Lanthanides are widely assumed not to form covalent bonds due to the localized nature of their 4f valence electrons. This work demonstrates that the ionic bond of Sm(II) with cyclononatetraenyl (η⁹-C(9)H(9)^(-)) in [Sm(η⁹-C(9)H(9))(2)] can be modulated and becomes more covalent by photon-induced transfer of Sm 4f electrons to Sm 5d orbitals. This photon-induced change in bonding properties facilitates a subsequent reconfiguration of [Sm(η(9)-C(9)H(9))(2)]. As a result, Sm-C bond …