INTERRUPTION OF CONJUGATION IN TRANSITION-METAL-BOUND POLYENES - A REINVESTIGATION OF THE X-RAY CRYSTAL-STRUCTURE OF (HEXAMETHYLBENZENE)TRICARBONYLCHROMIUM
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abstract
The lowering of symmetry in polyenes -bound to transition metals and transition-metal fragments has been the subject of controversy for over 20 years.1As the accuracy of X-ray crystallographic investigations improve, more accurate values for C-C bond lengths may be determined in polyene rings of organometallic compounds. Early room-temperature X-ray crystallographic studies of (C6H6)Cr(CO)3showed equal C-C bond lengths, within experimental error, in the complexed benzene ring.2, 3However, a more accurate low-temperature study, which combined X-ray and neutron diffraction, showed C-C bond length alternation.4 The average difference was 0.017 (2) A. A low-temperature X-ray study of (C6(CH3)6)Mo(CO)3indicated that the bond length differences in this analogue averaged 0.036 (9) A, 5 twice the value seen for (C6H6)Cr(CO)3. It has been postulated6 that long-short C-C bond length alternation should be seen for all (C6R6)M(CO)3(M = Cr, Mo, W) complexes. Small C-C bond length alternation has also been observed in X-ray crystallographic studies of (C5-(CH3)5)M(CO)2(M = Co, 7 Rh8) and (C5H5)M(CO)3(M = Mn, 9a Re9b). In almost all cases, room-temperature X-ray studies2, 3, 7, 8, 9 of these and similar complexes have not detected any consistent polyene distortion. The increased accuracy of the low-temperature studies is due to reduction of the carbon atom thermal motions, which obscure their exact positions in room-temperature X-ray studies. 1987, American Chemical Society. All rights reserved.
