Platinum ammonates

The primary compounds considered by Blomstrand, Jorgensen, and Werner, as they struggled to provide a theory for what we now know as coordination compounds were the cobalt ammonates. Another series known in the late 1800s was the platinum ammonate chlorides. Data for this series is given in the table at the top of the next page. 

Selective reduction of 11 to 12 is achieved in high yield by the use of 5% Rh-on-C in DMF containing NH OH. Reduction essentially stops after absorption of 3 mol of hydrogen. Yields were lower in ethanol. Platinum oxide in ammonical DMF showed fair selectivity, but Pd-on-C none. In a typical experiment, 0.1 mol of 11 in 250 ml DMF containing 3 ml 28% NH4OH solution and 0.7 g 5% Rh-on-Al O was reduced at 40 psig until 0.3 mol of hydrogen were absorbed (2). 

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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