Nido-metalloborane complexes: Synthesis and structural characterization of μ₂,η ⁴-hexahydrodiboratotetrakis(N,N′ -diarylformamidinato) ditantalum(III), aryl = p-tolyl and phenyl. The first structurally characterized complexes containing the μ₂,η ⁴-B₂H₆^2- ligand

The nido-metalloborane complexes Ta₂(μ₂η⁴,-B₂H ₆)(DTolF)₄·2Et₂O, 1, DToIF^- = [(p-tolyl)NCHN(ptolyl)]^-, and Ta₂(μ₂η⁴-B₂H ₆)(DPhF)₄·Et₂O, 2, DPhF^- = [phenylNCHNphenyl]^-, are formed as the products of tantalum pentachloride with lithium borohydride followed by the addition of LiDTolF or LiDPhF. Complexes 1 and 2 are the first structurally characterized compounds in which two metal atoms that are bonded to each other are also bridged by an hexahydroborate ligand through four B-H-M three-center bonds in a symmetric bonding mode. The Ta-Ta distances are 2.725(1) Å in 1 and 2.728(1), 2.736(1) Å in 2. Crystallographic data are as follows: compound 1, P1, a = 15.197(3) Å, b = 15.675(2) Å, c = 16.449(6) Å, α = 102.50(1)°, β = 99.04(1)°, γ= 117.10-(2)°, and Z = 2; compound 2, P1 with a = 11.547(5) Å, b = 20.03(1) Å, c = 24.934(8) Å, α = 107.60(1)°, β= 100.86(2)°, γ= 95.65(2)°, and Z = 4. In these molecules there are two ArNC(H)NAr^- ligand corrdinated in a chelating manner to each of two tantalum atoms. These two Ta(DToIF)₂ or Ta(DPhF)₂ fragments are united by a Ta=Ta bond (ca. 2.73 Å), and a B₂H₆ unit lies across the bond forming two B-H-Ta bridge bonds to each tantalum atom. The B₂H₆Ta₂ moiety constitutes a B₂Ta₂ nido-metalloborane, formally similar to B₄H₁₀.