The unusual 1,2,7-isomer of Re2Cl5(PMe3)3 (1) has been used as a starting material to prepare previously unknown dirhenium complexes. One-electron reduction of 1 by cobaltocene followed by nonredox substitution of the resulting anionic species with PMe3 led to the formation of a triply bonded 1,2,7,8-Re2Cl4(PMe3)4 (2). In the crystal structure of 2, phosphine ligands on both metal centers exhibit a cis arrangement with a P−Re−P angle of 93.3° in contrast to the well-known type of 1,3,6,8-isomers with a trans arrangement of the monodentate phosphines connected to each metal atom. Complex 2 represents the first example of an isomer of this type in the large M2X4(PR3)4 class of compounds (M = Re, Tc, W, Mo; X = Cl, Br, I; PR3 = monodentate phosphine). The one-electron oxidation product of 1 cocrystallized with one molecule of tetrabutylammonium chloride afforded Re2Cl6(PMe3)2 (3), for which all previous synthetic attempts had failed. This quadruply bonded complex exhibits an unusual 3-fold disorder of the Re2 unit with equal populations for all three orientations. We also report that when the “classic” reaction of octachlorodirhenate anion, Re2Cl8 2-, with trimethylphosphine is carried out in benzene at room temperature the reduction processes do not occur and the product is a novel paramagnetic complex, Re2Cl6(PMe3)4 (4), which does not have a metal−metal bond (the Re−Re separation is 3.8476(4) Å). Another interesting feature of compound 4 is that the PMe3 ligands have a cis disposition at each rhenium center and are located in the same plane as the metal atoms and bridging chlorine ligands. For such a ligand arrangement the molecule of 4 is the only example of a nonmetal−metal-bonded dinuclear compound with monodentate phosphine ligands.

Reaction of the molybdenum(III) complex Mo2Cl6(THF)3 with diethylamine at room temperature causes reduction of the starting compound to produce the dimolybdenum(II) complex Mo2Cl4(NHEt2)4 (1). It was shown that the Cl- anions released by the substitution with amine can react with more of the starting material to give side products of molybdenum(III). Four such products were isolated and characterized:  [NH2Et2][MoCl4(NHEt2)2] (2), [NH2Et2][Mo2Cl7(NHEt2)2] (3), [NH2Et2]2[MoCl5(NHEt2)] (4), and [NH2Et2]3[Mo2Cl8(NEt2)] (5). The crystal structures of all complexes 1−5 have been investigated by X-ray diffraction. The structure of 1 consists of two trans-MoCl2(NHEt2)2 units joined by a quadruple Mo−Mo bond (2.1330(6) Å). The crystallographic parameters for compounds 1−5 are as follows:  for 1, monoclinic space group C2/c with a = 20.846(1) Å, b = 7.044(1) Å, c = 19.868(3) Å, β = 118.72(2)°, and Z = 4; for 2, triclinic space group P1̄ with a = 9.820(3) Å, b = 10.305(1) Å, c = 11.510(1) Å, α = 95.779(8)°, β = 97.80(2)°, γ = 111.17(1)°, and Z = 2; for 3, monoclinic space group P21/n with a = 13.847(5) Å, b = 10.448(4) Å, c = 22.091(4) Å, β = 96.68(2)°, and Z = 4; for 4, monoclinic space group P21/c with a = 11.710(2) Å, b = 12.363(2) Å, c = 30.272(7) Å, β = 95.87(2)°, and Z = 8; for 5, monoclinic space group P21/n with a = 10.335(1) Å, b = 18.061(1) Å, c = 16.775(1) Å, β = 95.558(8)°, and Z = 4.

