The pyrolysis of [NEt(4)](2)[Bi4Fe4(CO)(13)] in MeCN cleanly produces the square-pyramidal cluster [NEt(4)](2)[Fe-3(CO)(9)-Bi-2] An X-ray crystallographic study was carried out on this compound at 223 K (orthorhombic space group P2(1)2(1)2(1) (NO. 19) With a=7.280(1) Angstrom, b=20.642(4) Angstrom, c=22.365(4) Angstrom, V=3360.9(10) Angstrom(3), and Z=4) showing it to belong to the square-pyramidal class of 50 electron E(2)M(3) clusters. When the E atom bears a substituent, the E ... E distances are shorter, and the M(basal)-E-M(basal) 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 Huckel 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.