Alternatives to Triple-Decker Sandwich Structures for Binuclear Cyclooctatetraene First-Row Transition Metal Complexes of the Type (C8H8)3M2

The geometries and thermochemistry of the binuclear cyclooctatetraene metal complexes (C8H8)3M2 (M = Ti–Ni), including the experimentally known titanium and chromium derivatives, have been investigated by density functional theory. Two general types of (C8H8)3M2 structures are found, namely, trans structures in which both outer C8H8M units are located on opposite sides of the central cyclooctatetraene ring to construct triple-decker sandwich structures and cis structures in which both outer C8H8M units are located on the same side of the central cyclooctatetraene ring. The experimentally known (C8H8)3Ti2 triple-decker sandwich triplet spin state structure with outer octahapto η8-C8H8 rings and a central bis­(trihapto) η3,η3-C8H8 ring is the lowest energy isomer in the titanium system and represents an example of a trans structure. The very different experimentally known (C8H8)3Cr2 structure, with outer tetrahapto η4-C8H8 rings, a central bent bis­(pentahapto) η5,η5-C8H8 ring, and a short Cr  Cr distance suggesting multiple bonding, is the lowest energy isomer in the chromium system and an example of a cis structure. The low-energy (C8H8)3M2 structures exhibit a variety of coordination modes of the cyclooctatetraene ligands depending on the electronic requirements of the central metal atoms to approach the favored 18-electron configurations. Thus, fully bonded octahapto η8-C8H8 rings are found only in the earliest transition metals titanium and vanadium. The suggestion by Skell and co-workers of a triple-decker sandwich structure with three octahapto η8-C8H8 rings for the experimentally observed unstable singlet green C8H82– dianion is not supported by our theoretical results that instead predict a structure analogous to the isoelectronic singlet (C8H8)3V2. Partially bonded hexahapto and tetrahapto C8H8 rings with one or two uncomplexed carbon–carbon double bonds, respectively, are common features in low-energy structures of the later transition metals.