Selected lanthanocene anions M(C8H8)(2)(-) (M = Ce, Nd, Tb, Yb) and their neutral counterparts have been investigated with large-scale complete-active-space self-consistent field calculations followed by multireference configuration interaction and averaged coupled-pair functional calculations. Relativistic effects were included by means of energy-consistent ab initio pseudopotentials. It is shown that, in contrast to the neutral actinocenes (e.g., M = Th, U), all corresponding lanthanocenes are best described as resulting from trivalent rather than tetravalent central atoms. In addition, the independent-particle model fails to predict the unusual electronic ground states of the Ce, Nd, and Tb systems. The electron affinities of the still hypothetical lanthanocenes beyond the well-known cerocene are found to be almost independent of the central metals and to be substantially higher than those of the corresponding actinocenes. The differences in the electronic structure between lanthanocenes and actinocenes can be explained by the different impacts of relativistic effects, especially on the metal d and f orbitals.