Direct synthesis of alkali trifluorocobaltate(II) monohydrates, A[CoF3]·H2O (A = NH4, Na, or K). First synthesis and structural assessment of mixed-ligand fluoro complexes of cobalt(II), [Co(NH3)4F2] and A[CoF(SO4)(H2O)2](A = NH4, Na, or K)
Abstract
Alkali trifluorocobaltate(II) monohydrates, A[CoF3]·H2O (A = NH4, Na, or K), have been synthesised directly from the reaction of Co(OH)2 and alkali fluorides in 40% HF followed by precipitation with ethanol at pH <3. While a higher pH (5–6), maintained by the addition of AOH, contaminates the product for A = Na or K, a molecular complex [Co(NH3)4F2] is formed in the case where aqueous ammonia is used to raise the pH. The synthesis of mixed fluorocobaltates(II) of the type A[CoF(SO4)(H2O)2](A = NH4, Na, or K) has been achieved by reacting Co(OH)2 with AF and A2SO4, in the ratio 1 : 2 : 1, in the presence of a very small amount of 40% HF at a steam-bath temperature. The compounds were isolated by the addition of ethanol. The compounds have been characterised from the results of elemental analyses, pyrolysis at 120 °C, magnetic susceptibility measurements, i.r., laser Raman, and electronic spectroscopic studies. While the magnetic moments and electronic spectra of [Co(NH3)4F2] and [CoF(SO4)(H2O)2]– suggest an octahedral or a distorted-octahedral environment for cobalt(II), i.r. and laser Raman spectra provide evidence for the occurrence of a bridging bidentate sulphate ligand in the latter.