Transition metal carboxylate complexes are
coordination complex
A coordination complex consists of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of bound molecules or ions, that are in turn known as ''ligands'' or complexing agents. Many ...
es with
carboxylate
In organic chemistry, a carboxylate is the conjugate base of a carboxylic acid, (or ). It is an ion with negative charge.
Carboxylate salts are salts that have the general formula , where M is a metal and ''n'' is 1, 2,...; ''carboxylat ...
(RCO
2−)
ligand
In coordination chemistry, a ligand is an ion or molecule (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 electr ...
s. Reflecting the diversity of carboxylic acids, the inventory of metal carboxylates is large. Many are useful commercially, and many have attracted intense scholarly scrutiny. Carboxylates exhibit a variety of coordination modes, most common are κ
1- (O-monodentate), κ
2 (O,O-bidentate), and bridging.
Acetate and related monocarboxylates
Structure and bonding
Carboxylates bind to single metals by one or both oxygen atoms, respectively κ
1- and κ
2-. In terms of
electron count
Electron counting is a formalism used for classifying compounds and for explaining or predicting electronic structure and chemical bond, bonding. Many rules in chemistry rely on electron-counting:
*Octet rule is used with Lewis structures for mai ...
ing, κ
1- and κ
2- are "X", a pseudohalide, and "L-X ligands", i.e. a combination of a Lewis base (L) and a pseudohalide (X). Carboxylates are classified as hard ligands, in
HSAB
HSAB concept is a jargon for "hard and soft (Lewis) acids and bases". HSAB is widely used in chemistry for explaining stability of compounds, reaction mechanisms and pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemic ...
theory.
File:BasicFeacetate.png, Basic ferric acetate
File:Ag2(OAc)2.png, Silver acetate
File:BasicZnAcetate.png, Basic zinc acetate
Zinc acetate is a salt with the formula Zn(CH3CO2)2, which commonly occurs as the dihydrate Zn(CH3CO2)2·2H2O. Both the hydrate and the anhydrous forms are colorless solids that are used as dietary supplements. When used as a food additive, it ...
File:Mo2(OAc)4.svg, Molybdenum(II) acetate
Molybdenum(II) acetate is a coordination compound with the formula Mo2(O2CCH3)4. It is a yellow, diamagnetic, air-stable solid that is slightly soluble in organic solvents. Molybdenum(II) acetate is an iconic example of a compound with a metal-met ...
, illustrating the Mo-Mo quadruple bond.
File:DasCubane.svg, oO(acetate)sub>4, the " Das cubane"
For simple carboxylates, the acetate complexes are illustrative. Most transition metal acetates are mixed ligand complexes. One common example is hydrated
nickel acetate
Nickel(II) acetate is the name for the coordination compounds with the formula Ni(CH3CO2)2·x H2O where x can be 0, 2, and 4. The green tetrahydrate Ni(CH3CO2)2·4 H2O is most common. It is used for electroplating.
Synthesis and structure
The ...
, Ni(O
2CCH
3)
2(H
2O)
4, which features intramolecular hydrogen-bonding between the uncoordinated oxygens and the protons of
aquo ligand
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 ...
s. Stoichiometrically simple complexes are often multimetallic. One family are the
basic metal acetates, of the stoichiometry
3O(OAc)6(H2O)3">3O(OAc)6(H2O)3sup>n+.
Homoleptic complexes
Homoleptic carboxylate complexes are usually
coordination polymer
A coordination polymer is an inorganic or organometallic polymer structure containing metal cation centers linked by ligands. More formally a coordination polymer is a coordination compound with repeating coordination entities extending in 1, 2, o ...
s. But exceptions exist.
*A molecular monocarboxylate is
silver acetate
Silver acetate is an coordination compound with the empirical formula CH3CO2Ag (or AgC2H3O2). A photosensitive, white, crystalline solid, it is a useful reagent in the laboratory as a source of silver ions lacking an oxidizing anion.
Synthesis an ...
, Ag
2(OAc)
2.
*Molecular diacetates are more common. Several diacetates adopt the
Chinese lantern structure
In chemistry, the Chinese lantern structure is a coordination complex where two metal atoms are bridged by four bidentate ligands. This structure type is also known as a paddlewheel complex. Examples include chromium(II) acetate, molybdenum(II) ac ...
. Well studied examples include the dimetal tetraacetates (M
2(OAc)
4) where M = Cu(II), Rh(II), Cr(II), and Mo(II).
Platinum diacetate and
palladium diacetate feature Pt
4 and Pd
3 cores, further illustrating the tendency of acetate ligands to stabilize multimetallic structures.
*Mononuclear triacetates include derivatives of
1-adamantanecarboxylic acid
1-Adamantanecarboxylic acid is an organic compound with the formula . A white solid, it is the simplest carboxylic acid derivative of adamantane. The compound is notable for its synthesis by carboxylation of adamantane. 1-Adamantanecarboxylic ac ...
, which have the formula
2CC10H11)4">(O2CC10H11)4sup>− (M = Co, Ni, Zn).
Reactions and applications
Attempts to prepare some carboxylate complexes, especially for electrophilic metals, often gives oxo derivatives. Examples include the oxo-acetates of Fe(III), Mn(III), and Cr(III).
