Low-melting complexes with cationic side chains – Phosphonium-, ammonium- and imidazolium-tagged coordination compounds

• Cationic metal complexes bearing cationic side chains sometimes are MeILs or exceptionally low-melting.
• The type of counterion influences the melting point of a transition metal complex.
• The melting point is also influenced by the type of cationic tag, the length of the side chain and the linking group between the tag and the complex core.

Citing 120 references, this article gives an overview of low-melting cationically tagged coordination compounds of transition metal ions. In three sections, focussing on Werner-type complexes, carbonyls and ferrocenes, ammonium-, phosphonium- and imidazolium-tagged complexes that have Tfus < 200 °C are summarised. Many of these compounds are identified as Ionic Liquids. Influences of the ligand, the metal ion and the anionic counterion on the melting point of the complexes are discussed. Together about 220 coordination compounds of about 100 ligands are described and shown in 11 figures.