Expanding Rare-Earth Oxidation State Chemistry to Molecular Complexes of Holmium(II) and Erbium(II)

Synthesis, Structure, and Reactivity of the Sterically Crowded Th3+ Complex (C5Me5)3Th Including Formation of the Thorium Carbonyl, [(C5Me5)3Th(CO)][BPh4]

The Th³⁺ complex, (C₅Me₅)₃Th, has been isolated despite the fact that tris(pentamethylcyclopentadienyl) complexes are highly reactive due to steric crowding and few crystallographically characterizable Th³⁺ complexes are known due to their highly reducing nature. Reaction of (C₅Me₅)₂ThMe₂ with [Et₃NH][BPh₄] produces the cationic thorium complex [(C₅Me₅)₂ThMe][BPh₄] that can be treated with KC₅Me₅ to generate (C₅Me₅)₃ThMe, 1. The methyl group on (C₅Me₅)₃ThMe can be removed with [Et₃NH][BPh₄] to form [(C₅Me₅)₃Th][BPh₄], 2, the first cationic tris(pentamethylcyclopentadienyl) metal complex, which can be reduced with KC₈ to yield (C₅Me₅)₃Th, 3. Complexes 1-3 have metrical parameters consistent with the extreme steric crowding that previously has given unusual (C₅Me₅)^- reactivity to (C₅Me₅)₃M complexes in reactions that form less crowded (C₅Me₅)₂M-containing products. However, neither sterically induced reduction nor (η¹-C₅Me₅)^- reactivity is observed for these complexes. (C₅Me₅)₃Th, which has a characteristic EPR spectrum consistent with a d¹ ground state, has the capacity for two-electron reduction via Th³⁺ and sterically induced reduction. However, it reacts with MeI to make two sterically more crowded complexes, (C₅Me₅)₃ThI, 4, and (C₅Me₅)₃ThMe, 1, rather than (C₅Me₅)₂Th(Me)I. Complex 3 also forms more crowded complexes in reactions with I₂, PhCl, and Al₂Me₆, which generate (C₅Me₅)₃ThI, (C₅Me₅)₃ThCl, and (C₅Me₅)₃ThMe, 1, respectively. The reaction of (C₅Me₅)₃Th, 3, with H₂ forms the known (C₅Me₅)₃ThH as the sole thorium-containing product. Surprisingly, (C₅Me₅)₃ThH is also observed when (C₅Me₅)₃Th is combined with 1,3,5,7-cyclooctatetraene. [(C₅Me₅)₃Th][BPh₄] reacts with tetrahydrofuran (THF) to make [(C₅Me₅)₃Th(THF)][BPh₄], 2-THF, which is the first (C₅Me₅)₃M of any kind that does not have a trigonal planar arrangement of the (C₅Me₅)^- rings. It is also the first (C₅Me₅)₃M complex that does not ring-open THF. [(C₅Me₅)₃Th][BPh₄], 2, reacts with CO to generate a product characterized as [(C₅Me₅)₃Th(CO)][BPh₄], 5, the first example of a molecular thorium carbonyl isolable at room temperature. These results have been analyzed using density functional theory calculations.

Advances in organometallic synthesis with mechanochemical methods

Solvent-based syntheses have long been normative in all areas of chemistry, although mechanochemical methods (specifically grinding and milling) have been used to good effect for decades in organic, and to a lesser but growing extent, inorganic coordination chemistry. Organometallic synthesis, in contrast, represents a relatively underdeveloped area for mechanochemical research, and the potential benefits are considerable. From access to new classes of unsolvated complexes, to control over stoichiometries that have not been observed in solution routes, mechanochemical (or 'M-chem') approaches have much to offer the synthetic chemist. It has already become clear that removing the solvent from an organometallic reaction can change reaction pathways considerably, so that prediction of the outcome is not always straightforward. This Perspective reviews recent developments in the field, and describes equipment that can be used in organometallic synthesis. Synthetic chemists are encouraged to add mechanochemical methods to their repertoire in the search for new and highly reactive metal complexes and novel types of organometallic transformations.

