Palladium(II) cyanide

Palladium(II) cyanide describes any chemical species with the empirical formula Pd(CN)_n^(2-n)-. The dicyanide (n = 2, CAS: [2035-66-7]) is a coordination polymer of minor significance. It likely has a structure consisting of square planar Pd(II) centers linked by cyanide ligands, which are bonded through both the carbon and nitrogen atoms.

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The slightly more topical palladium(II) cyanide is the dianion [Pd(CN)₄]^2-. This compound is renowned for its extraordinary stability, with a predicted stability constant of 10⁶⁵ to 10⁷⁵ as defined by the following process:

[Pd(H₂O)₄]²⁺ + 4 CN^- → [Pd(CN)₄]^2- + 4 H₂O

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” 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:

rate = k₂[M(CN)₄^2-][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 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.

References

• R. D. Hancock, A. Evers “Formation Constant of Pd(CN)₄^2-” Inorganic Chemistry, Volume 15, 995-6 (1976).
• J. J. Pesek, W. R. Mason “Cyanide Exchange Kinetics for Planar Tetracyanometalate Complexes by Carbon-13 NMR” Inorganic Chemistry 1983, 22, 2958-2959.