Krogmann's salt

Krogmann's salt is a mixed-valence square planar coordination complex of platinum and cyanide bonded through linear platinum metal chains, sometimes described as molecular wires.

Although the term Krogmann’s salt most commonly refers to a platinum metal complex of the formula K2[Pt(CN)4X0.3] where X is usually bromine (or sometimes chlorine), a number of non-stoichiometric metal salts containing the anionic complex Pt(CN)42- can also be characterized under the blanket term “Krogmann’s salts.”

Modeled as an infinite one-dimensional molecular chain of platinum atoms, the high anisotropy and restricted dimensionality of Krogmann’s salt and related compounds are becoming increasingly attractive properties for many facets of nanotechnology.

History
Krogmann’s salt was first synthesized by Dr. Klaus Krogmann in the late 1960s at the University of Stuttgart in Germany. Dr. Krogmann published the original journal article documenting the synthesis and characterization of the salt in 1969.

Structure and physical properties
Krogmann’s salt is a series of partially oxidized tetracyanophatinate complexes linked by the platinum-platinum bonds on the top and bottom sides of the [Pt(CN)4]n- anions. This salt forms infinite stacks in the solid state based on the overlap of the dz 2 orbitals.

Krogmann’s salt has a tetragonal crystal structure with a Pt-Pt distance of 2.880 angstroms, which is much shorter than the metal-metal bond distances in other planar platinum complexes such as Ca[Pt(CN)4]·5H2O (3.36 angstroms), Sr[Pt(CN)4]·5H2O (3.58 angstroms), and Mg[Pt(CN)4]·7H2O (3.16 angstroms).2,3 The Pt-Pt distance in Krogmann's salt is only 0.1 angstroms longer than in platinum metal.

Each unit cell 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 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 dz 2 orbitals, Krogmann’s salt is an excellent conductor of electricity. 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[Pt(CN)4] and K2[Pt(CN)4Br2] in water to give copper-colored needles of K2[Pt(CN)4]Br0.32·2.6 H2O.


 * 5K2[Pt(CN)4] + K2[Pt(CN)4Br2] + 15.6 H2O → 6K2[Pt(CN)4]Br0.32·2.6 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.

Uses
Although there was a large body of research and literature generated on molecular wire-type metal complexes through the mid-1980s, interest in stacked metal-metal bonds saw a decline until only very recently.

Due to the explosion of nanotechnology in the last few years, many researchers have taken a renewed interest in Krogmann’s salt and its related compounds due to their high anisotropy, restricted dimensionality, and unique conductance properties.

A new group of platinum chains based on alternating cations and anions of [Pt(CNR)4]2+ (R = iPr, c-C12H23, p-(C2H5)C6H4) and [Pt(CN)4]2- is undergoing current research.1 These may be able to be used as vapochromic sensor materials, or materials which change color when exposed to different vapors.

Similar to Krogmann’s platinum salt, it has been shown that it is possible to stabilize metal chains with only unsaturated hydrocarbons, or olefins. Current research indicates that mononuclear Pd0 and PdII react with conjugated polyenes to give linear chains of Pd-Pd bonds protected by a “π-electron sheath.”

Not only do these olefin-stabilized metal chains constitute a significant contribution to the field of organometallic chemistry, both the complex’s metal atom structures and the olefin ligands themselves can conduct a current.1,11 The prospect of creating molecular wires of conducting organic and inorganic constituents has intriguing possibilities for future research, especially in microbiology, nanotechnology, and organic circuitry.