Ring Size\nEffects on the Structures of Sandwich Compounds\nwith a Stoichiometry of C<sub>12</sub>H<sub>12</sub>M (M = Ti–Ni)

Ring\nsize effects on geometries and electronic structures\nwere\ninvestigated for the (C<sub><i>n</i></sub>H<sub><i>n</i></sub>)M(C<sub><i>m</i></sub>H<sub><i>m</i></sub>) (<i>n</i> = 4, 5, or 6; <i>m</i> = 8,\n7, or 6; <i>m</i> + <i>n</i> = 12; M = Ti–Ni)\nsystems using density functional theory. The lowest-energy C<sub>12</sub>H<sub>12</sub>M structures for the early transition metals titanium,\nvanadium, and chromium are the experimentally known singlet (η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)Ti(η<sup>7</sup>-C<sub>7</sub>H<sub>7</sub>), doublet (η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)V(η<sup>7</sup>-C<sub>7</sub>H<sub>7</sub>), and singlet\n(η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)<sub>2</sub>Cr, respectively.\nThe likewise experimentally known singlet (η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)<sub>2</sub>Ti, doublet (η<sup>6</sup>-C<sub>6</sub>H<sub>6</sub>)<sub>2</sub>V, and singlet (η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>)Cr(η<sup>7</sup>-C<sub>7</sub>H<sub>7</sub>) are the second-lowest-energy structures with only\na small energy difference between the two vanadium structures. For\nthe later transition metals, dibenzenemetal complexes are the lowest-energy\nC<sub>12</sub>H<sub>12</sub>M species with two fully bonded hexahapto\nbenzene rings in the lowest-energy manganese and iron derivatives\nand one hexahapto and one dihapto benzene ring in the lowest-energy\ncobalt and nickel derivatives. The lowest-energy (C<sub>5</sub>H<sub>5</sub>)M(C<sub>7</sub>H<sub>7</sub>) structures for the later transition\nmetals iron, cobalt, and nickel have partially bonded nonplanar C<sub>7</sub>H<sub>7</sub> rings with one or two uncomplexed CC\nbonds. The (C<sub>4</sub>H<sub>4</sub>)M(C<sub>8</sub>H<sub>8</sub>) (M = Ti–Ni) structures with the metal sandwiched between\nfour- and eight-membered rings were found to be much higher in energy\nthan their (C<sub>5</sub>H<sub>5</sub>)M(C<sub>7</sub>H<sub>7</sub>) and (C<sub>6</sub>H<sub>6</sub>)<sub>2</sub>M isomers.