Contributions to the Chemistry of Boron, 204 [1] An Unusual [2+2]-Cycloaddition Reaction of Dicobalt Octacarbonyl

Co2(CO)8 has proved a key material for the preparation of a variety of cobalt carbonyl com­ plexes. Although most of its reactions involve oxi­ dation, reduction, disproportionation or substitu­ tion [2], addition reactions are scarce. Moreover, Co2(CO)8 is one of the few compounds to form ad­ dition products with strong Lewis acids such as AlBr3 [3]. The acid adds to the oxygen atom of a bridging carbonyl group. In our studies on the chemistry of (9-fluorenylidene)(2,2,6,6-tetramethylpiperidino)borane (1) (hereafter abbreviated as tmpB = CR2) as a transi­ tion metal-complex ligand, we have synthesized mononuclear complexes of the type Fe(CO)4 (?7 2-tmpB = CR2) [4], Fe(CO)3(>74-tmpB = CR2) [5], or Fe(CO)3(?72-tmpB = CR2)L [5, 6]. In a similar way, it was anticipated that binuclear metal car­ bonyls would also react with 1 to yield products analogous to those obtained from Co2(CO)8 and unsaturated compounds such as C F2= C F 2, e .g . (CO)4Co(CF2)2Co(CO)4 [7], However, we have ob­ served a completely different behaviour to be reported here.