital, as expected for a d configuration. Upon one-electron stepwise reduction, the spectroelectrochemical measurements show a progressive red shift in the carbonyl stretching, in agreement with the increased population of the LUMO centered on the iron units. Such a trend is also confirmed by the X-ray structure of the direduced compound [Au{η-Fe(CO)}{η-Fe(CO)(μ-CO)}], featuring the cleavage of one Au-Fe bond.