Kläui ligand

The Kläui ligand is the anion {(C₅H₅)Co[(CH₃O)₂PO]₃}⁻. The ligand, popularized by Wolfgang Kläui, binds metals and metalloids via a facial O₃ donor set. Related tridentate anionic ligands include trispyrazolylborate and cyclopentadienyl anions.

The ligand is derived from the cationic complex of trimethylphosphite {(C₅H₅)Co[P(OCH₃)₃]₃}²⁺ via an Arbuzov reaction. Using other phosphites and other cyclopentadienyl ligands, a large variety of derivatives are possible. The parent acid {(C₅H₅)Co[(CH₃O)₂PO]₃}H is highly soluble in water (270 g/100 mL). Its pK_a is about 2. Many complexes have been described, including bis(chelate) complexes of the type {M[{(C₅H₅)Co[(CH₃O)₂PO]₃}₂]ⁿ⁺ (M = Co(II), Mn(II), Bi(III), etc.).