Inorg. Chem. zyxwvu 1986, 25, zyxwvu 3641-3649 3641 The 'H spectrum (CDC13, room temperature) is rather complex and displays six broad singlets at zyxwvutsrq 6 2.36, 2.21, 2.07, 1.92, 1.59, and 1 zyxwvutsrqpo .55 (of approximately equal intensity) in accord with the six pairs of inequivalent methyl groups that are seen to be present in the molecular structure. The absence of splitting by the 31P nuclei is a phenomenon for which there are several precedents, though no simple explanation. In M~~Cl~(dmpm)~'~ the CH3 resonances appear as singlets, and a similar observation has been made for Mn2(C0)6(dmpm)z.20 Other examples of small or nondetectable 31P-1H coupling of this sort will be found in other reports.z1*22 The methylene IH resonances are seen as an AB pattern centered at 6 4.40 zyxwv (JAB zyxw = 13.4 Hz) and a broad peak at 6 3.95, in an intensity ratio of 2:l. The absence of an AB pattern in the second resonance may be due to a slow motion of the unique methylene group from side to side. Unfortunately, the solubility of this compound is too low to permit low-temperature NMR studies. Acknowledgments. We thank the National Science Foundation for the support of this research through grants to R.A.W. (Grant No. CHE85-06702) and F.A.C. (Grant No. CHE82-11407). Re2CI,(p-dmpm),, 103852-11-5; (n-Bu4N),Re2CI8, 14023-10-0; Re2CI4(P-n-Pr3),, 523 59-07-6. Registry No. Supplementary Material Available: Tables of torsion angles and an- isotropic displacement parameters for both structures and figures showing contoured Fourier sections from both structures (IO pages); tables of observed and calculated structure factors for both structures (35 pages). Ordering information is given on any current masthead page. (19) Cotton, F. A.; Walton, R. A,, unpublished work. (20) Lemke, F. R.; Kubiak, C. P. zyxwvutsrqpo Inorg. Chim. Acta 1986, 113, 125. (21) King, R. B.; Raghuveer, K. S. Inorg. Chem. 1984, 23, 2482. (22) Wong, W. K.; Chiu, K. W.; Wilkinson, G.; Hursthouse, M. B. Polyhedron 1985, 4, 603. A. J.; Motevalli, M.; Contribution from the Department of Chemistry and Laboratory for Molecular Structure and Bonding, Texas A&M University, College Station, Texas 77843 Quadruply Bonded M~~I,.,(dppe)~ (dppe = Bis(dipheny1phosphino)ethane): Twisted and Eclipsed Rotational Conformations and Their Significance F. Albert Cotton,* Kim R. Dunbar, and Marek Matusz Received April 22, 1986 The structural characterizations of two crystallographic forms of p-M~~I,(dppe)~ are reported, and the relationship between the degree of torsional twist that the molecules exhibit in the solid state and the energy of the 6 - 6* transition is discussed. The title compound can be prepared by reaction of the quadruply bonded Mo2(~-O2CCH3), with Me3SiI in toluene in the presence of 2 q u i v of dppe and can be crystallized from many solvents. A solution of the compound in dichloromethane-methanol produces green monoclinic crystals of the dichloromethane solvate ~~-Mo~I,(~~~~),.~/~CH~CI~ (l), in which there are twisted molecules, la, located on a general position, and eclipsed molecules, lb, which reside on crystallographic inversion centers. The crystal belongs to the space group P2,/n with unit cell dimensions a = 13.600 (4) A, b = 17.755 (5) A, c = 30.30 (1) A, p = 98.74 (2)O, V = 8426 (4) A3, and Z = 6. The asymmetric unit is defined by 1.5 molecular formulas of Mo214(dppe)2 and one CH2C12 molecule. The structure is unusual in that l a possesses a mean torsional twist angle of zyxwvu x = 27.9O and l b assumes the fully eclipsed orientation with an average x = Oo, where x is the angle of internal rotation away from the eclipsed conformation. A significant difference exists in the degree of MG - Mo interaction for l a and l b as is evident from the Mo-Mo bond distances of 2.180 (4) and 2.129 (5) 8, for the cocrystallized molecules l a and lb, respectively. A