Chromium ammonates

Many of the following powdered metals reacted violently or explosively with fused ammonium nitrate below 200°C aluminium, antimony, bismuth, cadmium, chromium, cobalt, copper, iron, lead, magnesium, manganese, nickel, tin, zinc also brass and stainless steel. Mixtures with aluminium powder are used as the commercial explosive Ammonal. Sodium reacts to form the yellow explosive compound sodium hyponitrite, and presence of potassium sensitises the nitrate to shock [1], Shock-sensitivity of mixtures of ammonium nitrate and powdered metals decreases in the order titanium, tin, aluminium, magnesium, zinc, lead, iron, antimony, copper [2], Contact between molten aluminium and the salt is violently explosive, apparently there is a considerable risk of this happening in scrap remelting [3],... 

This is known as pinacol coupling. The use of Zn-Cu couple to couple unsaturated aldehydes to pinacols was recorded as early as 1892. Subsequently chromium and vanadium and some ammonical-TiCl3 based reducing agents were used. 

In the second half of the century, other ammonates, particularly those of chromium and platinum, were prepared. Despite various attempts, however, no theoretical basis was developed to account satisfactorily for these wondrous compounds. 

Werner decided that the idea of a single fixed valence could not apply to cobalt and other similar metals. Working with the cobalt ammonates and other related series involving chromium and platinum, he proposed instead that these metals have two types of valence, a primary valence hauptvalen and a secondary valence nebenvalen . The primary, or ionizable, valence corresponded to what we call today the oxidation state, for cobalt, it is the 3+ state. The secondary valence is more commonly called the coordination number, for cobalt, it is 6. Werner maintained that this secondary valence was directed toward fixed geometric positions in space. 

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