Studies of covalent bonding to f transition elements

Abstract

In an effort to understand the bonding in lanthanide (Ln) and actinide (An) compounds, we have carried out a multidisciplinary study of their complexes with sulphur donor ligands. Metals studies to date: Y(III), Ln and An analogue with no available f orbitals; Ln(III), diamagnetic and paramagnetic 4f ions (Ln = LaLu); Th(IV) and UO 2+ 2, diamagnetic 5f ions. Choice of ligands: Dithiophosphinate ions, R 2PS − 2 , were chosen as the polarisable nature of the donor atoms should enhance any covalent contribution to the bonding and these ligands also contain convenient nmr probes. X-ray diffraction: Metalsulphur bonding has been established in all systems studied with coordination numbers of 6 and 8 (Ln), 8 (Ln), 8(Th) and 5 (uranyl), and the coordination polyhedra analysed. Nmr of diamagnetics: Nuclear spin-spin coupling between 89Y and 31P demonstrates the presence of a covalent interaction without f orbital participation. Important spin-spin coupling, 4J pp, through the uranyl center in the complexes UO 2(S 2PR 2) 2·L may be understood on the basis of metal-ligand π interactions. Nmr of paramagnetics: For the eight coordinate lanthanides a change in solution structure from dodecahedral (DOD) to square antiprismatic (SAP) is shown to occur at holmium. Hyperfine coupling to 31P has been measured and shown to be twice as great for DOD structures as for the SAP analogues. Its variation with the substituents at phosphorus may be understood on the basis of changes in hybridisation due to the electronic requirement of the substituents. Theoretical calculations: Preliminary results obtained from MS Xα calculations demonstrate an appreciable electron transfer form the ligands to the metal in the model compound [Ce(S 2PH 2) 4 −. The calculated electron populations resulting from this donation are 6s 0.186p 0.565d 1.034f 0.77.