INTERCONVERTING EDGE-SHARING AND FACE-SHARING BIOCTAHEDRAL DINUCLEAR TUNGSTEN(III) COMPLEXES - PREPARATIONS, PROPERTIES, AND STRUCTURES OF W2CL6L4, WHERE L = PME3, PET3, AND P(N-BU)3, W2CL6L3, WHERE L = PET3 AND P(N-BU)3, AND [HPET3]+[W2CL7(PET3)2]-

In tetrahydrofuran solution the sodium salt of the [W2Cl7(THF)2]- anion and the tertiary phosphines PR3 (greater-than-or-equal-to 4 equiv) react at room temperature to give red crystalline compounds W2Cl6(PR3)4, where R = Me, Et, and n-Bu. The compounds where R = Me and Et have been characterized by single-crystal X-ray crystallography and were shown to contain an edge-shared bioctahedral (ESBO) geometry Cl2P2W(mu-Cl)2WCl2P2 with W-W = ca. 2.72 angstrom. The arrangement of the terminal ligands is such that at one W atom there are cis Cl and trans PR3 ligands while at the other W atom the order is reversed. Crystal data: for W2Cl6(PMe3)4 at -152-degrees-C, a = 11.049(2) angstrom, b = 18.004(5) angstrom, c = 13.667(4) angstrom, beta = 96.29(1)degrees, Z = 4, d(calcd) = 2.18 g cm-3, and space group P2(1)/a; for W2Cl6(PEt3)4 at -141-degrees-C, a = 11.220(2) angstrom, b = 18.462(3) angstrom, c = 18.519(3) angstrom, Z = 4, d(calcd) = 1.84 g cm-3, and space group P2(1)/n. In hydrocarbon solutions the ESBO complexes exist in equilibrium with face-shared bioctahedral (FSBO) complexes W2Cl6(PR3)3 and free PR3, where R = Et and n-Bu. Crystal data for W2Cl6(PEt3)3.CH2Cl2 at -144-degrees-C: a = 14.097(2) angstrom, b = 12.992(2) angstrom, c = 18.798(3) angstrom, beta = 97.96(1)degrees, Z = 4, d(calcd) = 1.99 g cm-3, and space group P2(1)/n. The molecule contains a central FSBO P2ClW(mu-Cl)3 WCl2P core that has a virtual mirror plane of symmetry and the NMR data obtained are consistent with this isomeric form being present in solution. The W-W distance 2.471(1) angstrom in the FSBO complex is notably shorter than those in the ESBO complexes. The equilibrium W2-Cl6(PEt3)4 reversible W2Cl6(PEt3)3 + PEt3, has been studied in toluene-d8 by NMR spectroscopy over the temperature range +22 to -15-degrees-C. From measurements of K(eq), the thermodynamic parameters were evaluated: DELTAH-degrees = -2.5 (+/- 0.5) kcal mol-1 and DELTAS-degrees = -4 (+/- 2) eu. The formation of the ESBO complex W2Cl6(PEt3)4 from the FSBO complex W2Cl6(PEt3) 3 was studied in the presence of an excess of PEt3 in the temperature range +22 to - 10-degrees-C. Under these conditions the reaction showed a first-order dependence on [PEt3]. The activation parameters, DELTAH(double dagger) = +8 (+/- 1) kcal mol-1 and DELTAS(double dagger) = -50 (+/- 8) eu, are consistent with a bimolecular reaction. An impurity often encountered in hydrocarbon solutions of W2Cl6(PEt3)n, where n = 4 or 3, was shown to be [HPEt3] + [W2Cl7(PEt3)2]-. Crystal data at -166-degrees-C: a = 12.011(4) angstrom, b = 33.54(1) angstrom, c = 16.685(6) angstrom, beta = 107.73(1)degrees, Z = 8, d(calcd) = 2.015 g cm-3, and space group P2(1)/a. In the solid state there is an enantiomeric pair of gauche-[W2Cl7(PEt3)2]- anions each weakly associated with the countercation [HPEt3]+ by hydrogen bonding. The FSBO PCl2W(mu-Cl)3 WClP2 cores and the W-W distances of 2.438(2) and 2.427(2) angstrom for the two enantiomers are very similar. A preparation of [HPEt3]+[W2Cl7(PEt3)2]- involved either (i) addition of [HPEt3]+Cl- (1 equiv) to W2Cl6(PEt3)4 in hydrocarbon solutions or (ii) the reaction between NaW2Cl7(THF)5, PEt3 (4 equiv), and H2O (2 equiv) in tetrahydrofuran. These findings are discussed in terms of the factors influencing the interconversion of d3-d3 FSBO and ESBO complexes and are comparerd with the recent studies of Cotton and Mandal [Inorg. Chem. 1992, 31, 1267).