SYNTHESIS AND CRYSTAL-STRUCTURE OF THE 1ST NEUTRAL BIS(ETA-5-C2B4-CARBORANE)CHROMIUM(IV) SANDWICH COMPLEX

Oxidation of the lithium salt of the paramagnetic chromium(III) sandwich complex, Li+(THF)4{1,1'-commo-Cr[2,3-(SiMe3)2-2,3-C2B4H4]2}- (2) with anhydrous PbCl2 in a molar ratio 2:1 in dry hexane/THF solution produced the novel, diamagnetic and neutral chromium-(IV) sandwich complex 1,1'-commo-Cr[2,3-(SiMe3)2-2,3-C2B4H4]2 (1) as a dark red, air-sensitive crystalline solid in 63% yield. Complex 1 was characterized by H-1, B-11, and C-13 NMR spectra, mass and IR spectra, and also by X-ray crystallography. The crystal structure of 1 reveals that the central chromium metal, in a formal oxidation state of 4+, lies at a crystallographic center of symmetry and adopts an essentially eta-5-bonding posture with respect to each of the C2B3 faces. However, the chromium is slipped significantly toward the cage carbons giving an average Cr-B distance of 2.321 (7) angstrom, which is considerably longer than the average Cr-C distance of 2.179 (5) angstrom, thus showing a trend opposite to those observed in the structures of main-group heterocarboranes.