4] First example of a discrete pure metal sulfide cluster: [Mo,S,,]'-, A. Miiller, S. Pohl, M. Dartmann, J. P. Cohen, J. M. Bennett, R. M.Kirchner, 2. Naturforsrh. B34 (1979) 434; of a polythiometallate: [W,S,]'-: E. Koniger-Ahlborn, A. Miiller, Angew. Chem. 87 (1975) 598; Angew. Chem. Znt. Ed. Engl. 14(1975) 573:A. Miiller, H. Bogge, E. Krickemeyer, G. Henkel, B. Krebs, 2. Nuturforsrh. 8 3 7 (1982) 1014; of a heterometal thioanion: [Ni(WS,),]'': A. Miiller, E. Diemann, J. Chem. Sor. Chem. Commun. (1971) 65.[5] l a : P i ; a =713.4(1), b = 925.6(2), c = 984.1 (2) pm, I = 87.31 (2). p = 84.83(2),y=70.57(1)", V = 6 1 0 . 1 x 1 0 6 p m 3 , Z = 1 , p , , , d = 2 . 7 0 g c m~3 , R = 0.035 for 2485 independent reflections (Fo > 4u(F0)). 1 b: C2/c; a=t379.1(3), b = 1378.1(4), c = 1452.1(4)pm, p=114.07(2)". V = 2519.7 x lo6 pm3, 2 = 4, pCsld = 2.62 g~m -~, R = 0.058 for 2380independent reflections (Fo > 4a(F0)). Further details of the crystal structure investigation of 1 a, 1 b. and 2 are available on request from the Fachinformationszentrum Karlsruhe, Gesellschaft fur wissenschaftlich-technische Information mbH, W-7514 Eggenstein-Leopoldshafen 2 (FRG), on quoting the depository number CSD-55 361, the names of the authors and the journal citation. The measured (Cu,Jpowderdiffraction diagram of 1 (ca. 75 YO 1 a and ca. 25 % 1 b) is in good agreement as regards reflection sequence and intensities with that calculated for 1 a and 1 b from data of the X-ray structure analysis (most intense reflections corresponding to interplanar distances d of 980, 930, 873, 670, 663, 645, 637, 631, 593, and 575 pm). Chimia 40 (1986) 50 and unpublished results.[8] The relatively short distance between the two ReV centers (electron configuration d2) (263.6(1) pm in l a and 263.2(1) pm in 1 b) points, according to the argument put forward by Hoffmann et al. and Cotton [6], to the presence of a formal M=M double bond. Example: [W$,(dtc),] with a W-W distance of 253.0(2) pm; cf. A. Bino, E A. Cotton, 2. Dori, J. C. Sekutowski, Inorg. Chem. 17 (1978) 2946; cf. also L. Wei, T. R. Halbert, H. H. Murray 111. E. I. Stiefel, J Am. Chem. SOC. 112 (1990) 6431 as well as F. A. Cotton, R. A. Wolton: Multiple Bonds between Metal Atoms, Wiley, New York 1982, p. 290.[9] a) Cf., e.g., Hartree-Fock-Slater orbital energies: S-3p: 10.3; M o 4 d : 7.2; W-5d: 9.3; Re-5d: 10.6 eV (J. J. Yeh, I. Lindau, At. Data Nucl. Data Tables 32 (1985) 1); b) X, calculations were carried out in spin-unpolarized form using the XASW program by M. Cook and D. A. Case (D. A. Case, Annu. Rev. Phys. Chem. 33 (1982) 151; Quantum Chemistry Program Exchange, Bloomington, IN 47405 (USA), Program 465). In contrast to the markedly localized Re-Re u-bonding orbital (€(a) = -7.7 eV with 61 % Re contribution) the expected Re-Re z-bonding orbital splits significantly due to strong Re-S interactions (E(n,) = -8.1 eV with 35 % Re contribution and E(n,) =-5.0 eV with 34 % Re contribution, whereby the energetically destabilized orbital (z2) lies at almost the same energy with the Re-Re d* an...