Ring Size Effects on the Structures of Sandwich Compounds with a Stoichiometry of C₁₂H₁₂M (M = Ti-Ni)

Ring size effects on geometries and electronic structures were investigated for the (C H )M(C H ) (n = 4, 5, or 6; m = 8, 7, or 6; m + n = 12; M = Ti-Ni) systems using density functional theory. The lowest-energy C₁₂H₁₂M structures for the early transition metals titanium, vanadium, and chromium are the experimentally known singlet (η⁵-C₅H₅)Ti(η⁷-C₇H₇), doublet (η⁵-C₅H₅)V(η⁷-C₇H₇), and singlet (η⁶-C₆H₆)₂Cr, respectively. The likewise experimentally known singlet (η⁶-C₆H₆)₂Ti, doublet (η⁶-C₆H₆)₂V, and singlet (η⁵-C₅H₅)Cr(η⁷-C₇H₇) are the second-lowest-energy structures with only a small energy difference between the two vanadium structures. For the later transition metals, dibenzenemetal complexes are the lowest-energy C₁₂H₁₂M species with two fully bonded hexahapto benzene rings in the lowest-energy manganese and iron derivatives and one hexahapto and one dihapto benzene ring in the lowest-energy cobalt and nickel derivatives. The lowest-energy (C₅H₅)M(C₇H₇) structures for the later transition metals iron, cobalt, and nickel have partially bonded nonplanar C₇H₇ rings with one or two uncomplexed C=C bonds. The (C₄H₄)M(C₈H₈) (M = Ti-Ni) structures with the metal sandwiched between four- and eight-membered rings were found to be much higher in energy than their (C₅H₅)M(C₇H₇) and (C₆H₆)₂M isomers.