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Significant orbital mixing is rare in lanthanide complexes because of the limited radial extent of the 4f orbitals, which results in a generally small stabilization due to 4f orbital interactions. Nevertheless, even a small amount of additional stabilization could enhance lanthanide separations. One lanthanide complex in which orbital mixing has been extensively studied both experimentally and computationally is cerocene, COT2Ce, where COT is cyclooctatetraene. This compound has a singlet ground state with a low-lying, triplet excited state. Previous fluorescence studies on trimethylsilyl-substituted cerocenes indicate the triplet state is 0.4 eV higher in energy than the singlet state. In addition, computational studies predict that the triplet is 0.3 to 1 eV higher in energy than the singlet. The synthesis of highly pure COT2Ce by Walter and Andersen allowed its physical properties to be accurately measured. Using these measurements, we evaluate the stabilization of the 4f orbitals using two, independent approaches. A Hubbard model is used to evaluate the stabilization of the ground state due to orbital mixing. This stabilization, which is also the singlet-triplet gap, is -0.29 eV using this model. This gap was also from the temperature independent paramagnetism of COT2Ce, which yielded a value of -0.32 eV.

Significant orbital mixing is rare in lanthanide complexes because of the limited radial extent of the 4f orbitals, which results in a generally small stabilization due to 4f orbital interactions. Nevertheless, even a small amount of additional stabilization could enhance lanthanide separations. One lanthanide complex in which orbital mixing has been extensively studied both experimentally and computationally is cerocene, COT2Ce, where COT is cyclooctatetraene. This compound has a singlet ground state with a low-lying, triplet excited state. Previous fluorescence studies on trimethylsilyl-substituted cerocenes indicate the triplet state is 0.4 eV higher in energy than the singlet state. In addition, computational studies predict that the triplet is 0.3 to 1 eV higher in energy than the singlet. The synthesis of highly pure COT2Ce by Walter and Andersen allowed its physical properties to be accurately measured. Using these measurements, we evaluate the stabilization of the 4f orbitals using two, independent approaches. A Hubbard model is used to evaluate the stabilization of the ground state due to orbital mixing. This stabilization, which is also the singlet-triplet gap, is -0.29 eV using this model. This gap was also from the temperature independent paramagnetism of COT2Ce, which yielded a value of -0.32 eV.

New synthetic procedures for the preparation of Ce(cot)2, cerocene, from [Li(thf)4][Ce(cot)2], and Ce2(cot)3 in high yield and purity are reported. Heating solid Ce(cot)2 yields Ce2(cot)3 and COT while heating Ce2(cot)3 with an excess of COT in C6D6 to 65oC over four months yields Ce(cot)2. The solid state magnetic susceptibility of these three organocerium compounds shows that Ce(cot)2 behaves as a TIP (temperature independent paramagnet) over the temperature range of 5-300 K, while that of Ce2(cot)3 shows that the spin carriers are antiferromagnetically coupled below 10 K; above 10 K, the individual spins are uncorrelated, and [Ce(cot)2]- behaves as an isolated f1 paramagnet. The EPR at 1.5K for Ce2(cot)3 and [Ce(cot)2]- have ground state of MJ= +- 1/2. The LIII edge XANES of Ce(cot)2 (Booth, C.H.; Walter, M.D.; Daniel, M.; Lukens, W.W., Andersen, R.A., Phys. Rev. Lett. 2005, 95, 267202) and 2Ce2(cot)3 over 30-500 K are reported; the Ce(cot)2 XANES spectra show Ce(III) and Ce(IV) signatures up to a temperature of approximately 500 K, whereupon the Ce(IV) signature disappears, consistent with the thermal behavior observed in the melting experiment. The EXAFS of Ce(cot)2 and Ce2(cot)3 are reported at 30 K; the agreement between the molecular parameters for Ce(cot)2 derived from EXAFS and single crystal X-ray diffraction data are excellent. In the case of Ce2(cot)3 no X-ray diffraction data are known to exist, but the EXAFS are consistent with a "triple-decker" sandwich structure. A molecular rationalization is presented for the electronic structure of cerocene having a multiconfiguration ground state that is an admixture of the two configurations Ce(III, 4f1)(cot1.5-)2 and Ce(IV, 4f0)(cot2-)2; the multiconfigurational ground state has profound effects on the magnetic properties and on the nature of the chemical bond in cerocene and, perhaps, other molecules.

The complexes [Ce(tmtaa)2], [Ce(tmtaa)(tmtaaH)] and [Ce2(tmtaa)3(thf)2] are obtained from Ce[N(SiMe3)2]3 and tmtaaH2, the macrocyclic ligand 6,8,15,17-tetramethyldibenzotetraaza[14]-annulene, depending on the stoichiometry, solvent and temperature. The crystal structure of Ce(tmtaa)2 is isostructural with Zr(tmtaa)2, however magnetic susceptibility measurements in the range 5-300 K show that Ce(tmtaa)2 is not diamagnetic, but is a temperature-independent paramagnet (TIP), similar to Ce(cot)2, cerocene.

The paramagnetic 1:1 coordination complexes of (C5Me5)2Yb with a series of 4,4'-disubstituted bipyridines, bipy-X, where X is Me, tert-Bu, OMe, Ph, CO2Me, and CO2Et have been prepared. All of the complexes are paramagnetic and the values of the magnetic susceptibility as a function of temperature show that these values are less than expected for the cation, [(C5Me5)2Yb(III)(bipy-X)]+, which have been isolated as the cation-anion ion-pairs [(C5Me5)2Yb(III)(bipy-X)]+[(C5Me5)2YbI2]f fnfn where X is CO2Et, OMe and Me. The 1H NMR chemical shifts (293 K) for the methine resonances located at the 6,6' site in the bipy-X ring show a linear relationship with the values of chiT (300 K) for the neutral complexes which illustrates that the molecular behavior does not depend upon the phase with one exception, viz., (C5Me5)2Yb(bipy-Me). Single crystals of the 4,4'-dimethylbipyridine complex undergo an irreversible, abrupt first order phase change at 228 K that shatters the single crystals. The magnetic susceptibility, represented in a delta vs. T plot, on this complex, in polycrystalline form undergoes reversible abrupt changes in the temperature regime 205 - 212 K, which is suggested to be due to the way the individual molec ular units pack in the unit cell. A qualitative model is proposed that accounts for the sub-normal magnetic moments in these ytterbocene-bipyridine complexes.

Reduction of the uranium metallocene, [eta5-1,2,4-(Me3C)3C5H2]2UCl2 (1), Cp'2UCl2, in the presence of 2,2'-bipyridyl and sodium naphthalene gives the dark green metallocene complex, Cp'2U(bipy) (6), which reacts with p-tolylazide or pyridine-N-oxide to give Cp'2U=N(p-tolyl) (7) or Cp'2U(O)(py) (8), respectively. The Lewis acid, BPh3, precipitates Ph3B(py) and gives the base-free oxo, Cp'2UO (10), which crystallizes from pentane. The oxometallocene 10 behaves as a nucleophile with Me3SiX reagents but it does not exhibit cycloaddition behavior with acetylenes, suggesting that the polar resonance structure, Cp'2U+-O- dominates the double bond resonance structure Cp'2U=O.