Kläui ligand
# Kläui ligand

The Kläui ligand is the anion ${(C_5H_5)Co[(CH_3O)_2PO]_3}^−.$ The ligand, popularized by Wolfgang Kläui, binds metals and metalloids via a facial $O_3$ donor set. Related tridentate and tripodal anionic ligands include trispyrazolylborates.

    General structure of a metal center coordinated to a $κ^3-Kläui$ ligand.

The ligand is derived from the cationic complex of trimethylphosphite ${(C_5H_5)Co[P(OCH_3)_3]_3}^{2+}$ via an Arbuzov reaction. Using other phosphites and other cyclopentadienyl ligands, a large variety of derivatives are possible. The parent acid ${(C_5H_5)Co[(CH_3O)_2PO]_3}H$ 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 ${M[{(C_5H_5)Co[(CH_3O)_2PO]_3}_2]^{n+}$ (M = Co(II), Mn(II), Bi(III), etc.).

