Transition Metal Nitrate Complex
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A transition metal nitrate complex is a
coordination compound 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 ' ...
containing one or more nitrate ligands. Such complexes are common starting reagents for the preparation of other compounds.


Ligand properties

Nitrate is isostructural with but less basic than
carbonate A carbonate is a salt of carbonic acid, (), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word "carbonate" may also refer to a carbonate ester, an organic compound containing the carbonate group ...
. Both nitrate and carbonate exhibit comparable coordination geometries. The nitrogen center of nitrate does not form bonds to metals. Being the conjugate base of a strong acid (
nitric acid Nitric acid is an inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into nitrogen oxide, oxides of nitrogen. Most com ...
, pKa = -1.4), nitrate has modest Lewis basicity. Two coordination modes are common: unidentate and bidentate. Unidentate nitrate is classified as X ligand in the
Covalent bond classification method The covalent bond classification (CBC) method, also referred to as LXZ notation, is a way of describing covalent compounds such as organometallic complexes in a way that is not prone to limitations resulting from the definition of oxidation state. ...
. With respect to
HSAB theory HSAB is an acronym for "hard and soft (Lewis) acids and bases". HSAB is widely used in chemistry for explaining the stability of compounds, reaction mechanisms and pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemical ...
, it is classified as hard. When bonded as a bidentate ligand, it is denoted κ2-NO3. Biidentate nitrate is classified as X-L ligand in the
Covalent bond classification method The covalent bond classification (CBC) method, also referred to as LXZ notation, is a way of describing covalent compounds such as organometallic complexes in a way that is not prone to limitations resulting from the definition of oxidation state. ...
. With respect to
HSAB theory HSAB is an acronym for "hard and soft (Lewis) acids and bases". HSAB is widely used in chemistry for explaining the stability of compounds, reaction mechanisms and pathways. It assigns the terms 'hard' or 'soft', and 'acid' or 'base' to chemical ...
, it is classified as hard. In some cases where it is bidentate, nitrate is bound unsymmetrically in the sense that one M-O distance is clearly bonding and the other is more weakly interacting. The MO-N distances for the coordinated oxygen are longer by about 10 picometers longer than the N-Oterminal bonds. This observation suggests that the terminal N-O bonds have double bond character.


Coordination complexes

With three terminal oxide groups, nitrate can in principle bind metals through many geometries. Even though the ligand is written as MNO3, the oxygen atoms are invariably coordinated. Thus, monodentate nitrate is illustrated by o(NH3)5NO3sup>2+, which could also be written as o(NH3)5ONO2sup>2+. Homoleptic metal nitrate complexes generally have O,O'-bidentate nitrate ligands.


Hydrates

Typical metal nitrates are
hydrated In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was understo ...
. Some of these salts crystallize with one or more nitrate ligands, but most are assumed to dissolve in water to give
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, often of the
stoichiometry Stoichiometry () is the relationships between the masses of reactants and Product (chemistry), products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must ...
(H2O)6sup>n+. * Cr(NO3)3(H2O)6 * Mn(NO3)2(H2O)4 * Fe(NO3)3(H2O)9H. Schmidt, A. Asztalos, F. Bok and W. Voigt (2012): "New Iron(III) Nitrate Hydrates: ·''x'' with ''x'' = 4, 5 and 6". ''Acta Crystallographica Section C: - Inorganic Compounds'', volume C68, pages i29-i33. * Co(NO3)2(H2O)2 * Ni(NO3)2(H2O)4 * Pd(NO3)2(H2O)2 * Cu(NO3)2(H2O)x * Zn(NO3)2(H2O)4 * Hg2(NO3)2(H2O)2


