From reaction of lanthanoid metals with bis (pentafluorophenyl)mercury and 3, 5-diphenylpyrazole (HPh2pz) in 1, 2-dimethoxyethane (dme) and treatment of the products with triphenylphosphine oxide, the complexes [ Ln (॑2-Ph2pz)3(OPPh3)2].(dme)n (Ln = Er or Nd, n = 1; Ln = La, n = 0.6) have been isolated. An X-ray crystal structure shows that the erbium complex [monoclinic, space group C2, a 22.08(3), b 14.30(1), c 13.37(1) Ԇ, ॆ 124.09(7)Ŷ, Z 2 2730 'observed' data refined to R 0.046] is monomeric, with eight coordination of the metal. There is a trigonal bipyramidal arrangement of the centre (Cen) of the N-N bonds of three ॑2-3, 5-diphenylpyrazolate groups (ऱCen-Er-Cen 360Ŷ; (Er -N) 2.344 Ԇ) and the oxygens of two equivalent trans triphenylphosphine oxide ligands [O- Er -O 176(1)Ŷ; Er -O 2.207(9)Ԇ]. Unit cell data indicate that the neodymium analogue [monoclinic, space group C2, a 23.01(3), b 14.94(2), c 13.82(2) Ԇ, ॆ 123.65(8)Ŷ] is isostructural with the erbium complex. An analogous reaction of lanthanum or neodymium metal, Hg(C6F5)2, and HPh2pz in tetrahydrofuran (thf) gives [ Ln (॑2-Ph2pz)3(thf)3]. thf (Ln = La or Nd). The X-ray crystal structure of the neodymium complex [orthorhombic, space group P212121, a 14.009(9), b 16.280(8), c 22.640(16)Ԇ, Z 4, 2967 'observed' data refined to R 0.046 reveals a nine-coordinate monomer with three ॑2-Ph2pz ligands [ Nd -N 2.420(10)-2.524(8)Ԇ] and three thf ligands [ Nd -O 2.497(8)-2.587(8)Ԇ]. There is a trigonal prismatic arrangement of the centres of the N-N bonds and the oxygen donor atoms.