Isolation of reactive Ln(ii) complexes with C5H4Me ligands (CpMe) using inverse sandwich countercations: synthesis and structure of [(18-crown-6)K(μ-CpMe)K(18-crown-6)][CpMe3LnII] (Ln = Tb, Ho) (Dedicated to Professor Geoffrey Cloke for enlivening organometallic chemistry by his always innovative contributions and for his groundbreaking discoveries using metal vapor chemistry.)

Although attempts to make the Y(ii) complex, (CpMe3Y)1− (CpMe = C5H4Me), by reduction of CpMe3Y with potassium were unsuccessful and the products of potassium reduction of CpMe3Ln for Ln = La and Pr led to ring-opening reduction of THF, we report that crystallographically-characterizable Ln(ii) complexes of Tb and Ho can be isolated by reducing CpMe3Ln(THF) with KC8 in THF in the presence of 18-crown-6 (18-c-6). X-ray crystallography revealed that these complexes are isolated with a methylcyclopentadienide inverse sandwich countercation: [(18-c-6)K(μ-CpMe)K(18-c-6)][CpMe3Ln] (Ln = Tb, Ho). Although reduction of CpMe3Ln with potassium in the presence of the 2.2.2-cryptand (crypt) chelate has not generally provided fully-characterizable, crystalline products, in the case of Dy, crystals of [K(crypt)][CpMe3Dy] could be isolated.