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A qualitative treatment of the bonding in Cp3M (Cp = í/-C5H5) compounds under C3" symmetry reveals that the Cp33" ligand field contains a high-lying a2 orbital which is restricted by symmetry to interact only with metals that contain f orbitals. Quantitative investigation of the electronic structure of "5f3" Cp3U via -SW molecular orbital calculations with quasi-relativistic corrections reveals that the Cp ligands donate electron density primarily into the U 6d orbitals while the three principally metal-based valence electrons are housed in the 5f orbitals. Electronic structure calculations of C13U show that although Cl can be considered isolobal with Cp, it is a poorer donor ligand. Calculations of Cp3U bonded to a fourth ligand L (L = H, CO, NO, OH) indicate that the -bonding framework is essentially the same for -neutral (H), -acidic (CO, NO), or -basic (OH) ligands: Electron density is donated from the orbital of the fourth ligand into a uranium orbital that is primarily 6dz2 in character with minor contributions from the 5fz3 orbital, the 7pz orbital, and the 7s orbital. In the -bonding framework, the U 5f orbitals are responsible for back-donation into the * orbitals of CO and NO, while acceptance of electron density from the orbitals of OH involves the U 6d orbitals and, to a lesser extent, the U 5f orbitals. The bonding scheme of Cp3UNO suggests that this molecule may prove to be a rather unusual example of a linear NO" ligand.Organoactinide chemistry has received considerable attention in the last two decades.3 As was the case for organotransition-(1) (a) Part 1:

Shortly after the discovery of ferrocene, Reynolds and Wilkinson demonstrated that actinide elements are capable of forming organometallic compounds containing the cyclopentadienyl (Cp) ligand. The compound they reported, [Cp,UCI], was the first of a host of organoactinide complexes that contain Cp and substituted Cp ligands. In 1968, Streitwieser and Miiller-Westerhoffsynthesized uranocene, [(C,H,),U], a molecule whose existence had been predicted five years earlier by R. D . Fischer. Because uranocene has no analogue in transition-metal chemistry, its synthesis stimulated new interest in organoactinide chemistry and in comparisons with organotransition-metal chemistry. Since that time, Cp and substituted Cp ligands, such as pentamethylcyclopentadienyl (Cp *), have become the most important and flexible ligands in actinide organometallic chemistry. In the past 15 years especially, the bonding of Cp ligands to actinide elements has been extensively probed by both experiment and theory. There are both significant similarities and equally significant differences in the bonding of the Cp ligand to the f-element metals compared to the d-element metals.

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