A growing number of metal–organic complexes are being employed as anion hosts. This tutorial review focuses on stable, mononuclear transition metal complexes with a well defined geometry that use a combination of electrostatic attraction and hydrogen bonds for anion binding. In these hosts, the metal provides positive charge and act as a scaffold that holds the ligands containing the hydrogen-bond donor groups in the appropriate positions. Issues particular to these hosts, such as the choice of the ligands, metal center and counteranion, and the stability of the host, are discussed using examples from the recent literature. This paper is addressed to readers interested in supramolecular chemistry and coordination or organometallic chemistry.

New anion hosts have been accessed using cationic organometallic fragments with sufficient kinetic stability as geometry-organizing cores, simple ditopic ligands featuring hydrogen bond donor groups, and the inert, low interacting, lipophilic BAr′₄ anion.

The behavior of the compound [RuCl(cym)(H₂biim)][BAr′₄] (cym = η⁶-para-isopropylmethylbenzene, Ar′ = 3,5-bis(trifluoromethyl)phenyl), synthesized from [{RuCl(cym)}₂(µ-Cl)₂], H₂biim and NaBAr′₄, has been studied as a receptor of anions both in solution and in the solid state.