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A series of lanthanide complexes bearing organic radical ligands, [Ln(Cp)(bipy·)] [Ln = La, Cp = Cp (1); Ln = Ce, Cp = Cp (2); Ln = Ce, Cp = Cp″ (3); Ln = Ce, Cp = Cp‴ (4)] [Cp = {CHBu-1,3}; Cp″ = {CH(SiMe)-1,3}; Cp‴ = {CH(SiMe)-1,2,4}; bipy = 2,2'-bipyridyl], were prepared by reduction of [Ln(Cp)(μ-I)] or [Ce(Cp‴)(I) (THF)] with KC in the presence of bipy (THF = tetrahydrofuran). Complexes 1-4 were thoroughly characterized by structural, spectroscopic, and computational methods, together with magnetism and cyclic voltammetry, to define an unambiguous Ln(III)/bipy· radical formulation. These complexes can act as selective reducing agents; for example, the reaction of 3 with benzophenone gives [{Ce(Cp")(bipy)}{κ-O,O'-OPhC(CH)CPhO}] (7), a rare example of a "head-to-tail" coupling product. We estimate the intramolecular exchange coupling for 2-4 using multiconfigurational and spin Hamiltonian methods and find that the commonly used Lines-type isotropic exchange is not appropriate, even for single 4f e/organic radical pairs.

The substituted cyclopentadienyl group 1 transfer agents KCp 00 , KCp 0 0 0 and KCp tt (Cp 00 = {C 5 H 3 (SiMe 3 ) 2 -1,3} À ; Cp 0 0 0 = {C 5 H 2 (SiMe 3 ) 3 -1,2,4} À ; Cp tt = {C 5 H 3 ( t Bu) 2 -1,3} À ) were prepared by modification of established procedures and the structure of [K(Cp 00 )(THF)] N ÁTHF (1) was obtained. KCp 00 and KCp tt were reacted variously with [Ln(I) 3 (THF) 4 ] (Ln = La, Ce) in 2 : 1 stoichiometries to afford monomeric [La(Cp 00 ) 2 (I)(THF)] (2aÁTHF) and the dimeric complexes [La(Cp 00 ) 2 (m-I)] 2 (2a), [Ce(Cp 00 ) 2 (m-I)] 2 (2b) and [Ce(Cp tt ) 2 (m-I)] 2 (3). KCp 0 0 0 was reacted with [Ce(I) 3 (THF) 4 ] to afford the mono-ring complex [Ce(Cp 0 0 0 )(I) 2 (THF) 2 ] (4), regardless of the stoichiometric ratio of the reagents. Complex 4 was reacted with [KN(SiMe 3 ) 2 ] to yield [Ce(Cp 0 0 0 ) 2 (I)(THF)] (5), [Ce(Cp 0 0 0 ){N(SiMe 3 ) 2 } 2 ] (6) and [Ce{N(SiMe 3 ) 2 } 3 ] by ligand scrambling. Complexes 1-6 have all been structurally authenticated and are variously characterised by other physical methods.

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