Cerocene Revisited: The Electronic Structure of and Interconversion Between Ce₂(C₈H₈)₃and Ce(C₈H₈)₂

New synthetic procedures for the preparation of Ce(cot) 2, cerocene, from [Li(thf) 4 ][Ce(cot) 2 ], and Ce 2 (cot) 3 in high yield and purity are reported. Heating solid Ce(cot) 2 yields Ce 2 (cot) 3 and COT while heating Ce 2 (cot) 3 with an excess of COT in C 6 D 6 to 65 °C over four months yields Ce(cot) 2 . The solid state magnetic susceptibility data of these three organocerium compounds show that Ce(cot) 2 behaves as a TIP (temperature independent paramagnet) over the temperature range of 5−300 K, while that of Ce 2 (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 f 1 paramagnet. The EPR spectra recorded at 1.5 K confirm that Ce 2 (cot) 3 and [Ce(cot) 2 ] − have a ground-state of M J = ± 1/2. The L III 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 Ce 2 (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 Ce 2 (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 Ce 2 (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, 4f 1 )(cot 1.5- ) 2 and Ce(IV, 4f 0 )(cot 2- ) 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.