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.