Quantum Chemical Calculations Predict the Diphenyl Diuranium Compound [PhUUPh] To Have a Stable 1Ag Ground StateG.L.M. and L.G. thank the Swiss National Foundation (grant no. 200021-111645/1). The authors thank P. P. Power and B. O. Roos for stimulating discussions

Use of sterically hindered ligands to maintain the integrity of normally unstable chemical entities is a widespread technique in modern synthetic chemistry. Bulky aryl and alkyl substituents have been designed to stabilize sensitive compounds, such as main-group-element dimers with multiple bonds, including the alkyne analogues of heavier Group 14 elements. 3] This methodology was recently employed in the generation of a new stable low-valent chromium dimer that, according to experimental and theoretical evidence, exhibited a Cr Cr quintuple bond. 5] The synthesis and characterization of this crystalline compound, [Ar’CrCrAr’] (Ar’= C6H3-2,6(C6H3-2,6-iPr2)2), has renewed interest in metalmetal multiple bonding. A subsequent computational study on a simplified model for [Ar’CrCrAr’] , [PhCrCrPh], predicted a fivefold bonding picture with filled bonding orbitals (spd) as the predominant electronic configuration in the singlet ground state. In contrast to the multiple bonding between early transition metals, like, for example, the quadruple bond in K2[Re2Cl8]·2H2O, [6] only rare examples of direct M M interactions are known in actinide chemistry. The only known bonds of such type occur in the hydride [H2UUH2] [7] or in the U2 species, experimentally trapped in argon matrices, which we have previously described through high-level calculations. In the U2 dimer, [9] two uranium atoms bind to form a quintuple bond, thus suggesting that this U2 unit could form the framework for the development of more diverse diuranium chemistry. The U2 2+ cation was also found to be metastable, exhibiting a large number of low-lying electronic states with a short bond length of about 2.30 9, compared to 2.43 9 in the neutral U2 molecule. The natural tendency of a uranium atom to be preferentially complexed by a ligand, rather than to explicitly form a direct U U bond, has to date precluded the isolation of stable uranium species exhibiting direct metal–metal bonding. From the experimental point of view, the synthesis of multiply bonded uranium compounds poses many challenges. Although the uranium ionic radius is not exceedingly large, the presence of many electrons combined with the preference for certain coordination modes with common ligands make the task of stabilizing the hypothetical U U bond difficult. With the relatively low first ionization energies of 584 (M! M) and 1420 kJmol 1 (M!M) and the ground-state electron configuration corresponding to [Rn]5f6d7s, uranium seems nevertheless to be a promising candidate to form multiply bonded species in actinide chemistry. Despite the fact that this most common actinide exhibits a large range of oxidation states (the most common is U, but less common oxidation states such as U, U, U, and U are also known), monovalent uranium ions have not yet been formally identified. Light atoms that are present in various organic ligands are known to bind tightly to uranium ions. Furthermore, hexaor tetrafluoro complexes with U and U are easily formed, and tetravalent uranium is also stable in hydroxides, hydrated fluorides, and phosphates. Hexavalent uranium is the most stable oxidation state, and the most commonly occurring uranium oxide is U3O8. Experimental studies taking advantage of the complex behavior of the f orbitals of uranium result in new variations of the already known moieties described above. For example, recent reports of uranium rings containing bimetallic nitride linkages or of [*] G. La Macchia, Prof. L. Gagliardi D partment de chimie physique Universit de Gen*ve 30, Quai Ernest Ansermet, 1211 Gen*ve (Switzerland) Fax: (+41)22-379-6518 E-mail: [email protected]