Tetracyanoplatinate salts

In the early stage of the development of molecular conductors based on metal complexes, partially oxidized tetracyanoplatinate salts (for example, KCP K2 [Pt(CN)4]Br0.30-3H2O) and related materials were intensively studied [6], In this system, the square-planar platinum complexes are stacked to form a linear Pt-atom chain. The conduction band originates from the overlap of 5dz2 orbitals of the central platinum atom and exhibits the one-dimensional character. 

Similar to (SN)t in their one-dimenston.il conductivity properties arc the stacked columnar complexes typified by [Pl(CN)j i. These square planar ions adopt a closely spaced parallel arrangement, allowing for considerable interaction among the d i orbitals of the platinum atoms. These orbitals are normally filled with electrons, so in order to get a conduction band some oxidation (removal of electrons) must take place. This may be readily accomplished by adding a little elemental chlorine or bromine to the pure tetracyanoplatinate salt to get stoichiometries such as K-.[Pt(CN)jBr0, in which the platinum has an average oxidation state of +2.3. The oxidation may also be accomplished electrolytically. as in the preparation of Rbj(Pt(CN)4J(FHF)04 (Fig. 16.8). which has a short Pt—Pi separation. The Pt—Pt distance is only 280 pm. almost as short as that (bund in platinum metal itself (277 pm) and in oxidized platinum "pop complexes (270 to 278 pm see Chapter l5).4- Cold-bronze materials of this type were discovered as early as 1842. though they have been little understood until recent times. The complexes behave not only as one-dimensional conductors, but... 

The optical properties of the one-dimensional d tetracyanoplatinate salts are very similar to those of the Au(CN)i salts. For example, the Pt(CN) ion in solution has no absorption bands below 35000 cm" but the solid salts are colored and show strong luminescence in the visible region. Yersin et al [47] have demonstrated that the emission and reflectivity energies vary linearly as R" . Day [48] has proposed that this behavior is characteristic of neutral Frenkel excitons. He has shown that the extrapolated energy of the transition at R=oo for the free ion should correspond to the solution transition energy case. 

Partially oxidized tetracyanoplatinates have attracted much attention in recent years because of their highly anisotropic physical properties.1 Although partially oxidized salts of K+, Na+, Rb and Mg+2 have been reported, the analogous bromide-deficient guanidinium salt is the only such compound containing an organic cation.2 A procedure for the preparation of this guanidinium salt has not been previously reported. 

According to some authorities, the first metal ammine to be isolated in the solid state was the reddish yellow hexaamminecobalt(III) oxalate [Co(NH3)6]2(C204)3, described in 1822 by Leopold Gmelin (1788-1853).16 Gmelin also discovered several new double salts potassium ferricyanide, or potassium hexacyanoferrate(III), K3[Fe(CN)6l>17 the cobalticyanides or hexacyanocobaltates(III), M3[Co(CN) ],18 and the platinocyanides or tetracyanoplatinates(II), M2[Pt(CN)4].19All these substances certainly deserve to rank among the earliest known coordination compounds. 

Fig. 16. Polarized reflectance (E c) and emission (E c, Elc) spectra of different single-crystal tetracyanoplatinates(n) at 295 K. For comparison the emission spectra of MgCP at 12 kbar are also reproduced. The emission intensities for different salts (and pressures) are not comparable. The excitation wavelengths (polarization E c) correspond approximately to the maxima of reflectivity. The absolute reflectivity error is 15%. References are given in Table 5...

As a second example of orbital interaction analysis of the COs, let us now consider the tetracyanoplatinate Pt(CN)4 chains. Although K2[Pt(CN)4] is an insulator, the salt can be cocrystallized with elemental bromine to produce partially oxidized salts, K2 [Pt(CN)4]Brj-3H20. These salts are metallic and have attracted a great deal of attention. The crystal structmes of these partially oxidized salts exhibit linear chains with a staggered arrangement of Pt(CN)4 units as shown in (52). All Pt atoms are equivalent and the Pt-Pt distance becomes shorter when the partial oxidation of Pt increases. 

Unlike the tetracyanoplatinates, the bis(oxalato)platinates, with the exception of the red form of the calcium salt, do not form metal chain structures unless they are partially oxidized. Metal chain substances containing nonstoichiometric mono- (e.g., HsO, Li, Na, K, Rb, NH4) or divalent (e.g., Mg, Ca, Si, Br, Mn, Co, Ni, Cu, Zn " ) cations are prepared with the nominal chain repeat unit being the [Pt(02C202)2] z = 1.6- to 1.7-) moiety. ... 

Earlandite structure, 849 Electrical conductivity metal complexes, 133 tetracyanoplatinates anion-deficient salts, 136 Electrical properties metal complexes, 133-154 Electrocatalysis, 28 Electrochemical cells, 1 Electrochemistry, 1-33 hydrogen or oxygen production from water coordination complex catalysts, 532 mineral processing, 831 reduction, 831 Electrodeposi (ion of metals, 1-15 mineral processing difficulty, 831 Electrodes clay modified, 23 ferrocene modified, 20 nation coated, 15 polymers on, 16 polyvinylferrocene coated, 19 poly(4-vinylpyridine) coated, 17 redox centres, 17 Prussian blue modified, 21 surface modified, 15-31 Electrolysis... 

One-dimensional metals of d -complexes with a quadric-planar surrounding are good electrical conductors (For a very recent review on polymers with transition metal chains see J.K. Bera, K.R. Dunbar, Angew. Chem. 2002, 114, 4633). Partial doping of tetracyanoplatinate (Kroogmann salt) with columnar stacks results in metallic-like conductivity [7]. A linear chain polymer containing Rh-Rh bonds has been obtained by galvanostatic reduction of... 

The partially oxidized tetracyanoplatinate materials and a number of anion radical salts based on tetracyanoquinodimethane satisfy these requirements. Additional experimental work is in order to apply these concepts to the synthesis of new materials. 

Summary. The potassium salts of tetracyanoplatinates are by far the most extensively studied highly conducting inorganic one-dimensional materials. The magnesium, sodium, guanidinium, and rubidium salts have been reported, but they have not been characterized to the extent that detailed comparisons can be made as a function of counter ions. From preliminary... 

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