The preparation, properties, and n.m.r. spectra of 1 : 1 triphenylphosphine oxide adducts of [M{N(SiMe₃)₂}₃](M = La Eu, or Lu) and dimeric peroxo-species [M₂{N(SiMe₃)₂}₄(O₂)(PPh₃O)₂](M = La, Pr, Sm, Eu, or Lu) are described. The crystal and molecular structures of [La{N(SiMe₃)₂}₃(PPh₃O)](1) and [La₂{N(SiMe₃)₂}₄(O₂)(PPh₃O)₂](2) have been determined bysingle-crystal X-ray diffraction methods from data measured on a manual diffractometer and refined by least-squares to R 0.116 for 2 770 observed reflections for (1) and to R 0.113 for 2 153 data for (2). Both species crystallise in space group P with cell parameters: (1); a= 19.92, b= 12.64, c= 12.48 Å, α= 120.5, β= 87.3, γ= 102.7°, Z= 2; (2); a= 13.55, b= 18.54, c= 12.54 Åα= 90.8, β= 121.7, γ= 115.3°, Z= 1. Complex (1) is monomeric and the lanthanum atom has slightly distorted L_aN₃O tetrahedral geometry. The La–O–P unit is almost linear, with La–O 2.39(2)Å; La–N distances are 2.38(2)–2.41(2)Å. Complex (2) is a peroxo-bridged centrosymmetric dimer in which the peroxo-function acts as a symmetrical doubly bidentate bridge linking two La{N(SiMe₃)₂}₂(PPh₃O) units. The metal atom can again be considered to have distorted tetrahedral co-ordination if the O₂ function is assumed to occupy one co-ordination site. The La–O(O₂) distances are both 2.33(3)Å, whilst La–O(PPh₃O) is 2.42(2) and La–N 2.37(2) and 2.49(3)Å. The O–O separation is 1.65(4)Å, but this may be artificially lengthened by the effects of crystal decomposition.