First uranium–rhodium bond shows that shorter is not stronger

Researchers in the UK have made the first two uranium–rhodium complexes and found their uranium–rhodium bonds to be among the shortest heterometallic uranium bonds ever reported.¹ 

Building on their recent success forming uranium complexes with nickel, palladium and platinum,² Polly Arnold’s group at the University of Edinburgh used a carefully designed bidentate phosphinoaryloxide ligand (Ar^PO^–) to create two distinct uranium–rhodium complexes: a tetrametallic dimer, [I₂U(OAr^P)₂RhI]₂, and a monomeric complex with three phosphinoaryloxide ligands and a bridging iodide. Although the two uranium–rhodium bonds are of similar length (2.760Å in the dimeric complex and 2.763Å in the monomeric one), electrochemical studies show that the bond stabilities are very different. 

For the dimeric complex, electrochemical studies show that the reduction is localised on the uranium centres, implying that there is no electronic communication between uranium and rhodium. In fact, the complex dissociates in solution. The monomeric complex, however, does show evidence of a weak interaction and is stable in solution.

Arnold proposes that the three phosphinoaryloxides in the monomeric complex force the filled rhodium d_z2-orbital towards the uranium centre – as seen in the previously reported complexes² – to create a weak interaction between the metals. A frontier orbital delocalised across the two metals through the bridging iodide – not seen in the dimer – then stabilises the longer uranium–rhodium bond, showing us that shorter metal–metal distances do not necessarily result in strong metal–metal bonds.