AN EFFICIENT REDUCTION PROCESS LEADING TO TITANIUM(II) AND NIOBIUM(II) - PREPARATION AND STRUCTURAL CHARACTERIZATION OF TRANS-MCL(2)(PY)(4) COMPOUNDS, M=TI, NB, ACID MN

An easy way to produce the scarcely known mononuclear divalent coordination compounds of titanium and niobium by reduction of higher-valent species with potassium graphite, KC8, in THF or THF-pyridine solutions is described. Also reported is the preparation and X-ray structural characterization of several trans-dichlorotetrakis(pyridine)metal(II) species: trans-TiCl2(py)(4) . THF, 1 . THF, and three crystal forms of trans-NbCl2(py)(4). The latter crystallize unsolvated in a tetragonal, 2t, and a monoclinic, 2m, form but the third type, 2 . 0.5THF, has THF in the crystals. The propeller-like arrangement of the pyridine planes in the niobium polymorphs is very similar, but differs considerably from that of 1 . THF in which the pairs of trans pyridine molecules are coplanar. For comparison purposes, we also report the structure of trans-MnCl2(py)4. The variation of the M-N bond distances for the high-spin trans-MCl(2)(py)(4) species of the first transition series shows the pattern expected from ligand field considerations with maxima at titanium and manganese and minima at vanadium and nickel.