903
Stereochemically Nonrigid Organometallic Molecules. XI. The Molecular Structure of ( 1,3,5,7-Tetrarnethylcyclooctatetraene)chromium Tricarbony1lv2 M. J. Bennett, F. A. Cotton, and Josef Takats Contribution from the Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139. Received September 1, 1967 Abstract: The crystal and molecular structures of (1,3,5,7-tetrarnethylcyclooctatetraene)chromiumtricarbonyl have bFen determined by singlFcrysta1X-ray methods. The crystal data are: space grcup, P2,/c; a = 7.512 f 0.004 A, b = 18.485 ==! 0.003 A, c = 11.173 i 0.006 A, /3 = 114" 5' d= 3'; V = 1416 AS; density measured by flotation, 1.37 i 0.02 g cm-3. Using a total of 1484 independent, nonzero reflections collected on a manually operated counter-diffractometer with Mo Ka:radiation, the structure was solved using Patterson and Fourier methods and refined by least squares to final unweighted and weighted residuals of 0.041 and 0.053, respectively. All heavyatom (Cr, C, 0) positions were refined with anisotropic temperature factors, and the positional parameters for the hydrogen atoms were also refined using isotropic temperature factors. The molecule has a structure generally but it is less regular. From the positions of the methyl carbon atoms and ring similar to that of (COT)MO(CO)~, hydrogen atoms, the type of hybridization at the ring carbon atoms may be inferred.
T
he fact that many x complexes formed by cgclooctatetraene with metal carbonyl moieties, that is, compounds of the general formula C8HsMZ(CO),, are stereochemically nonrigid, or fluctional, is now well known. Aside from the various molecules in which the metal atom(s) is iron3-" or ruthenium,12 there are the interesting compounds CsHsM(C0)a in which M is Cr3 or M o , ~ "as~ well as their 1,3,5,7-tetramethylcyclooctatetraene (TMCOT) analogs, l 4 (TMCOT)M(C0)3, in which M may be Cr,15 M o 14,15or W.16 Both the (COT)M(C0)3 and (TMdOT)M(CO)a compounds have been shown to exhibit complex, welldefined proton nmr spectra (limiting low-temperature spectra) at temperatures not too far (ca. -30") below room temperature. These limiting low-temperature spectra have been interpreted in terms of the instantaneous structures shown schematically as I and 11. an X-ray crystallographic In the case of C~H&IO(CO)~, studylo has confirmed the essential correctness of I and provided detailed dimensions. In view of the very complex behavior of the proton (1) This study has been supported by the National Science Foundation under grant No. 7034X and by a grant from The Petroleum Research Fund, administered by the American Chemical Society. (2) Preceding paper in this series (part X): M. J. Bennett, F. A. Cotton, and P. Legzdins, J . Am. Chern. Soc., 89, 6797 (1967). (3) C. G.Kreiter, A. Maasbol, F. A. L. Anet, H. D. Kaesz, andS. Winstein, ibid., 8 8 , 3444 (1966). (4) F.A. Cotton, A. Davison, and J. W. Faller, ibid., 88, 4507 (1966). (5) C. E.Keller, B. A. Shoulders, and R. Pettit, ibid., 88, 4760 (1966). (6) F. A. L. Anet, H. D. Kaesz, A. Maasbol, and S. Winstein, ibid., 89. 2489 (1967). F. A. L. Anet, ibid., 89,2491 (1967). (8) C. E. Keller, G. F. Emerson, and R. Pettit, ibid., 87, 1388 (1965). (9) A. Carbonaro, A. Greco, and G. Dall'Asta, Tetrahedron Letters, 2037 (1967). (10) E. B. Fleischer, et al., J . A m . Chem. Soc., 88, 3158 (1966). (11) B. Dickens and W. N. Lipscomb, J . Chem. Phys., 37, 2084 (1962). (12) (a) M. I. Bruce, M. Cooke, M. Green, and F. G. A. Stone, Chem. Commun., 523 (1967); (b) W. K. Bratton, F. A. Cotton, A. Davison, A. Musco, and J. W. Faller, Proc. Narl. Acad. Sci. U. S., 58, 1324 (1967). (13) S. Winstein, H.D. Kaesz, C. G. Kreiter, and E. C . Friedrich, J . Am. Chem. Soc., 87,3267 (1965). (14) F. A. Cotton, J. W. Faller, and A. Musco, ibid., 88, 4506 (1966). (15) Following paper (part XII): F. A. Cotton, J. W. Faller, and A. Musco, ibid., in press. (16) J. S. McKechnie and I. C. Paul, ibid., 88, 5927 (1966).
