Treatment of the trichlorides EuCl3 and YbCl3 with Na(H3BNMe2BH3) in tetrahydrofuran (THF) results in a reduction to the corresponding divalent europium and ytterbium N,N-dimethylaminodiboranate (DMADB) complexes Eu(H3BNMe2BH3)(2)(THF)(2) (1) and Yb(H3BNMe2BH3)(2)(THF)(2) (2), which can be separated from trivalent Ln(H3BNMe2BH3)(3)(THF) byproducts by extraction and crystallization from pentane. No other lanthanide trihalides react with Na(H3BNMe2BH3) to afford divalent products. Compounds 1 and 2 can also be prepared from the divalent lanthanide iodides Eul(2) and Ybl(2) in higher yield and without the need to separate them from trivalent species. Treatment of 1 and 2 with an excess of 1,2-dimethoxyethane (DME) in pentane affords the new species Eu(H3BNMe2BH3)(2)(DME)(2) (3) and Yb(H3BNMe2BH3)(2)(DME) (4). Compound 1 is dinuclear: each metal center is bound to two chelating DMADB ligands, one of which also bridges to the other metal. Overall, the coordination geometry about each Eu atom can be described as a distorted pentagonal bipyramid, with five B atoms from the DMADB ligands occupying the equatorial sites and two THF molecules occupying the axial sites. Unlike 1, compound 2 is monomeric owing to the smaller radius of Yb-II versus Eu-II; the B and O atoms describe a distorted cis octahedron. The Eu(DME) complex 3 is also monomeric; both DMADB ligands and both DME molecules chelate to the metal center. The four B atoms and the four O atoms describe a distorted square antiprism, with the O atoms occupying one square face and the B atoms occupying the other. In addition to X-ray crystallographic studies, IR, NMR, and mass spectrometric data are reported for all four new compounds.