Theoretical studies sandwich complexes

Another type of interaction is the association of radical ions with the parent compounds. Recently (118), a theoretical study was reported on the interaction of butadiene ions with butadiene. Assuming a sandwich structure for the complex, the potential curve based on an extended Hiickel calculation for two approaching butadienes (B + B) revealed only repulsion, as expected, while the curves for B + and B + B" interactions exhibit shallow minima (.068 and. 048 eV) at an interplanar distance of about 3.4 A. From CNDO/2 calculations, adopting the parameter set of Wiberg (161), the dimer cation radical, BJ, appears to be. 132 eV more stable than the separate B and B species, whereas the separate B and B species are favored by. 116eV over the dimer anion radical, BJ. This finding is consistent with experimental results formation of the dimer cation radical was proved in a convincing manner (162) while the attempts to detect the dimer anion radical have been unsuccessful. With other hydrocarbons, the reported formation of benzene dimer anion radical (163) represents an exceptional case, while the dimeric cation radical was observed... 

Theoretical studies of polymetallocenylene-related polymers have been carried by Burdett and Canadell [67]. There are two different electron counts associated with a bandgap corresponding to iron and chromium polymers in the band structure of a naphthalene-bridged sandwich complex polymer, 16 [67], whereas one bandgap is found in the band structure of polyferroce-nylene [60]. This is due to a three-orbital interaction between the and d y of metal and the LUMO of naphthalene, causing three bands in the band structure. 

Yang, L. Li, X. Ding, Y Sun, C. Theoretical study on a class of organometaUic complexes based on all-metal aromatic through sandwiching stabilization. Eur. J. Inorg. Chem. 2008, 2099-2106. 

In theoretical and experimental studies of XPS and vibrational spectra, Rossi (67), Ho (59) and Silverman (68) have advocated open-faced PMDA sandwich n-complexes, where the Cr atom is located on the principal molecular axis above the central arene ring. Formation of such species implies an unusually high selectivity of chromium for the arene part of the PMDA group, despite kinetic and thermodynamic considerations to the contrary. Ho et al. also claim that chromium prefers these PMDA binding sites over ODA up to nearly monolayer coverages of metal (59). 

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