Palladium and Bromine
The dibromide is well established, and there is an extensive chemistry of [PdBr4]2− and, to a lesser extent, of [PdBr6]2−. There seems to be no evidence for the existence of palladium(III) bromide or bromo complexes, nor for PdBr4.
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5 Palladium and Bromine The dibromide is weiL established, and there is an extensive chemistry of [PdBr4 j2- and, to a lesser extent, of [PdBr6j2-. There seems tobe no evidence for the existence of palladium(III) bromide or bromo complexes, nor for PdBr4 •
5.1
Palladium Dibromide, PdBr2 (see "Palladium" 1942, p. 287)
Formation and Preparation
The compound is made as a brown powder by the reaction of Br2-containing HBr on metallic palladium, or in crystalline black-brown form by a chemical transport procedure using the brown powder at 700°C in a sealed tube [1]. It can also be made by chemical transport from Pd and Al2 Br6 [2], and from palladium and bromine vapour at 500°C [3]. It is insoluble in water but dissolves in HBr to give a solution of "H 2 [Pd8r4 ]" [1]. The compound is probably involved du ring the electrodeposition and anodic dissolution of Pd in bromide electrolytes [4]. The distribution of trace metals du ring electrodialytic purification of PdBr2 solutions was studied and purification procedures described [5]. References:
Brodersen, K.; Thiele, G.; Gaedcke, H. (Z. Anorg. Allgem. Chem. 348 [1966]16217). Schäfer, H.; Nowitzki, J. (J. Less-Common Metals 61 [1978] 47/50). Williams, R. C; Gregory, N. W. (J. Phys. Chem. 73 [1969] 623/31). Nemova, V.A.; Kolpakova, N. A. (lzv. Tomsk. Politekh.Inst. 258 [1976]48/51; C.A. 8911978] No. 13923). [5] Kozin, L. F.; Grushina, N. V.; Saprykina, T. I.; Lysenko, A. I. (lzv. Akad. Nauk Kaz.SSR Ser. Khim. 28 [1978] 9/14; C.A. 90 [1979] No. 93200).
[1] [2] [3] [4]
Physical Properties Structure, Density. The crystals belong to the monoclinic P2 1 /c-C~h space group with Z = 4; a =6.59(2), b = 3.96(2), c = 25.22(3) A, f3 = 92.6°. The measured density is 5.38 g/cm3 , pyknometric density 5.35 g/cm3 . There is planar coordination with the four metal atoms forming a distorted quadrilateral with bridging Br atoms in infinite puckered chains, see Fig. 42a and 42b [1].
a)
b)
b c
fLa
Fig. 42.
Structure of PdBr2 ; a) dimensions in chain, b) the Pd-Br sheet.
Magnetic Properties. The compound is essentially diamagnetic, the small paramagnetism arising from temperature-independ ent paramagnetism; XM=+63x1o-e cgsu [1]. Gmelin 1-andbook Pd Main Vol. B 2
W. P. Griffith et al., Pd Palladium © Springer-Verlag Berlin Heidelberg 1989
177 Thermodynamic Functions. For the reactions Pd8r2(solid) ~Pd( solid)+ Br2 (gas) from 760 to 970 K, Ln K=-19840 1 -3.02 Ln T +40.124 and for Pd( solid)+ Br2 (gas) ~ PdBr2 (gas) from 1090 to 1155 K, ln K =17386 1 -1.006 ln T +13.278, the data being obtained from transpiration method studies [3].
r
r
From these data, ~H2!J 8 forformation of solid PdBr2 from solid Pd and liquid Br2 is -124 kJ/mol with S2!J8 =125 J ·deg- 1 • mol- 1 • There is evidence for the existence of monomeric, tetrameric and decameric species in the vapour phase [3]. The free energy ~Go has been calculated as -93.2 kJ/mol for PdBr2 [4]. The Lattice energy has been calculated as 2709 kJ/mol [2]. Electronic Spectra. For such spectra of PdBr2 in Br--containing media, see p. 1
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