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Krogmann's salt is a linear chain compound consisting of stacks of tetracyanoplatinate. Sometimes described as molecular wires, Krogmann's salt exhibits highly
anisotropic Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ver ...
electrical conductivity. For this reason, Krogmann's salt and related materials are of some interest in
nanotechnology Nanotechnology is the manipulation of matter with at least one dimension sized from 1 to 100 nanometers (nm). At this scale, commonly known as the nanoscale, surface area and quantum mechanical effects become important in describing propertie ...
.


History and nomenclature

Krogmann's salt was first synthesized by Klaus Krogmann in the late 1960s. Krogmann's salt most commonly refers to a platinum metal
complex Complex commonly refers to: * Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe ** Complex system, a system composed of many components which may interact with each ...
of the formula K2 t(CN)4X0.3where X is usually
bromine Bromine is a chemical element; it has chemical symbol, symbol Br and atomic number 35. It is a volatile red-brown liquid at room temperature that evaporates readily to form a similarly coloured vapour. Its properties are intermediate between th ...
(or sometimes
chlorine Chlorine is a chemical element; it has Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between ...
). Many other non-
stoichiometric Stoichiometry () is the relationships between the masses of reactants and products before, during, and following chemical reactions. Stoichiometry is based on the law of conservation of mass; the total mass of reactants must equal the total m ...
metal salts containing the
anionic An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...
complex t(CN)4sup>n− can also be characterized.


Structure and physical properties

Krogmann's salt is a series of partially
oxidized 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 ...
tetracyanoplatinate complexes linked by the platinum-platinum bonds on the top and bottom faces of the planar t(CN)4sup>n− anions. This salt forms infinite stacks in the solid state based on the overlap of the dz2 orbitals. Krogmann's salt has a
tetragonal In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the Cube (geometry), cube becomes a rectangular Pri ...
crystal structure with a Pt-Pt distance of 2.880
angstroms The angstrom (; ) is a unit of length equal to m; that is, one ten-billionth of a metre, a hundred-millionth of a centimetre, 0.1 nanometre, or 100 picometres. The unit is named after the Swedish physicist Anders Jonas Ångström (1814–1874) ...
, which is much shorter than the metal-metal bond distances in other planar platinum complexes such as Ca t(CN)4�5H2O (3.36 angstroms), Sr t(CN)4�5H2O (3.58 angstroms), and Mg t(CN)4�7H2O (3.16 angstroms). The Pt-Pt distance in Krogmann's salt is only 0.1
angstroms The angstrom (; ) is a unit of length equal to m; that is, one ten-billionth of a metre, a hundred-millionth of a centimetre, 0.1 nanometre, or 100 picometres. The unit is named after the Swedish physicist Anders Jonas Ångström (1814–1874) ...
longer than in platinum metal.Heger, G.; Deiseroth, H.J.; Schulz, H. "Combined X-ray and neutron diffraction study of K2 (Pt(CN)4)X0.3.3(H2O) with X= Br, Cl (KCP) between 31 K and room temperature" Acta Crystallographica B 1982, volume 24,1968-38. (1978) 34, p725-p731. Each
unit cell In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector In mathematics, a unit vector i ...
contains a site for Cl, corresponding to 0.5 Cl per Pt. However, this site is only filled 64% of the time, giving 0.32 Cl per Pt in the actual compound. Because of this, the
oxidation number In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to other atoms are fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. Concep ...
of Pt does not rise above +2.32. Krogmann's salt has no recognizable phase range and is characterized by broad and intense intervalence bands in its electronic spectra.


Chemical properties

One of the most widely researched properties of Krogmann's salt is its unusual electric conductance. Because of its linear chain structure and overlap of the platinum d_ orbitals, Krogmann's salt is an excellent conductor of
electricity Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...
. This property makes it an attractive material for nanotechnology.


Preparation

The usual preparation of Krogmann's salt involves the evaporation of a 5:1 molar ratio mixture of the salts K2 t(CN)4and K2 t(CN)4Br2in water to give copper-colored needles of K2 t(CN)4r0.32·2.6 H2O. ::5K2 t(CN)4+ K2 t(CN)4Br2+ 15.6 H2O → 6K2 t(CN)4r0.32·2.6 H2O Because excess PtII or PtIV complex crystallizes out with the product when the reactant ratio is changed, the product is therefore well defined, although
non-stoichiometric Non-stoichiometric compounds are chemical compounds, almost always solid inorganic compounds, having chemical element, elemental composition whose proportions cannot be represented by a ratio of small natural numbers (i.e. an empirical formula); ...
.


Uses

Krogmann's salt nor any related material has found any commercial applications.


References

{{potassium compounds Cyanides Potassium compounds Platinum compounds Metal halides Electrical conductors Mixed valence compounds Non-stoichiometric compounds