Reduction chemistry yields stable and soluble divalent lanthanide tris(pyrazolyl)borate complexes.

Reduction of the heteroleptic Ln(III) precursors [Ln(Tp) 2 (OTf)] (Tp = hydrotris(1-pyrazolyl)borate; OTf = triflate) with either an aluminyl(I) anion or KC 8 yielded the adduct-free homoleptic Ln(II) complexes dimeric 1-Eu [{Eu(Tp)( μ-κ 1 : η 5 -Tp)} 2 ] and monomeric 1-Yb [Yb(Tp) 2 ]. Complexes 1-Ln have good solubility and stability in both non-coordinating and coordinating solvents. Reaction of 1-Ln with 2 Ph 3 PO yielded 1-Ln(OPPh3)2. All complexes are intensely coloured and 1-Eu is photoluminescent. The electronic absorption data show the 4f-5d electronic transitions in Ln(II). Single-crystal X-ray diffraction data reveal first μ-κ 1 : η 5 -coordination mode of the unsubstituted Tp ligand to lanthanides in 1-Eu.