METAL-METAL BONDING IN FE2(CO)9 AND THE DOUBLE-BONDS FE(CO)4 = FE2(CO)8 AND (MU-CO) = FE2(CO)8 IN FE3(CO)12 AND FE2(CO)9 - SIMILARITIES AND DIFFERENCES IN THE ORGANIC/INORGANIC ISOLOBAL ANALOGS X = Y (X, Y ARE CH2, FE(CO)4, FE2(CO)8, C2H4, CO)

Fe3(CO)12 may be considered to consist of the fragments Fe(CO)4 and the bridged isomer 3 of Fe2(CO)8. The bonds between these two fragments are shown to be analogous to those between Fe(CO)4 and C2H4 in Fe(CO)4(C2H4) (3 is isolobal to ethylene) or alternatively to those between CO and Fe2(CO)8 in Fe2(CO)9. Using a symmetry analysis of the bonding which affords quantitative estimates of sigma and pi bond strengths as well as Pauli repulsion and electrostatic interaction between fragments, this analogy is analyzed quantitatively. The same analysis is applied to organic and mixed organic/inorganic systems which are isolobal analogues in the sense that they have a double bond (sigma and pi) formed by sigma and pi frontier orbitals of the fragments. The difference between organic and organometallic bonds is highlighted. It is stressed that, apart from the well-known frontier orbital interactions, the Pauli repulsion (or four-(three-)electron destabilizing interaction) between valence electrons on one fragment and subvalence or semi-core electrons on the other fragment is an important electronic structure factor in the molecular orbital theory of organic and inorganic chemistry. The Pauli repulsion brings atomic shell structure effects into play, such as, in the first row transition-metals, the similar spatial extent of the semi-core 3s, 3p shell and the valence 3d. Repulsion with the closed 3s, 3p shells causes bonds with inorganic fragments to stay long and weak compared to their organic analogues.