Expanding Thorium Hydride Chemistry Through Th²⁺, Including the Synthesis of a Mixed-Valent Th⁴⁺/Th³⁺ Hydride Complex

The reactivity of the recently discovered Th(2+) complex [K(18-crown-6)(THF)2][Cp″3Th], 1 [Cp'' = C5H3(SiMe3)2-1,3], with hydrogen reagents has been investigated and found to provide syntheses of new classes of thorium hydride compounds. Complex 1 reacts with [Et3NH][BPh4] to form the terminal Th(4+) hydride complex Cp″3ThH, 2, a reaction that formally involves a net two-electron reduction. Complex 1 also reacts in the solid state and in solution with H2 to form a mixed-valent bimetallic product, [K(18-crown-6)(Et2O)][Cp″2ThH2]2, 3, which was analyzed by X-ray crystallography, electron paramagnetic resonance and optical spectroscopy, and density functional theory. The existence of 3, which formally contains Th(3+) and Th(4+), suggested that KC8 could reduce [(C5Me5)2ThH2]2. In the presence of 18-crown-6, this reaction forms an analogous mixed-valent product formulated as [K(18-crown-6)(THF)][(C5Me5)2ThH2]2, 4. A similar complex with (C5Me4H)(1-) ligands was not obtained, but reaction of (C5Me4H)3Th with H2 in the presence of KC8 and 2.2.2-cryptand at -45 °C produced two monometallic hydride products, namely, (C5Me4H)3ThH, 5, and [K(2.2.2-cryptand)]{(C5Me4H)2[η(1):η(5)-C5Me3H(CH2)]ThH]}, 6. Complex 6 contains a metalated tetramethylcyclopentadienyl dianion, [C5Me3H(CH2)](2-), that binds in a tuck-in mode.

A novel samarium(ii) complex bearing a dianionic bis(phenolate) cyclam ligand: synthesis, structure and electron-transfer reactions

The reaction of the hexadentate dianionic 1,4,8,11-tetraazacyclotetradecane-based bis(phenolate) ligand, (tBu2ArO)2Me2-cyclam(2-), with [SmI2(thf )2] in thf resulted in the formation of the divalent samarium complex [Sm(κ(6)-{(tBu2ArO)2Me2-cyclam})] (1). X-ray diffraction studies revealed that after recrystallization from n-hexane/thf complex 1 has a monomeric structure and does not contain thf molecules coordinated to the Sm(II) center. However, UV-vis and (1)H NMR spectroscopy of 1 evidenced the formation of thf-solvated complexes in neat thf. Reductive studies show that complex 1 can act as a single electrontransfer reagent and form well-defined Sm(III) species. The reaction of 1 with several substrates, namely, TlBPh4, pyridine N-oxide, OPPh3, SPPh3 and bipyridines, are reported. Spectroscopy studies, including NMR, and single crystal X-ray diffraction data are in agreement with the formation of cationic Sm(III) species, monochalcogenide bridged Sm(III) complexes and Sm(III) complexes with bipyridine radical ligand, respectively.

Structures and Properties of the Products of the Reaction of Lanthanide Atoms with H2O: Dominance of the +II Oxidation State

The reactions of lanthanides with H2O have been studied using density functional theory with the B3LYP functional. H2O forms an initial Lewis acid-base complex with the lanthanides exothermically with interaction energies from -2 to -20 kcal/mol. For most of the Ln, formation of HLnOH is more exothermic than formation of H2LnO, HLnO + H, and LnOH + H. The reactions to produce HLnOH are exothermic from -25 to -75 kcal/mol. The formation of LnO + H2 for La and Ce is slightly more exothermic than formation of HLnOH and is less or equally exothermic for the rest of the lanthanides. The Ln in HLnOH and LnOH are in the formal +II and +I oxidation states, respectively. The Ln in H2LnO is mostly in the +III formal oxidation state with either Ln-O(-)/Ln-H(-) or Ln-(H2)(-)/Ln=O(2-) bonding interactions. A few of the H2LnO have the Ln in the +IV or mixed +III/+IV formal oxidation states with Ln=O(2-)/Ln-H(-) bonding interactions. The Ln in HLnO are generally in the +III oxidation state with the exception of Yb in the +II state. The orbital populations calculated within the natural bond orbital (NBO) analysis are consistent with the oxidation states and reaction energies. The more exothermic reactions to produce HLnOH are always associated with more backbonding from the O(H) and H characterized by more population in the 6s and 5d in Ln and the formation of a stronger Ln-O(H) bond. Overall, the calculations are consistent with the experiments in terms of reaction energies and vibrational frequencies.

