Coordination Number N from Werner to Bjerrum

It can still be seen in mineralogical museums how Berzelius classified many compounds as adducts of oxides, as when alum is written K20 A1203 4 S03 24 H20 whereas it was later written KA1(S04)2 12 H20 and the crystal structure indicates only half of the water molecule oxygen atoms directly connected to aluminum K[A1(0H2)6](S04)2(0H2)6. Actually, the large majority of minerals are mixed oxides (and though these formulae derive from the precursor ideas of electrovalent bonding, considering calcium sulfate as an adduct CaO S03, they have the undoubted 

One of the motivations for studying ammonia complexes is that (before deuterium and oxygen isotopes were available) they offered one of the rare opportunities to get an idea about the constitution of aqua ions in solution. However, this does not always proceed as smoothly as in the case of nickel(II). Bjerrum found that Zn(NH3)/2 does not react further with aqueous ammonia, as confirmed by crystal structures containing this tetrahedral complex. However, the zinc(II) aqua ions in crystals are octahedral with N = 6 like magnesium(II) and nearly all 3d group aqua ions. 

Such discrepancies may be quite frequent, especially among colorless closed-shell cations. Thus, ultra-violet spectroscopic evidence is obtained44 for the silver(I) aqua ion being tetrahedral Ag(OH2)4 whereas concentrated ammonia40 does not react with linear Ag(NH3)2+. It may be added that Raman spectra45 suggest the presence of AgfNHj) in liquid ammonia, as well as Ca(NH3)g 2 and Zn(NH3)4 2. 

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