Five series of novel mono(cyclooctatetraenyl) lanthanide(III) complexes have been synthesized and characterized. The organolanthanides (COT)Ln(HBpz3) (1), (COT)Ln[HB(3,5-Me2PZ)3] (2), (COT)Ln[CpCo(P(O)(OEt)2}3](3), (COT)Ln[Ph2P(NSiMe3)2](THF) (4) and (COT)Ln[MeOC6H4C(NSiMe3)2])THF) (5) (Ln = Ce, Pr, Nd, Sm) are readily accessible through metathetical reactions of [(COT)Ln(mu-CI)(THF)2]2 With the appropriate anionic ligands (COT = eta8-cyclooctatetraenyl, Cp = eta5-cyclopentadienyl, pz = pyrazol-1-yl).