The preparations and properties of some new four- and five-co-ordinate derivatives of tervalent thallium are described. The four-co-ordinate compounds, (C6F5)2LTlX (L=Ph3PO, Ph 3AsO, Ph3P, or Ph3As; X=Cl or Br) and (C 6F5)2(Ph3PO or Ph 3AsO)TlCO2CF3, have been prepared by reactions of the neutral ligands with the appropriate (C6F5) 2TlX compounds, and (C6F5)2Ph 3AsOT1NO3 has been obtained from the corresponding bromide and silver nitrate. The phosphine oxide and arsine oxide complexes are monomeric in benzene, but (C6F5)2(Ph 3P or Ph3As)TlX complexes dissociate, losing the neutral ligand. The complexes are non-electrolytes in acetone, but dissociate extensively into L and the corresponding (C6F5) 2TlX compounds in this solvent. The five-co-ordinate compounds, (C6F5)2L2TlNO3 (L=Ph 3PO or Ph3AsO), have been prepared similarly. They have low conductances in acetcne, but dissociate extensively into (C 6F5)2LTlNO, and L in acetone and benzene. However, infrared data show that thc crystalline compounds are not mixtures of (C6F5)2LTlNO3 and L, but contain five-coordinate thallium. The relative stabilities of the compounds, in terms of their dissociation in solution, are discussed.