Multinuclear NMR spectroscopy of the tetrahedral uranium(IV) complex tetrakis(trihydromethylborato)uranium

Abstract

The temperature dependence of the 1H, nB, and 13C NMR spectra of Td U(BH3CH3)4 in solution is reported. The paramagnetic shifts are interpreted as originating purely from spin delocalization mechanisms with no contribution from the metal-orbital dipolar interaction. It is shown that the temperature dependence of both *H shifts (bridging and terminal protons) is identical with that calculated from a polarization theory which assumes the shift is proportional to the average value of electron spin in the inner 5f orbitals. The proportionality constant is -5.64 MHz for the bridging protons and -0.59 MHz for the terminal protons. The temperature dependences of "B and 13C shifts are found to depart significantly from that predicted by the polarization theory with the largest deviations shown by the llB shifts. It is shown how those deviations can be accounted for by postulating a second spin delocalization through direct covalency involving molecular orbitals formed from the uranium 5f orbitals and ligand s and p orbitals.

Synthesis, Structure, and Reactions of Hydride, Borohydride, and Aluminohydride Compounds of the f-Elements

“…Treatment of U(BH 4 ) 4 with excess BMe 3 gave the methyl derivative U(BH 3 Me) 4 . However, the complexes An(BH 3 Me) 4 (An = Th, U, Np, and Pa) were best synthesized by reacting AnCl 4 or PaCl 5 with LiBH 3 Me (eq 35) …”