The first metal complexes containing the 1,4-cyclohexa-2,5-dienyl ligand (benzene 1,4-dianion); synthesis and structures of [K(18-crown-6)][Ln{η 5 -C 5 H 3 (SiMe 3 ) 2 -1,3} 2 (C 6 H 6 )](Ln = La, Ce)

The reaction of [Ln{η5-C5H3(SiMe3)2-1,3}3] or [(Ln{η5-C5H3(SiMe3)2-1,3}2](Ln = La, Ce) with K or C8K and 18-crown-6 in benzene at ambient temperature yields the crystalline, deep red salts [K(18-crown-6)][Ln{η5-C5H3(SiMe3)2-1,3}2(C6H6)](Ln = La 1, Ce 2), formulated as containing the LnIII(1,4-cyclohexa-2,5=dienyl) moiety, on the basis of their X-ray structures, spectra and hydrolysis to give cyclohexa-1,4-diene.