second crystal of M ~ J ~ ( d p p e ) ~ was grown from a dichloro- methane-toluene mixture and is designated as ,B'-M0~1,(dppe)~.C,H~ (2). The 8' form crystallizes in the monoclinic P2,/n space grou with the following unit cell dimensions: a = 12.015 (2) A, b = 25.048 (4) A, c = 19.407 (4) A, p = 95.3 (2)O, V = 5814 (2) 1 3, and Z = 4. The midpoints of the Mo-Mo bonds reside on a set of general positions. The molecule is internally rotated by an average value of x = 25.7O, and the Mo-Mo distance is 2.179 (3) A. The variation in 6 overlap in the three different types of molecules la, lb, and 2 was effectively probed by recording the visible spectra of the samples in the solid state. Two distinct bands are observed at 670 nm (14900 cm-I) and at 920 nm (10900 cm-I) in the spectrum of p-M~~I~(dppe),.~/~.CH~Cl~ and are assigned to the 6 - 6* (Alg - A,") transitions of molecules l b (x = OD) and l a (x = 27.9O), respectively; the intensity ratio is in approximate accord with the ratio of the 6/& overlap integrals. The solid-state visible spectrum of p'-Mo214(dppe)2C7H8 contains only one band at A , = 878 nm (1 1 400 cm-I), in agreement with the results of the X-ray structure, which shows that only one type of molecule is present. Dichloromethane solution spectra of both and p' forms are identical, with the 6 - 6* transition being located at 860 nm (1 1 600 cm-I). The electronic absorption properties of 1 and 2 as well as the results of "PI'HJ NMR spectroscopy and cyclic voltammetry studies are discussed. Introduction Compounds of the type Mo2X4(LL),, where X = C1 or Br and LL is a bridging bidentate phosphine RzP(CHz),PR2, have been interesting subjects for structural and spectroscopic studies.'-' X-ray crystallographic data for those with zyxwvutsr n = 1, i.e., the his- (diphenylphosphino)methane complexes, MozX4(dppm)2 (X = CIS, Br4, 19), have shown that the ligands adopt a fully eclipsed orientation across the two metal centers, with the dppm ligands in a trans bridging disposition. In the case of n = 2, as in M~~X~(dmpe)~ or M~~X~(dppe)~ (X = CI, Br; dmpe = Wdi- methylphosphino)ethane, dppe = bis(diphenylphosphino)ethane), the molecules exhibit various degrees of torsional twist about the Mo-Mo axis, resulting from the conformational preference of the fused, six-membered rings that are present in these P-Mo2X4(LL), molecules. It can be shown from the angular parts of the orbital wave functions that the strength of the 6 component of a quadruple bond should vary with cos (2x), where x is the angle of internal rotation away from the eclipsed conformation (x = O0).'O Extensive studies conducted in this group, based upon data from 10 related, structurally characterized compounds, have shown that the Mo- Mo distance in the Mo,X4(LL), class of molecules varies linearly with cos (2x)4.7 analysis of the variation in 6 - 6* transition (1) Cotton, F. A,; Powell, G. L. Inorg. Chem. 1983, 22, 1507. (2) Agaskar, P. A.; Cotton, F. A.; Fraser, I. F.; Peacock, R. D. J. Am. Chem. SOC. 1984, 106, 1851. (3) Agaskar, P. A.; Cotton, F. A. Inorg. Chem. 1984, 23, 3383. (4) Campbell, F. L., 111.; Cotton, F. A.; Powell, G. L. Inorg. Chem. 1984, 23, 4222. (5) Campbell, F. L., 111.; Cotton, F. A,; Powell, G. L. Inorg. Chem. 1985, 24, 177. (6) Agaskar, P. A.; Cotton, F. A.; Derringer, D. R.; Powell, G. L.; Root, D. R.; Smith, T. J. Inorg. Chem. 1985, 24, 2786. (7) Campbell, F. L., 111.; Cotton, F. A,; Powell, G. L. Inorg. Chem. 1985, 24, 4384. (8) Abbott, E. H.; Bose, K. S.; Cotton, F. A,; Hall, W. T.; Sekutowski, J. C. Inorg. Chem. 1978, 17, 3240. (9) Cotton, F. A,; Dunbar, K. R.; Poli, R. Inorg. Chem., in press. energy with strength (measured by M'-Mo bond length) (IO) Cotton, F. A,; Fanwick, P. E.; Fitch, J. W.; Glicksman, H. D.; Walton, R. A. J. Am. Chem. SOC. 1979, 101, 1752. 0020-1669/86/ 1325-3641$01.50/0 0 1986 American Chemical Society