Transition metal carbonate and bicarbonate complexes are coordination compounds containing carbonate (CO32-) and bicarbonate (HCO3-) as ligands. The inventory of complexes is large, enhanced by the fact that the carbonate ligand can bind metal ions in a variety of bonding modes. [1] [2] They illustrate the fate of low valent complexes when exposed to air.
![Structure of [Ru2(CO3)4Cl2] . Ru2(CO3)4Cl2.svg](http://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/Ru2%28CO3%294Cl2.svg/220px-Ru2%28CO3%294Cl2.svg.png)
Carbonate is a pseudohalide ligand. With a saturated pi-system, it has no pi-acceptor properties. With multiple electronegative elements, it is not strongly basic. The latter is consistent with the pKa’s of carbonic acid: pK1 = 6.77 and pK2 = 9.93.
To a single metal ion, carbonate is observed to bind in both unidentate (κ1-) and bidentate (κ2-) fashions. [5] In the covalent bond classification method, κ1-carbonate is anX ligand and κ2-carbonate is an X2 ligand. With two metals, the number of bonding modes increases because carbonate often serves as a bridging ligand. It can span metal-metal bonds as in [Ru2(CO3)4Cl2]5-, where again it functions as an (X)2 ligand. More commonly all three oxygen centers bind, as illustrated by [(C5H5)2Ti]2CO3. In such cases, carbonate is an LX ligand, providing 3e- to each metal. More complicated motifs have been characterized by X-ray crystallography including {(VO)6(μ-OH)9(CO3)4}5-.
The bonding modes of bicarbonate are more limited than those for carbonate, in part because it is less basic and in part because the proton occupies a metal-binding site. Typically bicarbonate is assumed to bind as an unidentate X ligand. Structural studies on such complexes are, however, rare. [6]
Carbonato complexes are prepared by salt metathesis reactions using alkali metal carbonate salts as precursors. In some cases, bicarbonate intermediates are implicated since carbonate does not exist in appreciable concentrations near neutral pH. The other chief route to metal carbonato complexes involves addition of CO2 to metal oxides. Such reactions may be catalyzed by water since the carbonation of metal hydroxides is particularly well established. Isotope labeling studies show that these reactions can proceed (and perhaps usually proceed) without scission of the M-OH bond (L = generic ligand):
Many esoteric routes have been demonstrated. For example, the deoxygenation of peroxycarbonate by tertiary phosphines:
Carbon dioxide undergoes disproportionation upon reaction with low-valence metals. [7]
Most fundamental reactivity of bicarbonate/carbonato complexes is their interconversion. This acid-base reaction has been examined mainly for unimolecular complexes. Such reactions are molecular versions of the familiar reaction of acids with carbonate minerals.
Protonation of carbonato complexes gives the corresponding bicarbonate. The structure of bicarbonate complex indicates that protonation occurs at the coordinated oxygen. [8] This process is the microscopic reverse or the first step in the carbonation of metal hydroxides. Protonation of bicarbonate ligands results in loss of carbon dioxide and formation of the metal hydroxide. Particularly well studied are the reactions of [Co(NH3)4(CO3)]+ and its ethylenediamine analogue carbonatobis(ethylenediamine)cobalt(III). [1]
Few homoleptic carbonato complexes have been characterized. One is [Zr(CO3)4]4-, featuring 8-ccordinate Zr(IV). [9] Tris(carbonato)cobalt(III) ([Co(CO)3]3-) is another example.
Metal carbonato and bicarbonate complexes are of no direct commercial importance. Several minerals are metal carbonates, and a few feature molecular carbonate complexes, e.g. hellyerite ([Ni2(CO3)2(H2O)8].H2O. [10]
In the biological sphere, zinc bicarbonate complexes are central intermediates in the action of the carbonic anhydrase: [11]
A carbonate is a salt of carbonic acid, H2CO3, characterized by the presence of the carbonate ion, a polyatomic ion with the formula CO2−3. The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate groupO=C(−O−)2.
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 functions as a base, a ligand, a nucleophile, and a catalyst. The hydroxide ion forms salts, some of which dissociate in aqueous solution, liberating solvated hydroxide ions. Sodium hydroxide is a multi-million-ton per annum commodity chemical. The corresponding electrically neutral compound HO• is the hydroxyl radical. The corresponding covalently bound group –OH of atoms is the hydroxy group. Both the hydroxide ion and hydroxy group are nucleophiles and can act as catalysts in organic chemistry.
Carbonic acid is a chemical compound with the chemical formula H2CO3. The molecule rapidly converts to water and carbon dioxide in the presence of water. However, in the absence of water, it is quite stable at room temperature. The interconversion of carbon dioxide and carbonic acid is related to the breathing cycle of animals and the acidification of natural waters.
Magnesium carbonate, MgCO3, is an inorganic salt that is a colourless or white solid. Several hydrated and basic forms of magnesium carbonate also exist as minerals.
Dicarbonyltris(triphenylphosphine)ruthenium(0) or Roper's complex is a ruthenium metal carbonyl. In it, two carbon monoxide ligands and three triphenylphosphine ligands are coordinated to a central ruthenium(0) center.