Metal acetates are common
catalyst
Catalysis () is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst (). Catalysts are not consumed in the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recyc ...
s or precatalysts. Particularly useful are the lipophilic complexes of
ethylhexanoate
2-Ethylhexanoic acid is the organic compound with the formula CH3(CH2)3CH(C2H5)CO2H. It is a carboxylic acid that is widely used to prepare lipophilic metal derivatives that are soluble in nonpolar organic solvents. 2-Ethylhexanoic acid is a colo ...
s, which are used as catalysts in oxidation reactions.
Synthesis
Many methods allow the synthesis of metal carboxylates. From preformed carboxylic acid, the following routes have been demonstrated:
:acid-base reactions:
:protonolysis:
:
oxidative addition
Oxidative addition and reductive elimination are two important and related classes of reactions in organometallic chemistry. Oxidative addition is a process that increases both the oxidation state and coordination number of a metal centre. Oxidat ...
:
From preformed carboxylate,
salt metathesis reaction
A salt metathesis reaction, sometimes called a double displacement reaction, is a chemical process involving the exchange of bonds between two reacting chemical species which results in the creation of products with similar or identical bonding a ...
s are common:
:
Metal carboxylates can be prepared by carbonation of highly basis metal alkyls:
:
Reactions
A common reaction of metal carboxylates is their displacement by more basic ligands. Acetate is a common
leaving group In chemistry, a leaving group is defined by the IUPAC as an atom or group of atoms that detaches from the main or residual part of a substrate during a reaction or elementary step of a reaction. However, in common usage, the term is often limited t ...
. They are especially prone to protonolysis, which is widely used to introduce ligands, displacing the carboxylic acid. In this way
octachlorodimolybdate
Potassium octachlorodimolybdate (systematically named potassium bis(tetrachloromolybdate)(''Mo''–''Mo'')(4−)) is an inorganic compound with the chemical formula . It is known as a red-coloured, microcrystalline solid. The anion is of historic ...
is produced from
dimolybdenum tetraacetate
Molybdenum(II) acetate is a coordination compound with the formula Mo2(O2CCH3)4. It is a yellow, diamagnetic, air-stable solid that is slightly soluble in organic solvents. Molybdenum(II) acetate is an iconic example of a compound with a metal-met ...
:
:
Acetates of electrophilic metals are proposed to function as bases in
concerted metalation deprotonation Concerted metalation-deprotonation (CMD) is a mechanistic pathway through which transition-metal catalyzed C–H activation reactions can take place. In a CMD pathway, the C–H bond of the substrate is cleaved and the new C–Metal bond forms thro ...
reactions.
Pyrolysis of metal carboxylates affords
acid anhydride An acid anhydride is a type of chemical compound derived by the removal of water molecules from an acid.
In organic chemistry, organic acid anhydrides contain the functional group R(CO)O(CO)R'. Organic acid anhydrides often form when one equivalent ...
s and the metal oxide. This reaction explains the formation of basic zinc acetate from anhydrous
zinc diacetate
Zinc acetate is a Salt (chemistry), salt with the chemical formula, formula Zn(CH3CO2)2, which commonly occurs as the Hydrate, dihydrate Zn(CH3CO2)2·2H2O. Both the hydrate and the anhydrous forms are colorless solids that are used as dietary supp ...
.
In some cases, monodentate carboxylates undergo O-alkylation to give esters. Strong alkylating agents are required.
Di- and polycarboxylates
Benzenedi- and tricarboxylates
Metal organic framework
A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typicall ...
s, porous, three-dimensional coordination polymers, are often derived from metal carboxylate clusters. These clusters, called secondary bonding units (SBU's), are often linked by the conjugate bases of benzenedi- and tricarboxylic acids.
Aminopolycarboxylates
A commercially important family of metal carboxylates are derived from
aminopolycarboxylate left, 120px, a metal complex with the EDTA anion
120px, Aspartic acid is an aminodicarboxylic acid and precursor to other ligands.
An aminopolycarboxylic acid (sometimes abbreviated APCA) is a chemical compound containing one or more nitrogen at ...
s, e.g.,
EDTA
Ethylenediaminetetraacetic acid (EDTA) is an aminopolycarboxylic acid with the formula H2N(CH2CO2H)2sub>2. This white, water-soluble solid is widely used to bind to iron (Fe2+/Fe3+) and calcium ions (Ca2+), forming water-soluble complexes eve ...
4-. Related to these synthetic chelating agents are the
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s, which form large families of
amino acid complex Transition metal amino acid complexes are a large family of coordination complexes containing the conjugate bases of the amino acids, the 2-aminocarboxylates. Amino acids are prevalent in nature, and all of them function as ligands toward the trans ...
es. Two amino acids, glutamate and aspartate, have carboxylate side chains, which function as ligands for iron in nonheme iron proteins, such as
hemerythrin
Hemerythrin (also spelled haemerythrin; grc, αἷμα, haîma, blood, grc, ἐρυθρός, erythrós, red) is an oligomeric protein responsible for oxygen (O2) transport in the marine invertebrate phyla of sipunculids, priapulids, brachiopo ...
.
References
{{Coordination complexes
Ligands
Coordination chemistry
Salts of carboxylic acids