Historical.
Cadmium acts so generally as a bivalent element that it is usually regarded as entering into combination only where it can play this rôle. The only compound described, in which it has apparently a lower valence than two, was prepared by Marchand[10]. It was obtained by heating cadmium oxalate to the melting point of lead when a green powder remained behind which resembled chromium oxide. When heated on the air it appeared to be decomposed into metal and oxide. When treated with mercury the compound was not altered. An analysis showed it to have the composition represented by the formula Cd₂O.
A. Vogel[11] has shown that the green powder described by Marchand consists of a mixture of the metal and oxide. When this mixture is treated with dilute acetic acid the metal remains behind as microscopic glistening globules. The lower the temperature at which the oxalate is decomposed the more oxide and the less metal were found in the product.
There was then no compound known in which cadmium acted as if its valence was less than two when this work was undertaken.
That it may act with a greater valence was shown by R. Haafs[12]. He found that when zinc hydroxide was treated with hydrogen dioxide certain compounds of zinc and oxygen were formed containing more oxygen than the normal oxide ZnO. The close resemblance between zinc and cadmium led him to try the same reaction with cadmium. Hydrogen dioxide was accordingly allowed to act on cadmium hydroxide and the resulting product analyzed. There were formed Cd₅O₈, Cd₃O₅ and Cd₄O₇. In no case was the compound CdO₂ obtained. These compounds are described as fairly stable even at a hundred degrees.