The first-ever actinide nitrosyl complex has been synthesized, combining uranium, a bulky cyclopentadienyl ligand, and nitric oxide (J. Am. Chem. Soc., DOI: 10.1021/ja2096128). Lanthanide and actinide complexes are known to bind small molecules that are electronically similar to NO•, but previous attempts to form nitrosyl complexes with lanthanides and actinides produced bimetallic oxide compounds. A group led by William J. Evans and Filipp Furche of the University of California, Irvine, and Jeffrey R. Long of UC Berkeley used a uranium compound with three polyalkylcyclopentadienyl ligands that they thought would sterically block formation of bimetallic species. Reacting [C5(CH3)4H]3U with NO•, they succeeded in forming [C5(CH3)4H]3UNO. They analyzed the product using spectroscopy, magnetic susceptibility, X-ray crystallography, and density functional calculations and found that, electronically, the complex contains U4+ bound to NO– as –U–N=O in a linear geometry. That combination of electronic structure and geometry is unusual, because other linear nitrosyl complexes are formally NO+. The complex demonstrates that actinides can bind and activate small molecules in ways that aren’t observed for transition metals, Evans says.