Palladium(II) dicyanide is the inorganic compound with the formula Pd(CN)
2. A grey solid, it is a
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 ...
. It was the first
palladium
Palladium is a chemical element with the symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1803 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas, which was itself na ...
compound isolated in pure form. In his attempts to produce pure platinum metal in 1804, W.H. Wollaston added mercuric cyanide to a solution prepared by dissolving impure platinum in
aqua regia. This precipitated palladium cyanide which was then ignited to recover palladium metal—a new element.
Structure
It had long been suspected that the structure of palladium cyanide consists of
square planar
The square planar molecular geometry in chemistry describes the stereochemistry (spatial arrangement of atoms) that is adopted by certain chemical compounds. As the name suggests, molecules of this geometry have their atoms positioned at the corne ...
Pd(II) centers linked by cyanide
bridging ligand
In coordination chemistry, a bridging ligand is a ligand that connects two or more atoms, usually metal ions. The ligand may be atomic or polyatomic. Virtually all complex organic compounds can serve as bridging ligands, so the term is usually r ...
s, which are bonded through both the carbon and nitrogen atoms. The CN vibration in the infrared spectra of Pd(CN)
2, at 2222 cm
−1, is typical of bridging cyanide ion. It is now known that the compound commonly known as "palladium(II) cyanide" is in fact a nanocrystaline material better described using the formula Pd(CN)
2.0.29H
2O. The interior of the sheets do indeed consist of square-planar palladium ions linked by head-to-tail disordered bridging cyanide groups to form 4,4-nets. These sheets are approximately 3 nm x 3 nm in size and are terminated by an equal number of water and cyanide groups maintaining the charge neutrality of the sheets. These sheets then stack with very little long range order resulting in Bragg diffraction patterns with very broad peaks. The Pd-C and Pd-N bond lengths, determined using total neutron diffraction, are both 1.98 Å.
Properties and reactions
Palladium dicyanide is insoluble in water with a
solubility product Solubility equilibrium is a type of dynamic equilibrium that exists when a chemical compound in the solid state is in chemical equilibrium with a solution of that compound. The solid may dissolve unchanged, with dissociation, or with chemical reacti ...
of log K
sp = -42.
[
The equilibrium constant for the competition reaction
:PdL2+ + 4CN− d(CN)4sup>2− + L
In the above equation, L is 1,4,8,11-tetraazaundecane ("2,3,2-tet")
was found to have a value of log K = 14.5.][
The affinity of Pd(II) for cyanide is so great that palladium metal is attacked by cyanide solutions:
:Pd(s) + 2 H+ + 4 CN− d(CN)4sup>2− + H2
This reaction is reminiscent of the "]cyanide process
Gold cyanidation (also known as the cyanide process or the MacArthur-Forrest process) is a hydrometallurgical technique for extracting gold from low-grade ore by converting the gold to a water-soluble coordination complex. It is the most commonly ...
" for the extraction of gold, although in the latter reaction O2 is proposed to be involved, to give H2O.NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy or magnetic resonance spectroscopy (MRS), is a spectroscopic technique to observe local magnetic fields around atomic nuclei. The sample is placed in a magnetic fiel ...
. That exchange occurs at all illustrates the ability of some compounds to be labile
Lability refers to something that is constantly undergoing change or is likely to undergo change.
Biochemistry
In reference to biochemistry, this is an important concept as far as kinetics is concerned in metalloproteins. This can allow for th ...
(fast reactions) but also stable (high formation constants). The reaction rate
The reaction rate or rate of reaction is the speed at which a chemical reaction takes place, defined as proportional to the increase in the concentration of a product per unit time and to the decrease in the concentration of a reactant per unit ...
is described as follows:
::rate = ''k''2 (CN)42−CN−], where ''k''2 120 M−1−s−1
The bimolecular kinetics implicate a so-called associative pathway. The associative mechanism of exchange entails rate-limiting attack of cyanide on d(CN)4sup>2−, possibly with the intermediacy of a highly reactive pentacoordinate species d(CN)5sup>3−. By comparison, the rate constant In chemical kinetics a reaction rate constant or reaction rate coefficient, ''k'', quantifies the rate and direction of a chemical reaction.
For a reaction between reactants A and B to form product C
the reaction rate is often found to have the f ...
for i(CN)4sup>2− is > 500,000 M−1−s−1, whereas t(CN)4sup>2−exchanges more slowly at 26 M−1s−1. Such associative reactions are characterized by large negative entropies of activation, in this case: -178 and -143 kJ/(mol·K) for Pd and Pt, respectively.
Pd(CN)2 has few uses. It has been demonstrated to facilitate the synthesis of alkenyl nitriles from olefins. and as a catalyst in the regioselective reaction between cyanotrimethylsilane and oxiranes.
See also
* Nickel dicyanide
References
{{Cyanides
Palladium compounds
Cyanides
Two-dimensional nanomaterials
Coordination polymers