Effects of the bridging ligands on the molecular and electronic structure of Fe2(CO)9 derivatives

This article reviews the structural and electronic properties of Fe2(CO)9 derivatives with any triad of sigma donor and pi acceptor bridges selected from the series CO, CH2, CF , SiMe2, GeMe2, InMe and CS. Based on the available structural data and computational results, the bridges other than CO can be catalogued in two different groups. The ligands of group I feature a pivotal carbon atom such as the CO ligand itself (i.e. L = CH2, CF2 and CS). Those of group II are characterized by different hetero-atoms at the core of the bridging group (i.e. L = SiMe2, GeMe2 and InMe). The geometry of the inner Fe(m-L)3Fe skeleton changes significantly between the members of the different series. In particular, the range of Fe-Fe distances can be as large as 52.3 pm, the higher and lower limits being found in the system with three InMe bridges and in that with one CO and two CF2 ligands, respectively. It turns out that ligands of group I favor the decrease of the Fe-Fe distance compared to the parent compound Fe2(CO)9. On the contrary, those of group II favor a larger intermetallic separation. By detailed analyses, the structural effects can be traced back to the s donor and p acceptor capabilities of the different bridges.