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A series of cerium(iv) mixed-ligand guanidinate-amide complexes, {[(Me3Si)2NC(N i Pr)2] x CeIV[N(SiMe3)2]3-x }+ (x = 0-3), was prepared by chemical oxidation of the corresponding cerium(iii) complexes, where x = 1 and 2 represent novel complexes. The Ce(iv) complexes exhibited a range of intense colors, including red, black, cyan, and green. Notably, increasing the number of the guanidinate ligands from zero to three resulted in significant redshift of the absorption bands from 503 nm (2.48 eV) to 785 nm (1.58 eV) in THF. X-ray absorption near edge structure (XANES) spectra indicated increasing f occupancy (n f) with more guanidinate ligands, and revealed the multiconfigurational ground states for all Ce(iv) complexes. Cyclic voltammetry experiments demonstrated less stabilization of the Ce(iv) oxidation state with more guanidinate ligands. Moreover, the Ce(iv) tris(guanidinate) complex exhibited temperature independent paramagnetism (TIP) arising from the small energy gap between the ground- and excited states with considerable magnetic moments. Computational analysis suggested that the origin of the low energy absorption bands was a charge transfer between guanidinate π orbitals that were close in energy to the unoccupied Ce 4f orbitals. However, the incorporation of sterically hindered guanidinate ligands inhibited optimal overlaps between Ce 5d and ligand N 2p orbitals. As a result, there was an overall decrease of ligand-to-metal donation and a less stabilized Ce(iv) oxidation state, while at the same time, more of the donated electron density ended up in the 4f shell. The results indicate that incorporating guanidinate ligands into Ce(iv) complexes gives rise to intense charge transfer bands and noteworthy electronic structures, providing insights into the stabilization of tetravalent lanthanide oxidation states.

A series of uranium(VI)−acetylide complexes of the generalformula UVI(O)(CC-C6H4-R)[N(SiMe3)2]3, with variation of the parasubstituent (R = NMe2, OMe, Me, Ph, H, Cl) on the aryl(acetylide) ring, wasprepared. These compounds were analyzed by 13C NMR spectroscopy, whichshowed that the acetylide carbon bound to the uranium(VI) center, U-CC-Ar, was shifted strongly downfield, withδ(13C) values ranging from 392.1to 409.7 ppm for Cl and NMe2 substituted complexes, respectively. Theseextreme high-frequency13C resonances are attributed to large negativeparamagnetic (σpara) and relativistic spin−orbit (σSO) shielding contributions,associated with extensive U(5f) and C(2s) orbital contributions to the U−Cbonding in title complexes. The trend in the 13C chemicalshift of the terminalacetylide carbon is opposite that observed in the series of parent(aryl)acetylenes, due to shielding effects of the para substituent. The 13Cchemical shifts of the acetylide carbon instead correlate with DFT computedU−C bond lengths and corresponding QTAIM delocalization indices or Wiberg bond orders. SQUID magnetic susceptibilitymeasurements were indicative of the Van Vleck temperature independent paramagnetism (TIP) of the uranium(VI) complexes,suggesting a magnetic field-induced mixing of the singlet ground-state (f0) of the U(VI) ion with low-lying (thermallyinaccessible) paramagnetic excited states (involved also in the perturbation-theoretical treatment of the unusually largeparamagnetic and SO contributions to the 13C shifts). Thus, together with reported data, we demonstrate that the sensitive 13CNMR shifts serve as a direct, simple, and accessible measure of uranium(VI)-carbon bond covalency.