A comprehensive review of structurally characterized rare-earth metal complexes containing anionic phosphorus ligands is presented. Since rare-earth elements form hard ions and phosphorus is considered as a soft ligand, the rare-earth metal phosphorus coordination is regarded as a less favorite combination. Three classes of phosphorus ligands, (1) the monoanionic organophosphide ligands (PR₂ ⁻) bearing one negative charge on the phosphorus atom; (2) the dianionic phosphinidene (PR²⁻) and P³⁻ ligands; and (3) the pure inorganic polyphosphide ligands (P_n ^x−), are included here. Particular attention has been paid to the synthesis, structure, and reactivity of the rare-earth metal phosphides.

The discovery of decamethyldizincocene [Zn₂(η⁵-Cp*)₂] (Cp* = C₅Me₅), the first complex containing a covalent zinc–zinc bond, by Carmona in 2004 initiated the search for this remarkable class of compounds. Low-valent organozinc complexes can either be formed by ligand substitution reactions of [Zn₂(η⁵-Cp*)₂] or by reductive coupling reactions of Zn(II) compounds. To the best of our knowledge, until now 25 low-valent Zn–Zn bonded molecular compounds stabilized by a variety of sterically demanding, very often chelating, organic ligands have been synthesized and characterized. There are two major reaction pathways of [Zn₂(η⁵-Cp*)₂]: it can either react with cleavage of the Zn–Zn bond or by ligand substitution. In addition, upon reaction with late transition metal complexes, [Zn₂(η⁵-Cp*)₂] was found to form novel intermetallic complexes with Cp*Zn and Cp*Zn₂ acting as unusual one-electron donor ligands. Very recently, the potential capability of [Zn₂(η⁵-Cp*)₂] to serve as a suitable catalyst in hydroamination reactions was demonstrated. Finally, the recent work on Cd–Cd bonded coordination compounds is reviewed.