Synthesis

Metal nitrate complexes are often prepared by treating metal oxides or metal carbonates with nitric acid. The main complication with dissolving metals in nitric acid arises from
redox Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...
reactions, which can afford either nitric oxide or nitrogen dioxide. Anhydrous nitrates can be prepared by the oxidation of metals with
dinitrogen tetroxide Dinitrogen tetroxide, commonly referred to as nitrogen tetroxide (NTO), and occasionally (usually among ex-USSR/Russian rocket engineers) as amyl, is the chemical compound N2O4. It is a useful reagent in chemical synthesis. It forms an equilibrium ...
(often as a mixture with nitrogen dioxide, with which it interconverts). N2O4 undergoes
molecular autoionization In chemistry, molecular autoionization (or self-ionization) is a chemical reaction between molecules of the same substance to produce ions. If a pure liquid partially dissociates into ions, it is said to be self-ionizing. In most cases the ox ...
to give O+ O3− with the former
nitrosonium The nitrosonium ion is , in which the nitrogen atom is bonded to an oxygen atom with a bond order of 3, and the overall diatomic species bears a positive charge. It can be viewed as nitric oxide with one electron removed. This ion is usually o ...
ion being a strong
oxidant An oxidizing agent (also known as an oxidant, oxidizer, electron recipient, or electron acceptor) is a substance in a redox chemical reaction that gains or "Electron acceptor, accepts"/"receives" an electron from a (called the , , or ''electr ...
. The method is illustrated by the route to β-Cu(NO3)2: : Cu + 2N2O4 → Cu(NO3)2 + 2NO Many metals, metal halides, and metal carbonyls undergo similar reactions, but the product formulas can be deceptive. For example from chromium one obtains Cr(NO3)3(N2O4)2, which was shown to be the salt (NO+)2 r(NO3)5sup>2-. Nitrogen oxides readily interconvert between various forms, some of which may act as completing ligands. The redox reaction of nitrosonium and the metal can give rise to nitrogen oxide which forms strong
metal nitrosyl complex Sodium nitroprusside, a medicinally significant metal nitrosyl-pentacyanoferrate (Fe-III) compound, used to treat complexes that contain nitric oxide">hypertension. Metal nitrosyl complexes are complex (chemistry)">complexes that contain nitri ...
es;
nitronium ion The nitronium ion, , is a cation. It is an onium ion because its nitrogen atom has +1 charge, similar to ammonium ion . It is created by the removal of an electron from the paramagnetic nitrogen dioxide molecule , or the protonation of nitri ...
s (NO2+) are similarly observed. In some cases, nitrate complexes are produced from the reaction of
nitrogen dioxide Nitrogen dioxide is a chemical compound with the formula . One of several nitrogen oxides, nitrogen dioxide is a reddish-brown gas. It is a paramagnetic, bent molecule with C2v point group symmetry. Industrially, is an intermediate in the s ...
with a metal dioxygen complex: : (PPh3 =
triphenylphosphine Triphenylphosphine (IUPAC name: triphenylphosphane) is a common organophosphorus compound with the formula P(C6H5)3 and often abbreviated to P Ph3 or Ph3P. It is versatile compound that is widely used as a reagent in organic synthesis and as a l ...
)


Reactions

Given nitrate's low basicity, the tendency of metal nitrate complexes toward
hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
is expected. Thus
copper(II) nitrate Copper(II) nitrate describes any member of the family of inorganic compounds with the formula Cu( NO3)2(H2O)x. The hydrates are hygroscopic blue solids. Anhydrous copper nitrate forms blue-green crystals and sublimes in a vacuum at 150-200 ° ...
readily dissociates in aqueous solution to give the aqua complex: : Cu(NO3)2 + 6 H2O → u(H2O)6NO3)2
Pyrolysis Pyrolysis is a process involving the Bond cleavage, separation of covalent bonds in organic matter by thermal decomposition within an Chemically inert, inert environment without oxygen. Etymology The word ''pyrolysis'' is coined from the Gree ...
of metal nitrates yields oxides. : Ni(NO3)2 → NiO + NO2 + 0.5O2 This reaction is used to impregnate oxide supports with nickel oxides.
Nitrate reductase Nitrate reductases are molybdoenzymes that reduce nitrate () to nitrite (). This reaction is critical for the production of protein in most crop plants, as nitrate is the predominant source of nitrogen in fertilized soils. Types Euka ...
enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
s convert nitrate to nitrite. The mechanism involves the intermediacy of Mo-ONO2 complexes.


References

{{Coordination complexes Nitrates