Synthesis and characterization of binuclear tantalum hydride complexes

Reduction of the quadruply-bridged (2Cl, 2H) tantalum(IV) dimer, Ta2Cl6 (PMe3)4H2 (2) with sodium amalgam in glyme or THF at 0[deg]C provides deep green Ta2Cl4(PMe3)4H2 (3) in 70% yield. Dimer 3 has a D2d Ta2Cl4(PMe3)4 substructure which closely resembles that of the quadruply metal-metal-bonded dimer W2Cl4(PMe3)4. The hydride ligands of 3 are located on a diagonal plane, bridging the two tantalum atoms and the Ta-Ta separation is 2.545(1) A. 3 reacts cleanly with Cl2, HCl and H2 in diethyl ether to provide the quadruply-bridged dimers 2, Ta2Cl5(PMe3)4H3 (4), and Ta2Cl4(PMe3)4H4 (5), respectively, in high yield. Dimer 5 can also be prepared in high yield via thermolysis of the tantalum(IV) hydride TaCl2H2(PMe3)4 (6) in refluxing methylcyclohexane. The X-ray structure of 5 shows that the ([mu]-H)4 group is staggered by 45[deg] with respect to the eclipsed pyramidal TaCl2(PMe3)2 end groups. The molecular symmetry of 5 is D2d and the Ta-Ta separation is 2.511(2)A. Multiple-scattering X[alpha] calculations on the model compounds Ta2Cl4(PH3)4H2 and Ta2Cl4(PH3)4 are used to elucidate the ground-state electronic structures of 3 and 5, and to probe the question of ([mu]-H)x rotation about the metal-metal bonds in these complexes. Crystal data (at 160[deg]C) are as follows: for 3, monoclinic space group C2/c, a = 18.371(5) A, b = 9.520(3) A, c = 18.942(6) A, [beta] = 125.36(2)[deg], V = 2701.8 A3, Z = 4,dcalc. = 1.991 g cm-3; for 5, tetragonal space group P4/nbm, a = b = 12.579(2) A, c = 10.205(2) A, V = 1614.7 A3, Z = 2, dcalc. = 1.670 g cm-3.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26987/1/0000554.pd