Four new quadruply-bonded dimolybdenum(II) complexes of the formula Mo 2 Cl 4 (NH 2 R) 4 (R ) Et (1), Pr n (2), Bu t (3), Cy (4)) have been prepared in excellent yield by reduction of the dimolybdenum(III) complex Mo 2 Cl 6 -(THF) 3 with 2 equiv of sodium amalgam in the presence of the appropriate primary amine. The molecular structures of 2-4 have been investigated by X-ray crystallography. Crystal data are as follows: for 2, orthorhombic space group Ccca with a ) 13.328(3) Å, b ) 26.639(5) Å, c ) 6.774(2) Å, and Z ) 4; for 3, monoclinic space group P2 1 /c with a ) 19.165(1) Å, b ) 20.858(1) Å, c ) 14.1400(8) Å, β ) 99.002(5)°, and Z ) 8; for 4, tetragonal space group P4 2 2 1 2 with a ) 15.556(4) Å, c ) 6.9368(8) Å, and Z ) 2. All of the non-centrosymmetric molecules 2-4 possess the same structure characterized by a Mo 2 Cl 4 N 4 core with D 2d virtual symmetry and slight deviation from the eclipsed geometry. The Mo-Mo bond lengths for 2, 3, and 4 are 2.118(2), 2.1322(6), and 2.117(1) Å, respectively, which are consistent with the Mo-Mo quadruple bond. In addition to the structural data, IR, UVvis, and 1 H NMR spectroscopy have been used to characterize the complexes 1-4. Without amalgam, the reactions of the starting material with amines also produce the reduced species but with low yields (not exceeding 25%). The main products of these interactions have been found to be mononuclear molybdenum(III) complexes MoCl 3 -(NH 2 R) 3 . This has been confirmed by a single-crystal X-ray diffraction study for mer-MoCl 3 (NH 2 Pr n ) 3 ‚ 1 / 6 THF (2a‚ 1 / 6 THF) with the following crystal data: triclinic space group P1 h, a ) 12.370(2) Å, b ) 17.977(2) Å, c ) 25.498(6) Å, R ) 95.32(1)°, β ) 103.21(2)°, γ ) 103.48(1)°, and Z ) 12.

The solid-state reaction of Rh2(O2CCF3)4 with C6Me6 yields a one-dimensional copolymer [Rh2(O2CCF3)4·C6Me6]∞ (1), which constitutes the first example of a dirhodium(II,II) carboxylate complex with arene. Compound 1 has been characterized by elemental analysis and IR and mass spectroscopy, and its structure has been determined by X-ray diffraction. In the crystal structure of 1 the alternating arrangement of Rh2(O2CCF3)4 and C6Me6 units forms infinite chains extended along the [100] direction. The coordination of hexamethylbenzene to the dirhodium unit is unsymmetrical, with two opposite edges of the arene ring approaching the axial sites of the dimers. This off-centered complexation causes no distortion of the aromatic system. The closest Rh−Carene contacts (2.770(6) and 2.787(6) Å) are the longest for any axial interactions to dirhodium carboxylates known to date. At the same time, the Rh−Rh distance (2.422(1) Å) is not as short as might have been expected in the case of total absence of axial donation.

Several new paramagnetic dirhenium(II,III) anions of the formula [Re2Cl6P2]- (P2 = (PPrn 3)2 (1), (PEt2Ph)2 (2), dppp (3)) have been isolated in the form of their tetrabutylammonium salts as the kinetic products in reactions of [Re2Cl8]2- with an appropriate phosphine ligand. All three complexes have been characterized in solution by cyclic voltammetry, ESR, and mass spectroscopy. Three different isomeric forms of the [Re2Cl6P2] core have been recognized in the solid state by X-ray crystallography. The known 1,7-isomer (C 2 h ) with a centrosymmetric disposition of phosphine groups was found for PPrn 3 (1a) and PEt2Ph (2) ligands. The compound [Bun 4N][Re2Cl6(PPrn 3)2] was also crystallized in the unprecedented form of a 1,6-isomer (C 2) (1b) which can be derived from 1a by a 90° rotation of the ligands around the metal−metal bond. For the diphosphine, a C s isomer of [Re2Cl6(dppp)]- (3) was discovered having the dppp ligand chelating on one rhenium atom. Crystallographic data are as follows:  for 1a, monoclinic space group P21/n with a = 12.758(1) Å, b = 15.755(5) Å, c = 24.160(3) Å, β = 104.22(1)°, and Z = 4; for 1b, monoclinic space group P21/c with a = 16.3355(8) Å, b = 12.656(1) Å, c = 22.892(2) Å, β = 94.735(6)°, and Z = 4; for 2, triclinic space group P1̄ with a = 11.868(1) Å, b = 12.125(2) Å, c = 15.375(3) Å, α = 88.01(1)°, β = 82.64(1)°, γ = 88.47(1)°, and Z = 2; for 3, triclinic space group P1̄ with a = 11.8986(4) Å, b = 11.913(1) Å, c = 18.383(2) Å, α = 102.23(2)°, β = 108.003(5)°, γ = 97.38(1)°, and Z = 2.