The Kläui ligand is the anion {(C<sub>5</sub>H<sub>5</sub>)Co[(CH<sub>3</sub>O)<sub>2</sub>PO]<sub>3</sub>}<sup>âÂÂ</sup>. The ligand, popularized by Wolfgang Kläui, binds metals and metalloids via a facial O<sub>3</sub> donor set. Related tridentate and tripodal anionic ligands include trispyrazolylborates.
The ligand is derived from the cationic complex of trimethylphosphite {(C<sub>5</sub>H<sub>5</sub>)Co[P(OCH<sub>3</sub>)<sub>3</sub>]<sub>3</sub>}<sup>2+</sup> via an Arbuzov reaction. Using other phosphites and other cyclopentadienyl ligands, a large variety of derivatives are possible. The parent acid {(C<sub>5</sub>H<sub>5</sub>)Co[(CH<sub>3</sub>O)<sub>2</sub>PO]<sub>3</sub>}H is highly soluble in water (270 g/100 mL). Its pK<sub>a</sub> is about 2. Many complexes have been described, including bis(chelate) complexes of the type {M[{(C<sub>5</sub>H<sub>5</sub>)Co[(CH<sub>3</sub>O)<sub>2</sub>PO]<sub>3</sub>}<sub>2</sub>]<sup>n+</sup> (M = Co(II), Mn(II), Bi(III), etc.).