Extending the Row of Lanthanide Tetrafluorides: A Combined Matrix-Isolation and Quantum-Chemical Study

Only the neutral tetrafluorides of Ce, Pr, and Tb as well as the [LnF7 ](3-) anions of Dy and Nd, with the metal in the +IV oxidation state, have been previously reported. We report our attempts to extend the row of neutral lanthanide tetrafluorides through the reaction of laser-ablated metal atoms with fluorine and their stabilization and characterization by matrix-isolation IR spectroscopy. In addition to the above three tetrafluorides, we found two new tetrafluorides, (3) NdF4 and (7) DyF4 , both of which are in the +IV oxidation state, which extends this lanthanide oxidation state to two new metals. Our experimental results are supported by quantum-chemical calculations and the role of the lanthanide oxidation state is discussed for both the LnF4 and [LnF4 ](-) species. Most of the LnF4 species are predicted to be in the +IV oxidation state and all of the [LnF4 ](-) anions are predicted to be in the +III oxidation state. The LnF4 species are predicted to be strong oxidizing agents and the LnF3 species are predicted to be moderate to strong Lewis acids.

Paving the way for the synthesis of a series of divalent actinide complexes: a theoretical perspective

Recently, the +2 formal oxidation state in soluble molecular complexes for lanthanides (La-Nd, Sm-Lu) and actinides (Th and U) has been discovered [W. J. Evans, et al., J. Am. Chem. Soc., 2011, 133, 15914; J. Am. Chem. Soc., 2012, 134, 8420; J. Am. Chem. Soc., 2013, 135, 13310; Chem. Sci., 2015, 6, 517]. To explore the nature of the bonding and stabilities of the low-valent actinide complexes, a series of divalent actinide species, [AnCp'3](-) (An[double bond, length as m-dash]Th-Am, Cp' = [η(5)-C5H4(SiMe3)](-)) have been investigated in THF solution using scalar relativistic density functional theory. The electronic structures and electron affinity properties were systematically studied to identify the interactions between the +2 actinide ions and Cp' ligands. The ground state electron configurations for the [AnCp'3](-) species are [ThCp'3](-) 6d(2), [PaCp'3](-) 5f(2)6d(1), [UCp'3](-) 5f(3)6d(1), [NpCp'3](-) 5f(5), [PuCp'3](-) 5f(6), and [AmCp'3](-) 5f(7), respectively, according to the MO analysis. The total bonding energy decreases from the Th- to the Am-complex and the electrostatic interactions mainly dominate the bonding between the actinide atom and ligands. The electron affinity analysis suggests that the reduction reaction of AnCp'3→ [AnCp'3](-) should become increasingly facile across the actinide series from Th to Am, in accord with the known An(iii/ii) reduction potentials. This work expands the knowledge on the low oxidation state chemistry of actinides, and further motivates and guides the synthesis of related low oxidation state compounds of 5f elements.

Evidence for 5d-σ and 5d-π covalency in lanthanide sesquioxides from oxygen K-edge X-ray absorption spectroscopy

The electronic structure in the complete series of stable lanthanide sesquioxides, Ln₂O₃ (Ln = La to Lu, except radioactive Pm), has been evaluated using oxygen K-edge X-ray absorption spectroscopy with a scanning transmission X-ray microscope (STXM).

Expanding the Chemistry of Molecular U(2+) Complexes: Synthesis, Characterization, and Reactivity of the {[C5 H3 (SiMe3 )2 ]3 U}(-) Anion

The synthesis of new molecular complexes of U(2+) has been pursued to make comparisons in structure, physical properties, and reactivity with the first U(2+) complex, [K(2.2.2-cryptand)][Cp'3 U], 1 (Cp'=C5 H4 SiMe3 ). Reduction of Cp''3 U [Cp''=C5 H3 (SiMe3 )2 ] with KC8 in the presence of 2.2.2-cryptand or 18-crown-6 generates [K(2.2.2-cryptand)][Cp''3 U], 2-K(crypt), or [K(18-crown-6)(THF)2 ][Cp''3 U], 2-K(18c6), respectively. The UV/Vis spectra of 2-K and 1 are similar, and they are much more intense than those of U(3+) analogues. Variable temperature magnetic susceptibility data for 1 and 2-K(crypt) reveal lower room temperature χM T values relative to the experimental values for the 5f(3) U(3+) precursors. Stability studies monitored by UV/Vis spectroscopy show that 2-K(crypt) and 2-K(18c6) have t1/2 values of 20 and 15 h at room temperature, respectively, vs. 1.5 h for 1. Complex 2-K(18c6) reacts with H2 or PhSiH3 to form the uranium hydride, [K(18-crown-6)(THF)2 ][Cp''3 UH], 3. Complexes 1 and 2-K(18c6) both reduce cyclooctatetraene to form uranocene, (C8 H8 )2 U, as well as the U(3+) byproducts [K(2.2.2-cryptand)][Cp'4 U], 4, and Cp''3 U, respectively.

