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 n H n )M(C m H m ) (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.