Octahedral–tetrahedral equilibria in aqueous cobalt(II) solutions at high temperatures

Evidence is presented to indicate that aqueous Co 2+ exists as the hexaaquo-ion in equilibrium with minor amounts (upper limits 0.08% at 298 K, 7% at 625 K, at 16–25 MPa) of tetraaquocobalt(II), with ΔH ~ +17 kJ mol −1 . The single visible absorption band of the supposed Co(H 2 O) 4 2+ has maxima at 552 nm and 486 nm in the intensity ratio 2:1. Hydrogen sulfate ion (up to 0.5 M at least) does not complex Co 2+ (aq) detectably in acidic media, 290–625 K, and sulfuric acid therefore holds promise as a non-complexing strong monobasic acid for high-temperature aqueous studies. In water containing 2.0 M or more Cl − , the tetrahedral form of cobalt(II) is CoCl 4 2− , ΔH for the octahedral → tetrahedral equilibrium being +62 kJ mol −1 ; forCoBr 4 2− , the corresponding ΔH is +70 kJ mol −1 , the greater endothermicity accounting entirely for the lower stability relative to CoCl 4 2− .