Uranocene structure

Many substituted uranocenes have been made and there is a substantial body of organometallic chemistry of uranocene derivatives now known 16, 17). Some of this chemistry will be mentioned in passing but wiU not be covered in a systematic way since other reviews of the organic chemistry are available 18). The only other actinide cyclooctatetraene complex structurally characterized to date is bis[(l,3,5,7-tetramethylcyclooctatetraenyl]uranium(IV) 19), which was of interest because the presence of methyl groups allowed the planarity and relative orientation of the dianion rings to be determined. Crystal and molecular parameters for these three actinide compounds are summarized in Table 1. 

It is interesting to compare these actinide(IV) cyclooctatetraene complexes with similar compounds of the group IVB transition elements Ti, Zr and Hf. Bis (cyclooctatetraene) complexes of aU three are known although structural data is only available for the first two. All would appear to involve both planar and non-planar COT rings and to exhibit a sHpped sandwich structure rather than the true sandwich structure of uranocene. 

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. 

Fig. 15.44 Structure of uranocene showing the two eclipsed cydooctatetmenyl rings.

Also crystallizing in space group P2 ]c, uranocene has two molecules per unit cell, so that the U(CsH8)2 molecule occupies a special position of site symmetry 1. In other words, the molecule has an eclipsed conformation, and it may be assigned to special position 2(a). Similarly, the two halves of the [Re2Clg]2- dianion in K2 [Rc2C lmolecular dimensions (indicating that the symmetry of the dianion is Z>4h within experimental error) and crystal structure are shown in Fig. 9.6.5. 

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... 

Both compounds have been obtained for most lanthanides. The anion in K(dme)2 CLn(C8Hg)2] has a uranocene -like sandwich structure (Section 13.7), whilst [(C8Hg)Ce(THF)2Cl]2 has a dimeric structure with bridging chlorides (Figure 6.19). All... 

Reaction of cyclooctatetraene with solutions of ytterbium and 2 moles of potassium in liquid ammonia gives K2 [M(C8H8)2] the solvate [K(dme)]2 [Yb(C8Hs)2] (DME = 1,2-dimethoxyethane) has a sandwich uranocene-type structure for the anion. 

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. 

C4Ph4)2 (M = Ni, Pd) (5,6), bis(benzene)chromium(0), Cr(ri -C6H6)2 (7), and bis(cyclooctatetraene)uranium(IV), uranocene or U(ri -C8H8)2 (8), the structures of which are shown in Figure 1. The synthesis of uranocene, and its structure and chemistry via-a-vis those of ferrocene, provided a remarkable example of the similarities and differences between organotransition metal chemistry and organo-/-element chemistry. 

Reducing the magnitude of the calculated pseudocontact shifts requires smaller values of the anisotropy term X -Xj / which can only result if Xj O. This result provides independent confirmation of the same result of Fischer cited above. We noted also that both the electronic structure of uranocene proposed by Warren (63), assuming a Jz= 4 ground state, and a recent model proposed by Fischer (15), assuming a Jz=3 ground state, show that XjJLs non-zero, and less than X(j, at 30°C. 

Preparation of ferrocene was reported at about the same time by two research groups, and a sandwich structure was proposed, based on ferrocene s physical properties (Kauffman, pp. 185-186). The sandwich structnre was confirmed by x-ray diffraction studies. Since then, other metallocenes composed of other metals and other carbon ring molecules, such as dibenzenechromium (see Figure 3) and uranocene (see Figure 4), have been prepared. 

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