Uranocene bonding

The nature and extent of /-orbital participation in the bonding of uranocene and other bis(cyclooctatetraenyl) actinides has never been satisfactorily established, although a good deal of effort has been expended on it. The X-ray structures do not resolve the issue because an ionically bonded model would also lead to a sandwich-type structure (for example, MgCp2 has essentially the same structure as ferrocene). Other physical techniques have been used, but the complexity of the electronic structures often leads to ambiguous interpretations. 

The electronic structures and bonding ia-,actinide compounds cannot be reliably treated without the inclusion of relativistic effects. A specific example is provided by uranocene, (QHg U,19 but there are many others.20 21... 

Bonding in uranocene has been the subject of controversy for some 30 years. It was early on pointed out that orbital symmetry interactions, involving f orbitals, could be drawn analogous to those in ferrocene (Figure 13.16). 

All of the (C8H8)2An complexes are exceedingly sensitive to oxygen. Metal-ring bonding appears to be more ionic in thoracene than uranocene. (C8H8)2Th is rapidly destroyed by protic reagents, whereas uranocene is hydrolyzed only slowly. 

Uranocene, U( -C8H8) provides an excellent example. The f ionization band, the first of the spectrum, shows two characteristic features, a delayed maximum and a giant resonance. Figure 13(a) shows the cross section of this band. The next two bands are primarily ring C 2p r bands of 62 symmetry. They show the characteristic decay associated with nodeless 2p orbitals (Figure 13(b)). The second band, however, has, superimposed on this decay, a maximum around 40 eV and a double resonance between 95 and 125 eV. This indicates partial 5f character and assigns the orbital to the e2 orbital. The third band is consequently associated with the C2g orbital and its higher IE is indicative of the fact that the U 6d orbitals form more effective bonds that the 5f in this instance. 

Nevertheless, the existence of biscyclooctatetraenyl metal complexes, bonded via the metal f-orbitals and the aromatic 7r-systems of the rings, was first predicted on the basis of Wolfsberg-Helmholz MO calculations by Fischer (7) in 1963, who treated the then hypothetical U(Cot)2 molecule, and demonstrated experimentally by Streit-wieser and Miiller-Westerhoff (8) in 1968. These authors succeeded in synthesising the uranium derivative, U(Cot)2, and gave to it the trivial name, uranocene, in order to underline its resemblance to the d-series metallocenes both structurally and, as they proposed, in its mode of metal-ring bonding. 

The IR and Raman spectra of (1) show characteristic bands of terminal and 7i-bonded, bridging aryl groups, both in the solid state and in solution.4 The Raman spectrum of uranocene, U(COT)2, at 77 K, gave ligand mode assignments, e.g. vCH at 3042 cm-1 and vCC at 1500 cm-1.5... 

The visible spectrum of 4 (Table 3) contains a strong band at 396 nm and tails off to longer wavelength with a series of shoulders much like a uranocene. The infrared spectrum of the compound is nearly identical to that of the thorium analog 2a and is consistent with tridentate bonding. We are, however, unable to reproduce the nmr spectrum reported by Marquet-Ellis. 

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