Carbon monoxide and isocyanide complexes of trivalent uranium metallocenes

Organic isocyanide complexes of trivalent uranium metallocenes of the type [Cp'U-3(CNR)], where Cp' = C5H5, MeC5H4, Me3CC5H4, and Me3SiC5H4 and R = Et ; Cp' = 1,3-(Me3Si)(2)C5H3 and R = tBu; Cp' = Me4C5H and R = 4-(MeO)C6H4 an d 2,6-Me2C6H3, have been isolated. When Cp' = MeC5H4 and R = 2,6-Me2C6H3, b oth 1:1 and 1:2 adducts are obtained. The IR spectra show that <(nu)over ba r>(CN) increases slightly for the alkyl isocyanide complexes and decreases slightly for the aryl isocyanide complexes relative to (n) over bar(CN) for the free ligands. The uranium metallocenes form carbon monoxide adducts in which (n) over bar(CO) moves to lower wavenumbers upon coordination (by 15 5 to 260 cm(-1)). In only one case, however, can an adduct be isolated in c rystalline form; [(C5Me4H)(3)U(CO)]. The <(nu)over bar>(CO) stretching freq uencies lie in the order 1,3-(Me3Si)(2)C5H3 > Me3SiC5H4 > Me3CC5H4 > C5Me4H ; [(C5Me4H)(3)U] is the best p donor in this series of metallocenes. Soluti on H-1 NMR spectra show that the adducts are fluxional and, in the case of [(C5Me4H)(3)U(L)] where L = CO or 4-(MeO)C6H4NC, two fluxional processes ca n be observed: dissociation of L. at relatively high temperature and cessat ion of ring rotation at low temperature. The crystal structures of the base -free metallocenes [{1,3-(Me3Si)(2)C5H3}(3)U] and [(C5Me4H)(3)U] show that these compounds are monomeric with idealized trigonal-planar geometry. The crystal structures of the 4-(MeO)-C6H4NC and CO adducts of the latter metal locene are also described. All of the experimental studies reported herein are consistent with the view that the uranium metallocenes are better pi do nors than their cerium analogues, and the pi-donating ability is dependent upon the ring substituents.