Cobalt cyanoaurate: Crystal structure of a component from cobalt-hardened gold electroplating baths

Abstract

Cobalt cyanoaurate (I) Co[Au(CN)2]2 is a new material derived from cobalt-hardened gold electroplating bath solutions that crystallizes in the hexagonal system with space group P6422 and six formula weights in a unit cell with a = 8.434±0.004 and c = 20.695±0.006 Å at 298 K. The integrated intensities of a full sphere of reciprocal space, within a radius of (sin ϑ)/λ = 0.85 Å−1, were measured using graphite monochromated MoKα with a CAD-4 diffractometer. The resulting total of 12 037 structure factors gave 1969 terms after averaging, of which 717 were accepted as reliably observed. Patterson and Fourier series were used to solve the crystal structure, which was refined by the method of least squares: the final agreement factor R = 0.0358. The atomic thermal vibrations are highly anisotropic. The Au(CN)−2 ion departs slightly but significantly from linearity with a C–Au–C angle of 173.1±1.5° and mean Au–C–N angle of 170.6±1.7°. The cyano groups also form tetrahedral Co(NC)2−4 ions: conversion to the octahedral Co(NC)2−6 ion occurs at high concentrations of K+ in solution. The crystal radius of high-spin Co(II) decreases by 0.15 Å as the coordination changes from octahedral to tetrahedral, for a two-coordinated N radius of 1.245 Å. The mean Au–C distance is 2.003±0.016 Å with C–N = 1.124±0.021 Å and Co–N = 1.973±0.013 Å. The mismatch between Au–Au separations in Co[Au(CN)2]2 and in (111) of elementary Au is less than 8%. The identical but rotated sequence of Au–Au separations at the c/3 and 2c/3 levels in Co[Au(CN)2]2 coincides within 2.4% with the corresponding spacings in the element normal to (111) at the a√3 and 2a√3 levels. These small mismatches could result in epitaxial overgrowth leading to gold hardening and incorporation of the observed impurities in the electrodeposit.