REACTIVITY OF THE UNSATURATED DIHYDRIDES [MN2(MU-H)2(CO)6(MU-L2)] (L2 = PH2PCH2PPH2, (ETO)2POP(OET)2) TOWARD BORON HYDRIDES - X-RAY CRYSTAL-STRUCTURE OF [MN2(MU-BH4)(MU-H)(CO)5(MU-PH2PCH2PPH2)], A MOLECULE DISPLAYING A NOVEL COORDINATION MODE OF THE TETRAHYDROBORATE LIGAND

The unsaturated dihydrides Mn2(mu-H)2(CO)6(mu-L2) (L2 = Ph2PCH2PPh2, dppm; (EtO)2POP(OEt)2, tedip) react with BH3.THF at room temperature to give the hexacarbonyl complexes Mn2(mu-BH4)(mu-H)(CO)6(mu-L2), containing a tetrahydroborate ligand symmetrically coordinated through two of its B-H bonds. The latter species lose easily a CO ligand to give the pentacarbonyl complexes Mn2(mi-BH4)(mu-H)(CO)5(mu-L2) containing an asymmetrically coordinated tetrahydroborate ligand. The latter L2 = dppm) crystallizes in the monoclinic space group P2(1)/a (a = 12.908(2) angstrom, b = 15.735(2) angstrom c = 15.960(1) angstrom, beta = 111.54(1)-degrees, V = 3015.2(8) angstrom3, Z = 4). The structure was refined to R = 0.031 (R(w) = 0.031) for 4968 reflections with I > 3sigma(I). The tetrahydroborate ligand binds the dimetal center through three of its hydrogen atoms which thus complete distorted octahedral environments around the manganese atoms. Binding to Mn(2) is made in a rather end-on way through H(3) (B-Mn(2) = 2.480 angstrom, B-H(3)-Mn(2) 118.1(20)-degrees) whereas binding to Mn(l) is made through H(2) and H(4) in a side-on way (B-Mn(1) = 2.048(4) angstrom, B-H-Mn(1) = 87.9(16) and 89.5(17)-degrees, respectively). Reaction of the manganese dihydrides with ''B3H7'' at room temperature gave a mixture of the hexacarbonyl and pentacarbonyl tetrahydroborate complexes, the latter as major products. Finally, reaction with BEt3 in the presence of water gave the ethoxy complexes Mn2(mu-H)(mu-OEt)(CO)6(mu-L2) in good yields, although the reaction pathway remains obscure. The structure and bonding in the tetrahydroborate complexes is compared with that of boron hydrides or metal clusters by using isolobal relationships.