Complexes Containing Seven- and Eight-Electron π-Ligands

Abstract

The most commonly encountered 7e ligands are the cycloheptatrienyl ligand (CHT), C₇H₇, and the azulenium ligand, C₁₀H₉. Cyclooctatetraene (COT), C₈H₈, represents an 8e ligand; it forms sandwich-type compounds with some actinides, for example, [U(COT)₂]. Coordinated η ⁷-C₇H₇ and η ⁸-COT molecules are planar, and all C-C distances are equal or differ only slightly.^(1–5) The interaction between CHT and COT and the metal is similar to the interaction between cyclopentadienyl or arene and the metal in complexes of the type Mcp₂ and M(arene)₂. However, due to the lowering of e ₁ and e ₂ orbital energy as a result of the ring size increase (Figure 9.4b), there is an increase of overlap between e ₂ (${d_x}_{^2 - {y^2}}$ and d _xy ) metal orbitals and ligand orbitals possessing the same symmetry (Figure 11.1). This effect causes an increase in the energy difference between the e ₂-type molecular orbitals and those of a ₁ type possessing predominantly d orbital character of the metal (Figure 9.4a).^{(6, 7)}