Isolation of +2 rare earth metal ions with three anionic carbocyclic rings: bimetallic bis(cyclopentadienyl) reduced arene complexes of La2+ and Ce2+ are four electron reductants

A new option for stabilizing unusual Ln²⁺ ions has been identified in the reaction of Cp'₃Ln, 1-Ln (Ln = La, Ce; Cp' = C₅H₄SiMe₃), with potassium graphite (KC₈) in benzene in the presence of 2.2.2-cryptand. This generates [K(2.2.2-cryptand)]₂[(Cp'₂Ln)₂(μ-η⁶:η⁶-C₆H₆)], 2-Ln, complexes that contain La and Ce in the formal +2 oxidation state. These complexes expand the range of coordination environments known for these ions beyond the previously established examples, (Cp''₃Ln)^1- and (Cp'₃Ln)^1- (Cp'' = C₅H₃(SiMe₃)₂-1,3), and generalize the viability of using three anionic carbocyclic rings to stabilize highly reactive Ln²⁺ ions. In 2-Ln, a non-planar bridging (C₆H₆)^2- ligand shared between two metals takes the place of a cyclopentadienyl ligand in (Cp'₃Ln)^1-. The intensely colored (ε = ∼8000 M^-1 cm^-1) 2-Ln complexes react as four electron reductants with two equiv. of naphthalene to produce two equiv. of the reduced naphthalenide complex, [K(2.2.2-cryptand)][Cp'₂Ln(η⁴-C₁₀H₈)].

2,2'-Bipyridyl formation from 2-arylpyridines through bimetallic diyttrium intermediate

Formation of dianionic 2,2′-bipyridyl-bridged dinuclear yttrium complexes proceeded upon treatment of (ArNCH₂CH₂NAr)Y(CH₂SiMe₃)(THF)₂ with 2-arylpyridine, in which mononuclear (2-pyridylphenyl)yttrium complexes were detected as key intermediates.

An alkylyttrium complex supported by an N,N′-bis(2,6-diisopropylphenyl)ethylenediamido ligand, (ArNCH₂CH₂NAr)Y(CH₂SiMe₃)(THF)₂ (1, Ar = 2,6-ⁱPr₂C₆H₃), activated an ortho-phenyl C–H bond of 2-phenylpyridine (2a) to form a (2-pyridylphenyl)yttrium complex (3a) containing a five-membered metallacycle. Subsequently, a unique C(sp²)–C(sp²) coupling of 2-phenylpyridine proceeded through a bimetallic yttrium intermediate, derived from an intramolecular shift of the yttrium center to an ortho-position of the pyridine ring in 3a, to yield a bimetallic yttrium complex (4a) bridged by two-electron reduced 6,6′-diphenyl-2,2′-bipyridyl. Aryl substituents at the ortho-position of the pyridine ring were key in order to destabilize the μ,κ²-(C,N)-pyridyldiyttrium intermediate prior to the C(sp²)–C(sp²) bond formation.

Neutral "Cp-Free" Silyl-Lanthanide(II) Complexes: Synthesis, Structure, and Bonding Analysis

Complexes featuring lanthanide silicon bonds represent a research area still in its infancy. Herein, we report a series of Cp-free lanthanide (+II) complexes bearing σ-bonded silyl ligands. By reactions of LnI₂ (Ln = Yb, Eu, Sm) either with a 1,4-oligosilanyl dianion [K-Si(SiMe₃)₂SiMe₂SiMe₂Si(SiMe₃)₂-K)] (1) or with 2 (Me₃Si)₃SiK (3) the corresponding neutral metallacyclopentasilanes ({Me₂Si(Me₃Si)₂Si}₂)Ln·(THF)₄ (Ln = Yb (2a), Eu (2b), Sm (2c)), or the disilylated complexes ({Me₃Si}₃Si)₂Ln·(THF)₃ (Ln = Yb (4a), Eu (4b), Sm (4c)), were selectively obtained. Complexes 2b, 2c, 4b, and 4c represent the first examples of structurally characterized Cp-free Eu and Sm complexes with silyl ligands. In both series, a linear correlation was observed between the Ln–Si bond lengths and the covalent radii of the corresponding lanthanide metals. Density functional theory calculations were also carried out for complexes 2a–c and 4a–c to elucidate the bonding situation between the Ln(+II) centers and Si.

