INDO SCF MO calculations have been carried out for the neutral metallocenes. MCp 2 (M = TiCu) and bis-benzene complexes, MBz 2 (M = ScCo) of the 3d series, and for certain mixed sandwich, CpMBz and CpMCh (Ch = C 7H 7), species. The results indicate that in all of there systems interactions between the metal orbitals and the π-orbitals of the ligand rings make the dominant contribution to the metal-ligand bonding, the most strongly involved levels being the metal 3d xz, 3d yz (e 1) 3d x 2- y 2, 3d xy (e 2), and 4p x, 4p y (e 1) orbitals. In addition signifcant, but generally smaller, interactions also take place between the metal 3d, 4s, and 4p orbitals and the carbon 2s and 2p x, 2p y orbitals of the σ-frameworks of the ringr Of the metal 3d orbitals the d z 2 (e 2) level it in all cases only very weakly concerned in the bonding, but the d xz, d yz (e 1) and d x 2- y 2) d xy (e 2 orbitals both interact strongly with the ligand π-orbitals. Moreover, calculations of the appropriate bond orders and overlap densities show the e 1 bonding to decrease and the e 2 interaction to increase systematically with increasing size of the ligand rings, predicted qualitatively by simple molecular orbital arguments. Thus in the MCp 2 series the bonding occurs almost entirely via the e 1 interaction, with a much smaller e 2 involvement, whilst for the MBz 2 systems the e 2 bonding is dominant and appreciably greater than the e 1 contribution. For the CpMBz systems the bonding of the Cp and Bz ligands similarly takes place primarily via the e 1 and e 2 orbitals respectively, whilst for the CpMCh complexes the Cp and Ch ligands again bond essentially via the e 1 and e 2 levels respectively, but the metal 3d-−e 2h interaction is here even stronger than the e 2 bonding in the MBz 2 species. Consequently the splitting patterns of the metal 3d levels in all these compounds, and hence many of the spectroscopic and magnetic properties, are determined almost exclusively by the dependence of the e 1 and e 2 interactions on the size of the ligand rings In the MCp 2 series the bond orders for both the e 1 and e 2 interactions show a maximum at ferrocene. in accordance with the known exceptional stability of this system, but for the MBz 2 compounds no clear maximum occurs, thus reflecting the experimental absence of any especially stable bis-arene derivative. Finally, the very low bond orders predicted for cuprocene correctly mirror the failure of attempts to obtain that system, whereas for CoBz 2 the calculated e 2 bond order is suffciently great to offset the low e 1 value due to the high population of that anti-bonding metal level, thus according with the experimental accessibility of a bis-arene cobalt(O) compound. ▪