The Unusual Stability of Homoleptic Di- and Tetravalent Chromium Alkyls

Reaction of CrCl<sub>3</sub>(THF)<sub>3</sub> with 3 equiv of R−M [R = Me<sub>3</sub>CCH<sub>2</sub> (<b>a</b>), Me<sub>3</sub>SiCH<sub>2</sub> (<b>b</b>)] afforded either the previously reported tetravalent homoleptic R<sub>4</sub>Cr (<b>1a</b>,<b> b</b>) [M = MgCl] or the unprecedented divalent and tetranuclear homoleptic [R<sub>2</sub>Cr]<sub>4</sub> (<b>2b</b>) [M = Li]. Both types of compounds display a remarkable chemical inertness toward proton transfer agents but readily react with ethylene.