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

Ring size effects on geometries and electronic structures were investigated for the (C H )M(C H ) ( = 4, 5, or 6; = 8, 7, or 6; + = 12; M = Ti-Ni) systems using density functional theory. The lowest-energy CHM structures for the early transition metals titanium, vanadium, and chromium are the experimentally known singlet (η-CH)Ti(η-CH), doublet (η-CH)V(η-CH), and singlet (η-CH)Cr, respectively. The likewise experimentally known singlet (η-CH)Ti, doublet (η-CH)V, and singlet (η-CH)Cr(η-CH) 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 CHM 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 (CH)M(CH) structures for the later transition metals iron, cobalt, and nickel have partially bonded nonplanar CH rings with one or two uncomplexed C=C bonds. The (CH)M(CH) (M = Ti-Ni) structures with the metal sandwiched between four- and eight-membered rings were found to be much higher in energy than their (CH)M(CH) and (CH)M isomers.