Metallocene analogs with split (2+4)-.pi.-electron ligands

Ligand conformations and spin states in sandwich-type complexes of the split (3+2) five-electron donor hydrocarbon ligand bicyclo[3.2.1]octa-2,6-dien-4-yl (bcod)

Sandwich-type structures are energetically preferred for the bis(bicyclo[3.2.1]octa-2,6-dien-4-yl) complexes of the first row transition metals (bcod)₂M including the experimentally known chromium and iron derivatives.

Structural and Spectroscopic Demonstration of Agostic C−C Interactions in Electron-Deficient Metallacyclobutanes and Related Cage Complexes: Possible Implications for Olefin Polymerizations and Metatheses

The reaction of the half-open titanocene, Ti(C5H5)(c-C8H11)(PMe3) (c-C8H11 = cyclooctadienyl), with two equivalents of PhC2SiMe3 leads to their incorporation and coupling to the dienyl fragment. One alkyne inserts into a C-H bond of the central CH2 group of the c-C8H11 ligand's edge-bridge, while the second undergoes a 5+2 coupling with the dienyl fragment, yielding coordinated sigma-allyl and olefin fragments, as demonstrated by X-ray diffraction. Together with the C5H5 and PMe3 coordinations, this leads to a 14-electron count. While the very electron-deficient titanium center passes up potential pi coordination of the allyl fragment, it instead engages in interactions with one or two C-C bonds, and perhaps a C-H bond, as revealed from the structural and spectroscopic data. Similar interactions have been found in electron-deficient metallacyclobutane complexes of titanium and zirconium, but not in the 18-electron molybdenum and tungsten analogues. These and other observations may have implications relating to metatheses and polymerizations of olefins.