An unusual reaction of A π-complexed lithio-arene: the characterization and structure of μ-(2,2'-difluoro-5,5'-dimethylbenzophenone)bis(tricarbonylchromium(0))

(π-Lithioarene)tricarbonychromium(0) complexes are suggested to be in equilibrium with carbene-bridged dimers at low temperature; the dimers can be trapped with acid chlorides or chloroformate esters yielding bis[(arene)tricarbonylchromium(0)]-carbinols or -ketones, respectively. Thus, the lithiation of (p-fluorotoluene)tricarbonylchromium(0) followed by addition of methyl chloroformate yields μ(2,2'-difluoro-5,5'-dimethylbenzophenone)bis(tricarbonylchromium(0)); crystals of C21H12Cr2F2O7 are monoclinic space group P21/c, with a = 7.170(2), b = 21.832(5), c = 13.428(2) Å, β = 98.91(2)°. The structure has been determined by the analysis of 1959 observed reflections recorded on a Syntex P21 automatic diffractometer and refined by full-matrix least-squares to R = 0.054, Rw = 0.045. The two Cr(CO)3 tripods are oriented differently with respect to the bound arene rings. This shows that the orientation of the Cr(CO)3 moiety is not governed primarily by the electronic properties of the ring substituents.