The solvothermal synthetic system Ln/Sb/Se (Ln = La, Eu) was investigated in different ethylene polyamines, and a series of lanthanum and europium selenidoantimonates [La(en)(2)(dien)(eta(2)-SbSe4)] (Ia), [La(dien)(2)(mu-eta(1), eta(2)-SbSe4)] (Ib), [La(trien)(2)(H2O)]SbSe4 (Ic), [La(en)(trien)(mu-eta(1), eta(2)-SbSe4)] (Id), [Eu(en)(2)(dien)(SbSe4)] (IIa), [Eu(en)(trien)(eta(2)-SbSe4)] (IIb), and [Eu(dien)(2)(eta(2)-SbSe4)] (IIc) (en = ethylenediamine, then = diethylenetriamine, trien = triethylenetetramine) were prepared. A systematic investigation of the crystal structures showed that the soft Lewis basic ligand [SbSe4](3-) can be tuned to coordinate to the hard Lewis acidic lanthanide ions as a monodentate ligand, mono-SbSe4; a bidentate chelating ligand, eta(2)-SbSe4; or a tridentate bridging ligand, mu-eta(1), eta(2)-SbSe4, by the ethylene polyamines or mixed ethylene polyamines used in the syntheses. The [SbSe4](3-) anion exhibited different coordination modes for La3+ and Eu3+ ions in the presence of the same ethylene polyamine because of the different coordination numbers of La3+ and Eu3+ ions.