Adventitious hydrolysis of a number of different complexes with the molecular formula Ln(Tp(Me2))(2)X [Tp(Me2) = (HB-(dmpz)(3)), where X is a basic anionic ligand] in various solvents, yielded crystals of highly insoluble dimers of the general formula [Ln(Tp(Me2))(mu-BOp(Me2))](2) (1) [Ln = La, Ce, Sm; BOp(Me2) = (HBO(dmpz)(2))(2-); dmpzH = 3,5-dimethylpyrazole]. The results of several single-crystal X-ray determinations are reported. One metal nitrogen distance, that lying across from the two negatively charged bridging oxygen atoms, is 0,06 Angstrom longer than the others, suggesting an unusual trans influence at a lanthanide center. The formation of 1 is proposed to involve the intermediacy of Ln(Tp(Me2))(2)OH formed by protonolysis with adventitious water.