By reactions of LnI₂ (Ln = Yb, Eu, Sm) with oligosilanyl anions or dianions, either neutral disilylated complexes or metallacyclopentasilanes were obtained.

Rare-earth metal π-complexes of reduced arenes, alkenes, and alkynes: bonding, electronic structure, and comparison with actinides and other electropositive metals

Rare-earth metal complexes of reduced π ligands are reviewed with an emphasis on their electronic structure and bonding interactions.

Synthesis, structure, and reactivity of crystalline molecular complexes of the {[C5H3(SiMe3)2]3Th}1- anion containing thorium in the formal +2 oxidation state

Reduction of the Th3+ complex Cp''3Th, 1 [Cp'' = C5H3(SiMe3)2], with potassium graphite in THF in the presence of 2.2.2-cryptand generates [K(2.2.2-cryptand)][Cp''3Th], 2, a complex containing thorium in the formal +2 oxidation state. Reaction of 1 with KC8 in the presence of 18-crown-6 generates the analogous Th2+ compound, [K(18-crown-6)(THF)2][Cp''3Th], 3. Complexes 2 and 3 form dark green solutions in THF with ε = 23 000 M-1 cm-1, but crystallize as dichroic dark blue/red crystals. X-ray crystallography revealed that the anions in 2 and 3 have trigonal planar coordination geometries, with 2.521 and 2.525 Å Th-(Cp'' ring centroid) distances, respectively, equivalent to the 2.520 Å distance measured in 1. Density functional theory analysis of (Cp''3Th)1- is consistent with a 6d2 ground state, the first example of this transition metal electron configuration. Complex 3 reacts as a two-electron reductant with cyclooctatetraene to make Cp''2Th(C8H8), 4, and [K(18-crown-6)]Cp''.

Experimental evidence of Er³⁺ ion reduction in the radiation-induced degradation of erbium-doped silica fibers

The gain of erbium-doped fiber amplifiers is damaged by irradiation partly because of creation of color centers responsible of excess absorption at pump and signal wavelengths. Based on the combination of thermally stimulated luminescence and spectrophotometry, this Letter demonstrates that a part of the gain loss should be associated with the reduction of the density of Er3+ ions by irradiation.

Synthesis and characterization of a uranium(II) monoarene complex supported by δ backbonding

The low-temperature (<-35 °C) reduction of the trivalent uranium monoarene complex [{((Ad,Me) ArO)3 mes}U] (1), with potassium spheres in the presence of a slight excess of 2.2.2-cryptand, affords the quantitative conversion of 1 into the uranium(II) monoarene complex [K(2.2.2-crypt)][(((Ad,Me) ArO)3 mes)U] (1-K). The molecular and electronic structure of 1-K was established experimentally by single-crystal X-ray diffraction, variable-temperature (1) H NMR and X-band EPR spectroscopy, solution-state and solid-state magnetism studies, and optical absorption spectroscopy. The electronic structure of the complex was further investigated by DFT calculations. The complete body of evidence confirms that 1-K is a uranium(II) monoarene complex with a 5f (4) electronic configuration supported by δ backbonding and that the nearly reversible, room-temperature reduction observed for 1 at -2.495 V vs. Fc/Fc(+) is principally metal-centered.

Coordination and redox isomerization in the reduction of a uranium(III) monoarene complex

Synthetic studies on the redox chemistry of trivalent uranium monoarene complexes were undertaken with a complex derived from the chelating tris(aryloxide)arene ligand ((Ad,Me) ArO)3 mes(3-) . Cyclic voltammetry of [{((Ad,Me) ArO)3 mes}U(III) ] (1) revealed a nearly reversible and chemically accessible reduction at -2.495 V vs. Fc/Fc(+) -the first electrochemical evidence for a formally divalent uranium complex. Chemical reduction of 1 indicates that reduction induces coordination and redox isomerization to form a uranium(IV) hydride, and addition of a crown ether results in hydride insertion into the coordinated arene to afford uranium(IV) complexes. This stoichiometric reaction sequence provides structural insight into the mechanism of arene functionalization at diuranium inverted sandwich complexes.

