In discussing ferrocene in the previous post, I mentioned Irving Langmuir’s 1921 postulate that filled valence shells in what he called complete molecules would have magic numbers of 2, 8, 18 or 32 electrons (deriving from the sum of terms in 2[1+3+5+7]). The first two dominate organic chemistry of course, whilst the third is illustrated by the transition series, ferrocene being an example of such. The fourth case is very much rarer, only one example ever having been suggested, it deriving from the actinides. In this post, I thought I would augment ferrocene (an 18-electron example) with beryllocene (an 8-electron example) and then speculate about 32-electron metallocenes.
The crystal structure of (nonamethyl)bis-cyclopentadienyl beryllium  illustrates the octet rule directly. Be is ionised to Be2+, the charge balanced by two cyclopentadienyl anions. The octet is formally filled by donation of six electrons from one Cp* anion, and only two from the other, filling the s and p shells of the metal (the 1 and 3 in the sum alluded to earlier). The ELF analysis suggests the molecule is less ionic than ferrocene. ELF disynaptic basis are located for all five Be-C bonds on the η-5 ring, and only one for the η-1 ring. The latter basin contains 1.87 electrons (a conventional electron pair bond), whilst the five former range range from 0.57 to 0.68 electrons, adding to 5.02. The formal octet is thus not entirely filled, but in this sense, it is less ionic than ferrocene. (See DOI 10042/to-8371 for details of the calculation).
Uranocene is a rather different beast. The ligands are not cyclopentadienyl, but cyclo-octatetraenyl. Uranium has a radon core, and a 5f3, 6d1 and 7s2 valence shell(s) electron configuration. Ionised to U4+, formally the 5f, 6d and 7p shells are all empty; a total of 14 + 10 + 6 electrons would be required to achieve a 32-electron filled shell , or 30 additional electrons. The two COT ligands, as di-anions (achieving aromaticity) could provide only 20. So uranocene (Cambridge refcode URACEN10, DOI 10.1021/ic50111a034) is far from the holy-grail of a 32-electron complete molecule.The QTAIM analysis of the electron density (the molecule itself is a triplet spin state) shows only six bonds from each COT ligand to the metal. The ELF analysis shows NO U-C disynaptic basins, unlike either beryllocene or ferrocene (the features surrounding the U derive from pseudopotential used for the calculation). This indicates that uranocene is the most ionic of the three metallocenes.
Could a molecule be contrived that might achieve (a formal) 32-electron filled 5f,6d,7p valence shell? One would probably need a ligand contributing 14 rather than 10 electrons whilst keeping the size of the ring manageable, quite a challenge. There may not be enough space for three 10-electron ligands. So, no examples of 32-electron metallocenes just yet then!
- J. Dognon, C. Clavaguéra, and P. Pyykkö, "Towards a 32-Electron Principle: Pu@Pb12 and Related Systems", Angewandte Chemie International Edition, vol. 46, pp. 1427-1430, 2007. http://dx.doi.org/10.1002/anie.200604198
- M.M. Conejo, R. Fernández, D. del Río, E. Carmona, A. Monge, and C. Ruiz, "Synthesis and structural characterization of Be(η5-C5Me5)(η1-C5Me4H). Evidence for ring-inversion leading to Be(η5-C5Me4H)(η1-C5Me5)", Chem. Commun., pp. 2916-2917, 2002. http://dx.doi.org/10.1039/b208972f