Cyanometallates or cyanometalates are a class of

coordination compound 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 ...

s, most often consisting only of

cyanide Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of a ...

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 electro ...

s. Most are anions. Cyanide is a highly basic and

small Small may refer to: Science and technology * SMALL, an ALGOL-like programming language * Small (anatomy), the lumbar region of the back * ''Small'' (journal), a nano-science publication * <small>, an HTML element that defines smaller text ...

ligand, hence it readily saturates the coordination sphere of metal ions. The resulting cyanometallate anions are often used as building blocks for more complex structures called

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, the best known example of which is

Prussian blue Prussian blue (also known as Berlin blue, Brandenburg blue or, in painting, Parisian or Paris blue) is a dark blue pigment produced by oxidation of ferrous ferrocyanide salts. It has the chemical formula Fe CN)">Cyanide.html" ;"title="e(Cyani ...

, a common dyestuff.*Dunbar, K. R. and Heintz, R. A., "Chemistry of Transition Metal Cyanide Compounds: Modern Perspectives", Progress in Inorganic Chemistry, 1997, 45, 283-391.

Examples

Homoleptic cyanometallates

''Homoleptic'' cyanometallates are complexes where the only ligand is cyanide. For transition metals, well known homoleptic cyanometallates are the hexacyanides. Hexacyanometalates are known for Ti(III), V(III), Cr(III), Cr(II), Mn(IV), Mn(III), Mn(II),

Fe(II) In chemistry, iron(II) refers to the element iron in its +2 oxidation state. In ionic compounds (salts), such an atom may occur as a separate cation (positive ion) denoted by Fe2+. The adjective ferrous or the prefix ferro- is often used to sp ...

, Fe(III), Co(III), Ru(III), Ru(II), Os(III), and Os(II). Other more labile derivatives are also known. The Cr(II), Mn(III), Mn(II), Fe(II), Fe(III), and Co(III) derivatives are

low-spin Spin states when describing transition metal coordination complexes refers to the potential spin configurations of the central metal's d electrons. For several oxidation states, metals can adopt high-spin and low-spin configurations. The ambiguity o ...

, reflecting the strong binding of cyanide, i.e. cyanide ranks highly in the

spectrochemical series A spectrochemical series is a list of ligands ordered by ligand "strength", and a list of metal ions based on oxidation number, group and element. For a metal ion, the ligands modify the difference in energy Δ between the d orbitals, called the ...

when significant backbonding can occur. With low d-electron counts, inversion of cyanometallates to nitrile complexes can occur. Lower metal oxidation states can be achieved with binding of Lewis acids to the terminal nitrogen lone pairs. : HexacyanidoferratIII_2

Several tetracyanometalates are also known, the best known being those of the d⁸ metals, Ni(II), Pd(II), and Pt(II). These species are square-planar and diamagnetic. In the case of nickel, i(CN)₄sup>4− is known as well as i2(CN)₆sup>4-, with a Ni(I)-Ni(I) bond. The coinage metals form stable dicyanometallates, u(CN)₂sup>−, g(CN)₂sup>−, and u(CN)₂sup>−. For heavier metals, other stoichiometries are known such as K₄Mo(CN)₈ and Potassium heptacyanorhenate. Some cyanometallates are clusters featuring metal-metal bonds, such as o₂(CN)₈sup>4−.

Heteroleptic cyanometallates

Mixed ligand cyanometallates with anywhere from one to five cyanide ligands have been prepared. One example is the zero-valent e(CO)₄(CN)sup>−. Heteroleptic cyanometallates are of interest outside of the research laboratory, with one example being the drug

sodium nitroprusside Sodium nitroprusside (SNP), sold under the brand name Nitropress among others, is a medication used to lower blood pressure. This may be done if the blood pressure is very high and resulting in symptoms, in certain types of heart failure, and ...

(Na₂FeNO(CN)₅). Other studies have demonstrated their competency as photoredox catalysts.

Synthesis

Because cyanide is a powerful

nucleophile In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

and a strong ligand, cyanometallates are generally prepared by the direct reaction of cyanide salts with simple metal salts. If other ligands are present on the metal, these are often displaced by cyanide. By far the largest application of cyanometalates is the production of u(CN)₂sup>− in the extraction of gold from low grade ores. This conversion involves oxidation of metallic gold into Au⁺: :4 Au + 8 CN⁻ + O₂ + 2 H₂O → 4 u(CN)₂sup>− + 4 OH⁻

Reactions

Redox

Because the M-CN bond is strong and delocalizes electron density to the ligands, several cyanometallates exhibit multiple redox states. A well known couple is e(CN)₆sup>3−/4−. Mn(IV), Mn(III), and Mn(II) are known for hexacyanomanganate. Few unidentate ligands allow similar redox transformations wherein both members of the redox couple are observable in solution. Another perhaps more dramatic example is the 2 e^– reduction of the square planar

tetracyanonickelate The cyanonickelates are a class of chemical compound containing anions consisting of nickel atoms, and cyanide groups. The most important of these are the tetracyanonickelates containing four cyanide groups per nickel. The tetracyanonickelates c ...

to its tetrahedral Ni(0) derivative: : i(CN)₄sup>2− + 2 e^– → i(CN)₄sup>4−

N-Centered reactions

Many characteristic reactions of metal cyanides arise from ambidentate nature of cyanide, i.e. both the nitrogen and the carbon extremities of the anion are basic. Thus cyanometalates can be alkylated to give

isocyanide An isocyanide (also called isonitrile or carbylamine) is an organic compound with the functional group –. It is the isomer of the related nitrile (–C≡N), hence the prefix is ''isocyano''.IUPAC Goldboo''isocyanides''/ref> The organic fragme ...

complexes.Fehlhammer, W. P. Fritz, M., "Emergence of a CNH and Cyano Complex Based Organometallic Chemistry", Chemical Reviews, 1993, volume 93, pp. 1243-80. Cyanide ligands are susceptible to protonation, hence many cyanometalates are highly

solvatochromic In chemistry, solvatochromism is the phenomenon observed when the colour due to a solute is different when that solute is dissolved in different solvents. The solvatochromic effect is the way the spectrum of a substance (the solute) varies when th ...

. The nitrogen terminus is a good ligand for other metals. The latter tendency is illustrated by the condensation of ferrocyanide salts with other metal ions to give polymers, such as Prussian blue. Such polymers feature Fe-CN-M linkages.

References

{{cyanides Transition metals Cyano complexes Cyanometallates