Tables of fractional atomiccoordinates, bond distances and angles, and thermal parameters (80 pages); tables of calculated and observed structure factors (129 pages). Ordering information is given on any current masthead page.

Amalgam reduction of the Nb(III) compounds Cp‘2Nb(Cl)(L) (1, L = CO; 2, L = PMe3; 3, L = CNtBu) gives rise to the electron-rich Nb(II) radicals Cp‘2Nb(L). In one case (L = CO) the intermediate radical was observed using ESR spectroscopy. The radicals undergo a facile reaction with elemental mercury to give the heterometallic compounds [Cp‘2Nb(L)]2Hg, one of which (L = PMe3) was characterized crystallographically; since this process is reversible, the mercury adducts constitute convenient sources of the niobium(II) radicals.

The niobocene formaldehyde complex Cp′ 2 Nb-(Cl)(η 2 -CH 2 O) (1, Cp′ ) η 5 -C 5 H 4 SiMe 3 ) reacts with [Ph 3 C][BF 4 ] to give an alkoxide (2) of formula [Cp′ 2 Nb-(Cl)(OCH 2 CPh 3 )][BF 4 ] and with HBF 4 ‚OEt 2 to give analogue 4 ([Cp′ 2 Nb(Cl)(OMe)][BF 4 ]); these processes constitute examples of electrophilic attack on the formaldehyde carbon, instead of the more typical addition at oxygen. Compound 2 is hydrolyzed to Cp′ 2 Nb(Cl)-(OBF 3 ) (3), a Lewis acid adduct of the corresponding metal oxide; both 2 and 3 have been characterized by X-ray crystallography.