Why are hexavalent uranium cyanides rare while U?F and U?O bonds are common and short?

Relativistic small-core pseudopotential B3LYP and CCSD(T) calculations and frozen-core PW91–PW91 studies are reported for the series UF4X2 (X=H, F, Cl, CN, NC, NCO, OCN, NCS and SCN). The bonding in UF6 is analyzed and found to have some multiple-bond character, approaching at a theoretical limit a bond order of 1.5. In addition to these σ and π orbital interactions, the electrostatic attraction is important. Evidence for π bonding in the other systems studied was also found. The triatomic pseudohalides as well as fluorine and chlorine are in this sense better ligands than cyanide. The –CN group is a σ donor and π acceptor, as uranium itself, and hence is unfit to bond to U(VI). The σ-bonded UH6 is octahedral.