Transition metal hydroxide complexes are

coordination complex A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...

es containing one or more

hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...

(OH⁻)

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

s. The inventory is very large.

Hydroxide as a ligand

Hydroxide is classified as an X ligand in the Covalent bond classification method. In the usual electron counting method, it is a one-electron ligand when terminal and a three-electron ligand when doubly bridging. From the electric structure perspective, hydroxide is a strong pi-donor ligand, akin to fluoride. One consequence is that few polyhydroxide complexes are low spin. Another consequence is that electron-precise hydroxide complexes tend to be rather nucleophilic.

Representative complexes

Many hydroxo complexes are prepared by treating metal halides with hydroxide salts. Hydrolysis of basic ligands (amides, alkyls) also produces hydroxide complexes. Only a few homoleptic hydroxide complexes are known. These include the d⁶ species and the d⁰ complexes . Many complexes are known where hydroxide shares the coordination sphere with other ligands, i.e., hydroxide is a

bridging ligand In coordination chemistry, a bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually r ...

. Examples are ³⁺ and its derivative ⁴⁺.

Reactions

Prominent reactions of metal hydroxides are their acid-base behavior. Protonation of metal hydroxides gives

aquo complex In chemistry, metal aquo complexes are coordination compounds containing metal ions with only water as a ligand. These complexes are the predominant species in aqueous solutions of many metal salts, such as metal nitrates, sulfates, and perchlorat ...

es: : where is the ligand complement on the metal M Thus, aquo ligand is a

weak acid Acid strength is the tendency of an acid, symbolised by the chemical formula , to dissociate into a proton, , and an anion, . The dissociation or ionization of a strong acid in solution is effectively complete, except in its most concentrated s ...

, of comparable strength to

acetic acid Acetic acid , systematically named ethanoic acid , is an acidic, colourless liquid and organic compound with the chemical formula (also written as , , or ). Vinegar is at least 4% acetic acid by volume, making acetic acid the main compone ...

(p''K''_a of about 4.8). In principle but not very commonly, metal hydroxides undergo deprotonation, yielding oxo complexes: : Characteristically, hydroxide ligands are compact and basic. They tend to function as

s. One manifestation of this property is the preponderance of di-and polymetallic hydroxide complexes. A practical consequence of this feature is the tendency of metal aquo complexes to form precipitates of meta hydroxides.

Bioinorganic chemistry

Hemerythrin Hemerythrin (also spelled haemerythrin; , ) is an oligomeric protein responsible for oxygen (O2) transport in the marine invertebrate phyla of sipunculids, priapulids, brachiopods, and in a single annelid worm genus, '' Magelona''. Myohemeryth ...

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

s responsible for

oxygen Oxygen is a chemical element; it has chemical symbol, symbol O and atomic number 8. It is a member of the chalcogen group (periodic table), group in the periodic table, a highly reactivity (chemistry), reactive nonmetal (chemistry), non ...

(O₂) transport in some animals have an diiron hydroxide active site. The hydroxide ligand engages the bound O2 through

hydrogen bonding In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...

. The nucleophilicity of hydroxo ligands is relevant to the role of some M-OH centers in enzymology. For example, in

carbonic anhydrase The carbonic anhydrases (or carbonate dehydratases) () form a family of enzymes that catalyst, catalyze the interconversion between carbon dioxide and water and the Dissociation (chemistry), dissociated ions of carbonic acid (i.e. bicarbonate a ...

, a zinc hydroxide binds

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

: : The

oxygen evolving complex The oxygen-evolving complex (OEC), also known as the water-splitting complex, is a water-oxidizing enzyme involved in the photo-oxidation of water during the light reactions of photosynthesis. OEC is surrounded by 4 core proteins of photosystem ...

(OEC) consists of a Mn-Ca-O-OH cluster that is responsible for the biosynthesis of O₂. It is proposed that the O-O bond forming step involves a hydroxide ligand.

Metalloproteinase A metalloproteinase, or metalloprotease, is any protease enzyme whose catalytic mechanism involves a metal. An example is ADAM12 which plays a significant role in the fusion of muscle cells during embryo development, in a process known as myoge ...

s catalyze the hydrolysis peptide bond. The catalytic center is such enzymes often involves metal hydroxides.

References

{{Coordination complexes Ligands