Clear‐Cut Lanthanide( III )/Actinide( III ) Differentiation in Coordination of Pyrazine to Tris(cyclopentadienyl) Complexes of Cerium and Uranium, Involving Reversible U III → U IV Oxidation

In contrast to the [Ce(C 5 H 4 R) 3 ] complexes (where R = t Bu, SiMe 3 ), which react with pyrazine to give the Lewis base adducts [Ce(C 5 H 4 R) 3 (pyz)] (where R = t Bu 1 , SiMe 3 2 ), the uranium analogues are oxidized by the azine molecule to the dinuclear U IV complexes [{U(C 5 H 4 R) 3 } 2 (μ‐pyz)] (R = t Bu 3 , SiMe 3 4 ). The U−N distances of approximately 2.32 Å in the crystal structures of 3 and 4 are characteristic of U IV −N bond lengths of terminally‐coordinated amide groups. The formation constants of 1 and 3 in THF at 23 °C are 0.28(6) and 8(1) × 10 3 , respectively, permitting the clear differentiation between the two metallocenes by means of the nitrogen ligand. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)