(Silox)(3)Nb(eta(2)-N,C-4-picoline) as a source of (silox)(3)Nb: structureof [(silox)(3)Nb](2)(mu-eta(2),eta(2)-C6H6) (silox = (Bu3SiO)-Bu-t)

Electrochemical and chemical reductions of (silox)(3)NbCl2 (1, -1.42 V vers us Ag/Ag+) and (silox)(3)TaCl2 (2, -2.18 V) generated [(silox)(3)NbCl2](-) (1(-)) and [(silox)(3)TaCl2](-) (2(-)). The anions were subject to Cl- loss , leading to (silox)(3)NbCl (3) and probable disproportionation (Ta). Compl ex 3 formed reversible solvent adducts (silox)(3)ClNbL (3-L, L = THF, py). Na/Hg reduction of (silox)(3)NbCl2 (1) in the presence of 4-picoline afford ed (silox)(3)Nb(eta(2)-N,C-4-NC5H4CH3) (6), which can be considered a sourc e of '(silox)(3)Nb', analagous to (silox)(3)Ta (4). Adducts such as (silox) (3)Nb(eta(2)-H2CCHR) (R = H, 7; Ph, 8) may be formed upon addition of olefi ns, and 6 abstracted an oxygen from N2O, NO or ethylene oxide to give (silo x)(2)Nb = O (9) and N-2 or C2H4, respectively, which are reactions also obs erved for 3. 4-Picoline may be abstracted from 6 by 4 to give {(silox)(3)Nb }(2)(mu:eta(2),eta(2)-C6H6) (11) and (silox)(3)Ta(eta(2)-N,C-4-NC5H4CH3) (1 2). An X-ray structure determination of 11 revealed a disordered bridge tha t was modeled as a mu:eta(2),eta(2)-benzene. Extended Huckel molecular orbi tal (EHMO) calculations showed that the mu:eta(2),eta-configuration was ene rgetically better than a mu:eta(6),eta(6)-alternative, but about the same a s a plausible mu:eta(3),eta(3)-arrangement. (C) 1999 Elsevier Science S.A. All rights reserved.