Coordinatively unsaturated organometallic system based on Tp ligand: tetrahedral TpRM-R′ and TpRM-M′Ln species

Chemistry of the highly coordinatively unsaturated, tetrahedral hydrocarbyl and dinuclear complexes bearing a hydrotris(pyrazolyl)borate (TpR) ligand, TpRM-R′ and TpRM-M′Ln, is reviewed. The organometallic TpR complexes are prepared by salt elimination between the corresponding halide and Grignard reagents or metalates and fully characterized by spectroscopic and crystallographic methods. Although the number of the valence electrons of the resultant species is much shorter than that expected for a coordinatively saturated species (for mononuclear species: 14–15e vs. 18e; for dinuclear species: 29–32e vs. 34e), they turn out to be thermally stable. In particular, the ethyl complexes TpiPr2M-CH2CH3 (M = Fe, Co) are stable with respect to β-hydride elimination. The tetrahedral structures of the obtained organometallic species cause a small ligand field splitting of the frontier orbitals to lead to a high spin configuration, which leaves no vacant coordination site, and this should be the origin of the thermal stability of the electron deficient species. Upon interaction with donors they are incorporated into the organometallic system via switching of the spin state, and selective reactions dependent on the nature of the donor molecules are observed for the dinuclear complexes. Thus the high spin species can be regarded as masked forms of coordinatively unsaturated intermediates, which are regarded as key intermediates of organometallic transformations.