Effect of Hybridization on Structure and Bonding of Cluster Compounds Possessing a Square-Pyramidal Fe₃(CO)₉E₂ Core (E = Element of Group 15 or 16)

The pyrolysis of [NEt4]2[Bi4Fe4(CO)13] in MeCN cleanly produces the square-pyramidal cluster [NEt4]2[Fe3(CO)9Bi2]. An X-ray crystallographic study was carried out on this compound at 223 K (orthorhombic space group P212121 (No. 19) with a = 7.280(1) Å, b = 20.642(4) Å, c = 22.365(4) Å, V = 3360.9(10) Å3, and Z = 4) showing it to belong to the square-pyramidal class of 50 electron E2M3 clusters. When the E atom bears a substituent, the E···E distances are shorter, and the Mbasal−E−Mbasal angles are larger than when a lone pair of electrons resides on E. It has become fashionable to attribute such changes in bond parameters to the formation of E−E bonding, but rehybridization of the main group element could also explain this phenomenon. In order to shed light on this fundamental aspect of bonding in E−M clusters, a detailed extended Hückel molecular orbital analysis was undertaken. It appears that weak E−E bonding may be present in some cases but that this is not the dominant effect.