1,4-BIS(TRIMETHYLSILYL)BUT-2-ENE-1,4-DIYL COMPLEXES OF LITHIUM, MAGNESIUM, ALUMINUM, AND GALLIUM

(Z)-1,4-Dilithio-1,4-bis(trimethylsilyl) has been prepared as a THF (t etrahydrofuran) adduct, [{(CH(Me(3)Si)CH)(2)Li-2(THF)(2)}(2)] (4), via demetalation of the (E)-but-2-ene precursor using n-BuLi in THF and b y reduction of (E,E)-1,4-bis(trimethylsilyl)buta-1,3-diene by lithium in THF. The complex is dimeric in the solid state with two lithium ato ms bridging the (Z)-but-2-ene-1,4-diyl units; the other two lithium at oms are external to the dianion moieties and are solvated by two THF m olecules, forming a Li(dianion)Li-2(dianion)Li inverted double-decker sandwich structure. Treatment of the (Z)-1,4-dilithio-1,4-bis(trimethy lsilyl)but-2-ene adduct of TMEDA (double bond N,N,N',N'-tetramethyleth ylenediamine), [(CH(Me(3)Si)CH)(2){Li(TMEDA)}(2)] (3), with (a) MgCl2 or i-PrMgCl in diethyl ether results in formation of the TMEDA adduct of the magnesacyclopent-3-ene, isolated as [{meso-(CH(Me(3)Si)CH)(2)Mg }(TMEDA)] (5) or (b) MeAlCl(2), Et(2)AlCl, or GaCl3 in diethyl ether r esults in stereospecific formation of the corresponding metallacyclope nt-3-enes, isolated as [{meso-HC(Me(3)Si)HC)(2)MR}(2)(TMEDA)] (M = Al, R = Me (6a), Et (6b); M = Ga, R = Cl (7)). Assignment of the meso iso mers for 5-7 is based on NMR data and X-ray crystal structure determin ation for 5 6b, and 7. Crystals of 4 are monoclinic, space group P2(1) /n (No. 14), with a = 9.874(5) Angstrom, b = 19.691(5) Angstrom, c = 1 2.563(7) Angstrom, beta = 94.46(3)degrees, V = 2435(2) Angstrom(3), an d Z = 2. Crystals of 5 are monoclinic, space group P2(1)/c (No. 14), w ith a = 13.535(5) Angstrom, b = 12.464(5) Angstrom, c = 16.128(7) Angs trom, beta = 101.69(3)degrees, V = 2660(10) Angstrom(3), and Z = 4. Cr ystals of 6b are orthorhombic, space group Pbca (No. 61), with a = 17. 689(2) Angstrom, b = 16.743(2) Angstrom, c = 14.091(5) Angstrom, V = 4 173(1) Angstrom(3), and Z = 4. Crystals of 7 are monoclinic, space gro up P2(1)/n (No. 14), with a = 14.118(5) Angstrom, b = 17.309(2) Angstr om, c = 16.000(6) Angstrom, beta = 91.96(2)degrees, V = 3908(2) Angstr om(3), and Z = 4.