[(Tp t Bu, Me )CrR]: A New Class of Mononuclear, Coordinatively Unsaturated Chromium(II) Alkyls with cis ‐Divacant Octahedral Structure

Reaction of CrCl 2 with Tp t Bu, Me K yielded [Tp t Bu, Me Cr(3‐ t Bu, 5‐MepzH)Cl] ( 1 ) and [Tp t Bu, Me CrCl] ( 2 ), while the same reaction in the presence of pyridine gave 1 and [Tp t Bu, Me Cr(py)Cl] ( 3 ). Alkylation of 3 with Grignard reagents produced the chromium(II) alkyls [Tp t Bu, Me CrR] ( 4 , R = Et; 5 , R = Ph; 6 , R = CH 2 SiMe 3 ), which reversibly added to pyridine to form the five‐coordinate adducts [Tp t Bu, Me Cr(py)R] ( 7 , R = Et; 8 , R = Ph; 9 , R = CH 2 SiMe 3 ). 1, 2, 4 , and 5 were structurally characterized by X‐ray diffraction. The four‐coordinate molecules 2, 4 , and 5 adopt a highly unusual cis ‐divacant octahedral coordination geometry, while 1 is the first five‐coordinate Tp t Bu, Me ‐complex of a first‐row transition metal. Despite their coordinative unsaturation, chromium alkyls 4‐6 do not polymerize ethylene, or even react with it. This observation is inconsistent with the catalytic activity commonly ascribed to divalent chromium in heterogeneous polymerization catalysts. Attempts to oxidize 4‐6 (e.g., with [Cp 2 Fe]BPh 4 ) to cationic chromium(II) alkyls failed, yielding [Tp t Bu, Me Cr(thf)][BPh 4 ] ( 10 ) instead.