Lanthanide bis((cyclooctane- 1,5-diyl)dihydroborate) complexes (THF)(4)Ln{(mu-H)(2)BC8H14}(2) (Ln = Eu, 1; Yb, 2) were synthesized by a metathesis reaction between (THF)(x)LnCl(2) and K[H2BC8H14] in THF in a 1:2 molar ratio. Attempts to prepare the monosubstituted lanthanide cyclic organohydroborates (THF)(x)LnCl{(mu-H)(2)BC8H14} were unsuccessful. On the basis of the molecular structure and IR spectrum of 1, there is an agostic interaction between Eu(II) and one of the alpha-C-H hydrogens from the {(mu-H)(2)BC8H14} unit. No such interaction was observed for 2. The coordinated THF in 1 and 2 can be removed under dynamic vacuum, but the solvent ligands remain bound to Yb when 2 is directly dissolved in Et2O or toluene. In strong Lewis basic solvents, such as pyridine or CH3CN, attack of the Yb-H-B bridge bonds results. Decomposition of 2 to the 9-BBN dimer in CD2Cl2 was observed by B-11 and H-1 NMR spectroscopies. Compound 2 was reacted with 2 equiv of the hydride ion abstracting reagent B(C6F5)(3) to afford the solvent-separated ion pair [Yb(THF)(6)][HB(C6F5)(3)](2) (3). Complexes 1, 2, and 3 were characterized by single-crystal X-ray diffraction analysis. Crystal data: 1 is orthorhombic, Pna2(1), a = 21.975(1) Angstrom, b = 9.310(1) Angstrom, c = 16.816(1) Angstrom, Z = 4; 2 is triclinic, P1, a = 9.862(1) Angstrom, b = 10.227(1) Angstrom, c = 10.476(1) Angstrom, alpha = 69.87(1)degrees, beta = 76.63(1)degrees, gamma = 66.12(1)degrees, Z = 1; 3(.)Et(2)O is triclinic, P1, a = 13.708(1) Angstrom, b = 14.946(1) Angstrom, c = 17.177(1) Angstrom, alpha = 81.01(1)degrees, beta = 88.32(1)degrees, gamma = 88.54(1)degrees, Z = 2.