Identification of the +2 oxidation state for uranium in a crystalline molecular complex, [K(2.2.2-cryptand)][(C5H4SiMe3)3U]

Flash reduction of Cp'3U (Cp' = C5H4SiMe3) in a column of potassium graphite in the presence of 2.2.2-cryptand generates crystalline [K(2.2.2-cryptand)][Cp'3U], the first isolable molecular U(2+) complex. To ensure that this was not the U(3+) hydride, [K(2.2.2-cryptand)][Cp'3UH], which could be crystallographically similar, the hydride complex was synthesized by addition of KH to Cp'3U and by reduction of H2 by the U(2+) complex and was confirmed to be a different compound. Density functional theory calculations indicate a 5f(3)6d(1) quintet ground state for the [Cp'3U](-) anion and match the observed strong transitions in its optical spectrum.

Completing the series of +2 ions for the lanthanide elements: synthesis of molecular complexes of Pr2+, Gd2+, Tb2+, and Lu2+

The first examples of crystallographically characterizable complexes of Tb(2+), Pr(2+), Gd(2+), and Lu(2+) have been isolated, which demonstrate that Ln(2+) ions are accessible in soluble molecules for all of the lanthanides except radioactive promethium. The first molecular Tb(2+) complexes have been obtained from the reaction of Cp'3Ln (Cp' = C5H4SiMe3, Ln = rare earth) with potassium in the presence of 18-crown-6 in Et2O at -35 °C under argon: [(18-crown-6)K][Cp'3Tb], {[(18-crown-6)K][Cp'3Tb]}n, and {[K(18-crown-6)]2(μ-Cp')}{Cp'3Tb}. The first complex is analogous to previously isolated Y(2+), Ho(2+), and Er(2+) complexes, the second complex shows an isomeric structural form of these Ln(2+) complexes, and the third complex shows that [(18-crown-6)K](1+) alone is not the only cation that will stabilize these reactive Ln(2+) species, a result that led to further exploration of cation variants. With 2.2.2-cryptand in place of 18-crown-6 in the Cp'3Ln/K reaction, a more stable complex of Tb(2+) was produced as well as more stable Y(2+), Ho(2+), and Er(2+) analogs: [K(2.2.2-cryptand)][Cp'3Ln]. Exploration of this 2.2.2-cryptand-based reaction with the remaining lanthanides for which Ln(2+) had not been observed in molecular species provided crystalline Pr(2+), Gd(2+), and Lu(2+) complexes. These Ln(2+) complexes, [K(2.2.2-cryptand)][Cp'3Ln] (Ln = Y, Pr, Gd, Tb, Ho, Er, Lu), all have similar UV-vis spectra and exhibit Ln-C(Cp') bond distances that are ~0.03 Å longer than those in the Ln(3+) precursors, Cp'3Ln. These data, as well as density functional theory calculations and EPR spectra, suggest that a 4f(n)5d(1) description of the electron configuration in these Ln(2+) ions is more appropriate than 4f(n+1).

Synthesis, electrochemistry, and reactivity of cerium(III/IV) methylene-bis-phenolate complexes

A series of cerium complexes containing a 2,2'-methylenebis(6-tert-butyl-4-methylphenolate) (MBP(2-)) ligand framework is described. Electrochemical studies of the compound [Li(THF)2Ce(MBP)2(THF)2] (1) reveal that the metal based oxidation wave occurs at -0.93 V vs Fc/Fc(+). This potential demonstrates significant stabilization of the cerium(IV) ion in the MBP(2-) framework with a shift of ∼2.25 V from the typically reported value for the cerium(III/IV) couple of E°' = +1.30 V vs Fc/Fc(+) for Ce(ClO4)3 in HClO4 solutions. Compound 1 undergoes oxidation to form stable cerium(IV) species in the presence of a variety of common oxidants. The coordination of the redox-active ligands 2,2'-bipyridine and benzophenone to 1 result in complexes in which no apparent metal-to-ligand charge transfer occurs and the cerium ion remains in the +3 oxidation state.