Abstract Bis[diisopropylN,N-diethylcarbamylmethylenephosphonate] samarium(III) nitrate, Sm[(i-C 3H 7O) 2P(O)CH 2C(O)N(C 2H 5) 2] 2(NO 3) 3 3 and bis[diisopropyl N,N-diethylcarbamylmethylenephosphonatel]- erbium(III) nitrate monohydrate, Er[(i-C 3H 7O) 2P(O)CH 2C(O)N(C 2H 5) 2] 2(NO 3)·(H 2O) 4 have been prepared from the combination of an ethanolic solution of Sm(NO 3) 3·5H 2O or Er(NO 3) 3·5H 2O with phosphonate ligand. The resulting complexes were isolated and characterized by infrared and 1H, 13C 1H and 31P 1H NMR Spectroscopies. In addition, single crystal X-ray analyses of 3 and 4 have been completed. The structure of 3 was determined at −17°C from 7634 independent reflections obtained with a computer automated diffractometer. The complex crystallizes in the monoclinic space group C2/c with a = 25.672(10) Å, b = 11.431(5) Å, c = 18.731(5) Å, β = 133.73(2)°, Z = 4, V = 3971.5 Å 3 and ϱ calcd = 1.50 g cm −3. The structure was solved with standard heavy atom methods and blocked least squares refinement of the structure converged with R F = 5.6% and R wF = 5.7%. The crystal contains monomeric units of 3 which have crystallographic C 2 symmetry. The samarium atom is bonded to both carbonyl and phosphoryl oxygen atoms of two phosphonate ligands and the oxygen atoms of three bidentate nitrate ions. The overall coordination number of the samarium atom is ten. Several important bond distances include SmO (carbonyl) 2.433(2) Å, SmO (phosphoryl) 2.418(3) Å, SmO (nitrate) avg 2.589(5) Å, PO (phosphoryl) 1.480(2) Å and CO (carbonyl) 1.261(3) Å. The structure of 4 was determined at 25°C from 8432 independent reflections. The compound crystallizes in the triclinic space group P 1 with a = 11.590(2) Å, b = 12.321(2) Å, c = 14.840(4) Å, α = 97.40(2)°, β = 91.85(2)°, λ = 95.17(1)°, Z = 2, V = 2090.9 Å 3 and ϱ calcd = 1.48 g cm −3. The structure was solved by standard heavy atom methods and final discrepancy indices R F = 7.7% and R wF = 7.6% were obtained. The erbium atom is bonded to the phosphoryl oxygen atoms of two phosphonate ligands, the oxygen atoms of three bidentate nitrate ions and the oxygen atom of a water molecule. The carbonyl oxygen atoms are hydrogen bonded to the hydrogen atoms of the coordinated water and the erbium atom coordination number is nine. Several important bond distances include ErO (phosphoryl) aug 2.275(6) Å, ErO (nitrate) aug 2.435(8) Å, ErO (water) 2.302(7) Å, PO (phosphoryl) 1.479(6) and 1.452(7) Å and CO (carbonyl) 1.240(11) and 1.239(11) Å.