Nickel(II) carbonate describes one or a mixture of inorganic compounds containing nickel and carbonate. From the industrial perspective, an important nickel carbonate is basic nickel carbonate with the formula Ni4CO3(OH)6(H2O)4. Simpler carbonates, ones more likely encountered in the laboratory, are NiCO3 and its hexahydrate. All are paramagnetic green solids containing Ni2+ cations. The basic carbonate is an intermediate in the hydrometallurgical purification of nickel from its ores and is used in electroplating of nickel.
In coordination chemistry, hapticity is the coordination of a ligand to a metal center via an uninterrupted and contiguous series of atoms. The hapticity of a ligand is described with the Greek letter η ('eta'). For example, η2 describes a ligand that coordinates through 2 contiguous atoms. In general the η-notation only applies when multiple atoms are coordinated. In addition, if the ligand coordinates through multiple atoms that are not contiguous then this is considered denticity, and the κ-notation is used once again. When naming complexes care should be taken not to confuse η with μ ('mu'), which relates to bridging ligands.
In organometallic chemistry, a migratory insertion is a type of reaction wherein two ligands on a metal complex combine. It is a subset of reactions that very closely resembles the insertion reactions, and both are differentiated by the mechanism that leads to the resulting stereochemistry of the products. However, often the two are used interchangeably because the mechanism is sometimes unknown. Therefore, migratory insertion reactions or insertion reactions, for short, are defined not by the mechanism but by the overall regiochemistry wherein one chemical entity interposes itself into an existing bond of typically a second chemical entity e.g.:
Transition metal hydrides are chemical compounds containing a transition metal bonded to hydrogen. Most transition metals form hydride complexes and some are significant in various catalytic and synthetic reactions. The term "hydride" is used loosely: some of them are acidic (e.g., H2Fe(CO)4), whereas some others are hydridic, having H−-like character (e.g., ZnH2).
Aluminium carbonate (Al2(CO3)3), is a carbonate of aluminium. It is not well characterized; one authority says that simple carbonates of aluminium are not known. However related compounds are known, such as the basic sodium aluminium carbonate mineral dawsonite (NaAlCO3(OH)2) and hydrated basic aluminium carbonate minerals scarbroite (Al5(CO3)(OH)13•5(H2O)) and hydroscarbroite (Al14(CO3)3(OH)36•nH2O).
Metal carbon dioxide complexes are coordination complexes that contain carbon dioxide ligands. Aside from the fundamental interest in the coordination chemistry of simple molecules, studies in this field are motivated by the possibility that transition metals might catalyze useful transformations of CO2. This research is relevant both to organic synthesis and to the production of "solar fuels" that would avoid the use of petroleum-based fuels.
Metal carbonyl hydrides are complexes of transition metals with carbon monoxide and hydride as ligands. These complexes are useful in organic synthesis as catalysts in homogeneous catalysis, such as hydroformylation.
Copper(II) carbonate or cupric carbonate is a chemical compound with formula CuCO
3. At ambient temperatures, it is an ionic solid consisting of copper(II) cations Cu2+
and carbonate anions CO2−
3.
The carbonic anhydrases form a family of enzymes that catalyze the interconversion between carbon dioxide and water and the dissociated ions of carbonic acid. The active site of most carbonic anhydrases contains a zinc ion. They are therefore classified as metalloenzymes. The enzyme maintains acid-base balance and helps transport carbon dioxide.
Transition metal carboxylate complexes are coordination complexes with carboxylate (RCO2−) ligands. 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 κ1- (O-monodentate), κ2 (O,O-bidentate), and bridging.
Transition metal oxalate complexes are coordination complexes with oxalate (C2O42−) ligands. Some are useful commercially, but the topic has attracted regular scholarly scrutiny. Oxalate (C2O42-) is a kind of dicarboxylate ligand. As a small, symmetrical dinegative ion, oxalate commonly forms five-membered MO2C2 chelate rings. Mixed ligand complexes are known, e.g., [Co(C2O4)(NH3)4]κ+.
A transition metal nitrate complex is a coordination compound containing one or more nitrate ligands. Such complexes are common starting reagents for the preparation of other compounds.
Cobalt compounds are chemical compounds formed by cobalt with other elements.
Sodium tris(carbonato)cobalt(III) is the inorganic compound with the formula Na3Co(CO3)3•3H2O. The salt contains an olive-green metastable cobalt(III) coordination complex. The salt, a homoleptic metal carbonato complex, is sometimes referred to as the “Field-Durrant precursor” and is prepared by the “Field-Durrant synthesis”. It is used in the synthesis of other cobalt(III) complexes. Otherwise cobalt(III) complexes are generated from cobalt(II) precursors, a process that requires an oxidant.
Transition metal sulfate complexes or sulfato complexes are coordination complexes with one or more sulfate ligands. Sulfate binds to metals through one, two, three, or all four oxygen atoms. Common are complexes where sulfate is unidentate or chelating bidentate. Examples are respectively [Co(tren)(NH3)(SO4)]+ (tren = tris(2-aminoethyl)amine) and Co(phen)2SO4. All four oxygen atoms of sulfate bond to metals in some Dawson-type polyoxometalates, e.g. [S2Mo18O62]4-.