Palladium(II) cyanide
Encyclopedia
Palladium cyanides are chemical species with the empirical formula
Pd(CN)n(2-n)-. The dicyanide (n = 2, CAS: [2035-66-7]) is a coordination polymer which was the first pure palladium compound isolated. In his attempts to produce pure platinum metal in 1804, W.H. Wollaston added mercuric cyanide to a solution of impure platinum metal in aqua regia
to precipitate palladium cyanide which was then ignited to recover palladium metal - a new element. It had long been suspected that the structure of palladium cyanide consists of square planar
Pd(II) centers linked by bridging cyanide ligand
s, which are bonded through both the carbon and nitrogen atoms. The CN stretch in the infrared spectra of Pd(CN)2, at 2222 cm-1, is typical of bridging cyanide ions. 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)2.0.29H2O. 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. .
The compound is rather insoluble in water with a solubility product of log Ksp = -42.
The slightly more topical palladium(II) cyanide is the dianion [Pd(CN)4]2-. The equilibrium constant for the competition reaction
was found to have a value of log K = 14.5. Combination with the formation of the palladium complex with the tetradentate ligand
gives
This appears to be the highest formation constant known for any metal ion.
The affinity of Pd for cyanide is so great that palladium metal is attacked by cyanide solutions:
This reaction is reminiscent of the “cyanide process
” for the extraction of gold, although in the latter reaction O2 is proposed to be involved, to give H2O.
Exchange of between free cyanide ion and [Pd(CN)4]2- has been evaluated by 13C NMR spectroscopy
. That exchange occurs at all illustrates the ability of some compounds to be labile (fast reactions) but also stable (high formation constants). The reaction rate
is described as follows:
The bimolecular kinetics implicate a so-called associative pathway. The associative mechanism of exchange entails rate-limiting attack of cyanide on [Pd(CN)4]2-, possibly with the intermediacy of a highly reactive pentacoordinate species [Pd(CN)5]3-. By comparison, the rate constant
for [Ni(CN)4]2- is > 500,000 M-1-s-1, whereas [Pt(CN)4]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.
In organic synthesis palladium cyanide is used in the synthesis of olefinic cyanides from olefins. and as a catalyst in the regioselective reaction between cyanotrimethylsilane and oxiranes .
Empirical formula
In chemistry, the empirical formula of a chemical compound is the simplest positive integer ratio of atoms of each element present in a compound. An empirical formula makes no reference to isomerism, structure, or absolute number of atoms. The empirical formula is used as standard for most ionic...
Pd(CN)n(2-n)-. The dicyanide (n = 2, CAS: [2035-66-7]) is a coordination polymer which was the first pure palladium compound isolated. In his attempts to produce pure platinum metal in 1804, W.H. Wollaston added mercuric cyanide to a solution of impure platinum metal in aqua regia
Aqua regia
Aqua regia or aqua regis is a highly corrosive mixture of acids, fuming yellow or red solution, also called nitro-hydrochloric acid. The mixture is formed by freshly mixing concentrated nitric acid and hydrochloric acid, usually in a volume ratio of 1:3, respectively...
to precipitate palladium cyanide which was then ignited to recover palladium metal - a new element. It had long been suspected that the structure of palladium cyanide consists of square planar
Square planar
The square planar molecular geometry in chemistry describes the stereochemistry that is adopted by certain chemical compounds...
Pd(II) centers linked by bridging cyanide ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...
s, which are bonded through both the carbon and nitrogen atoms. The CN stretch in the infrared spectra of Pd(CN)2, at 2222 cm-1, is typical of bridging cyanide ions. 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)2.0.29H2O. 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. .
The compound is rather insoluble in water with a solubility product of log Ksp = -42.
The slightly more topical palladium(II) cyanide is the dianion [Pd(CN)4]2-. The equilibrium constant for the competition reaction
- PdL2+ + 4CN- [Pd(CN)4]2- + L, L = 1,4,8,11-tetraazaundecane (2,3,2-tet)
was found to have a value of log K = 14.5. Combination with the formation of the palladium complex with the tetradentate ligand
- [Pd(H2O)4]2+ + L PdL2+ + 4 H2O, log K = 47.9
gives
- [Pd(H2O)4]2+ + 4CN- [Pd(CN)4]2- + 4H2O, log β4 = 62.3.
This appears to be the highest formation constant known for any metal ion.
The affinity of Pd for cyanide is so great that palladium metal is attacked by cyanide solutions:
- Pd(s) + 2 H+ + 4 CN- [Pd(CN)4]2- + H2
This reaction is reminiscent of the “cyanide process
Gold cyanidation
Gold cyanidation is a metallurgical technique for extracting gold from low-grade ore by converting the gold to a water soluble coordination complex. It is the most commonly used process for gold extraction...
” for the extraction of gold, although in the latter reaction O2 is proposed to be involved, to give H2O.
Exchange of between free cyanide ion and [Pd(CN)4]2- has been evaluated by 13C NMR spectroscopy
NMR spectroscopy
Nuclear magnetic resonance spectroscopy, most commonly known as NMR spectroscopy, is a research technique that exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of atoms or the molecules in which they are contained...
. That exchange occurs at all illustrates the ability of some compounds to be labile (fast reactions) but also stable (high formation constants). The reaction rate
Reaction rate
The reaction rate or speed of reaction for a reactant or product in a particular reaction is intuitively defined as how fast or slow a reaction takes place...
is described as follows:
-
- rate = k2[M(CN)42-][CN-], where k2 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 [Pd(CN)4]2-, possibly with the intermediacy of a highly reactive pentacoordinate species [Pd(CN)5]3-. By comparison, the rate constant
Arrhenius equation
The Arrhenius equation is a simple, but remarkably accurate, formula for the temperature dependence of the reaction rate constant, and therefore, rate of a chemical reaction. The equation was first proposed by the Dutch chemist J. H. van 't Hoff in 1884; five years later in 1889, the Swedish...
for [Ni(CN)4]2- is > 500,000 M-1-s-1, whereas [Pt(CN)4]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.
In organic synthesis palladium cyanide is used in the synthesis of olefinic cyanides from olefins. and as a catalyst in the regioselective reaction between cyanotrimethylsilane and oxiranes .