Abstract: The title complex was synthesized in an aqueous solution and its crystal structure was determined by X-ray diffraction. The crystal is rhombohedral, space group R3 with a=1.9470(3) nm, b=1.9470(3) nm, c=1.7683(3) nm, α=90°, β=90°, γ=120°, V=5.805(2) nm³, Z=3 , the final Rvalue is 0.0512. The complex contains a novel tetranuclear cation with four Er³⁺ ions connected by four μ₃-OH ions and six bridging carboxyl groups. Each erbium ion is coordinated by three μ₃-OH, three carboxylic oxygen atoms from three Lproline, and two or one oxygen atoms from water molecules.

Key words: Erbium complex, L-Proline, Crystal structure, Tetranuclear complex