NONREDOX REACTIONS OF THE DIRHENIUM(II) A-FRAME-LIKE MONOCARBONYL COMPLEXES RE2(MU-X)X3(MU-DPPM)2(CO) (X = CL OR BR) WITH ALKYNES - STRUCTURAL CHARACTERIZATION OF [RE2CL3(CO)(DPPM)2(ETA-2-MECCET)]PF6

The monocarbonyl complex Re2(mu-Cl)Cl3(mu-dPPM)2(CO) reacts with alkynes in the presence of TlPF6 at room temperature to afford the yellow-brown salts of composition [Re2Cl3(mu-dppm)2(CO)(eta2-R1CCR2)]PF6, where R1CCR2 = HCCH, MeCCMe, MeCCEt, HCC-n-Pr, EtCCEt, HCCPh, HCC-p-tol, or MeCCPh. The bromide complex [Re2Br3(mu-dppm)2(CO)(eta2-HCCH)]-PF6 has been prepared by a similar procedure. The identity of [Re2Cl3(mu-dppm)2(CO)(eta2-MeCCEt)]PF6 (2c) has been established by X-ray crystallography. This complex crystallizes in the space group P2(1)/n (No. 14) with a = 11.885 (2) angstrom, b = 19.702 (3) angstrom, c = 23.830 (3) angstrom, beta = 99.05 (1)-degrees, V = 5510(3) angstrom3, and Z = 4. The structure, which was refined to R = 0.029 (R(w) = 0.037) for 5069 data with I < 3.0 sigma(I), shows the presence of a eta2-2-pentyne ligand which lies perpendicular to the Re-Re vector; the Re-Re distance of 2.3407 (4) angstrom is in accord with the preservation of a Re-Re multiple bond. The structure of the cation can be represented as [(OC)Cl2Re(mu-dppm)2ReCl(eta2-MeCCEt)]+ and is the first example of the binding of an alkyne to a compound that contains an electron-rich metal-metal triple bond based upon the sigma2pi4delta2delta*2 electronic configuration.