Cobalt ammonates

From 1900 to 1950, the realm of inorganic chemistry continued to expand. In one of the most productive avenues of research, Alfred Werner worked through the first two decades of the new century to bring order to the mysterious set of cobalt ammonates and related compounds that had been painstakingly synthesized during the nineteenth century. His coordination theory gave us new ways to think about the structures, properties, and reactions of this new class of what he called coordination compounds. Coordination chemistry, one of the primary components of inorganic chemistry to this day, is the subject of Part I (Chapters 2-6) of this book. 

Law of conservation of matter Lavoisier 1799 Law of definite composition Proust 1798 First cobalt ammonates observed Tassaert... 

At the very end of the eighteenth century, Tassaert—a French chemist so obscure in the history of chemistry that his first name remains unknown—observed that ammonia combined with a cobalt ore to yield a reddish-brown product. This was most likely the first known coordination compound. Throughout the first half of the nineteenth century, many other, often beautifully crystalline examples of various cobalt ammonates were prepared. These compounds were strikingly colored, and the names given to them—for example, roseo-, luteo- (from the Latin luteus, meaning deep yellow ), and purpureocobaltic chlorides—reflected these colors. 

The Cobalt Ammonate Chlorides (Data Available to Blomstrand, Jorgensen, and Werner)... 

In 1869 Christian Wilhelm Blomstrand first formulated his chain theory to account for the cobalt ammonate chlorides and other series of ammonates. Blomstrand, knowing that the fixed valence of cobalt was established at 3, chained together cobalt atoms, divalent ammonia groups, and monovalent chlorides to produce a picture of... 

Representations of the cobalt ammonate chlorides by Blomstrand and Jorgensen (o) Blomstrand s representation of C0CI3 6NH3 (b) Jorgensen s representations of four members of the series with the iridium substituted for the intended cobalt in compound (4). (Adapted from F. Basolo and R. C. Johnson, Coordination Chemistry, 2nd edition, p.6. Copyright 1986. Reprinted by permission of Science Reviews 2000 Ltd., www.sciencereviews2000. co.uk.)... 

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. 

Figure 2.3 shows Werner s early proposals for the bonding in the cobalt ammonates. He said that the cobalt must simultaneously satisfy both its primary and secondary valences. The solid lines show the groups that satisfy the primary valence, and the dashed lines, always directed toward the same fixed positions in space, show how the secondary valence was satisfied. In compound (1), all three chlorides satisfy only the primary valence, and the six ammonias satisfy only the secondary. In compound (2), one chloride must do double duty and help satisfy both valences. The chloride that satisfies the secondary valence (and is directly bound to the Co ion) was concluded to be unavailable for precipitation by silver nitrate. Compound (3) has two chlorides doing double duty and only one available for precipitation. Compound (4), according to Werner, should be a neutral compound with no ionizable chlorides. This was exactly what Jorgensen had foimd with the iridium compound. 

Werner s representations of the cobalt ammonate chlorides. The solid lines... 

Today, the molecular formulas of coordination compounds are represented in a manner that makes it clearer which groups are part of the coordination sphere and which are not. As indicated in the introduction to this chapter, the metal atom or ion and the ligands coordinated to it are enclosed in brackets. It follows that the cobalt ammonate chlorides can be represented as... 

The Blomstrand-Jorgensen chain theory was the most successful of the early theories that attempted to explain the known series of cobalt ammonates. This theory combined trivalent cobalt atoms, divalent ammonia radicals, and monovalent chlorides to produce structures that accounted for some of the formulas, conductivities, and reactions of these compounds. However, when an analog of a critical compound was finally synthesized, the chain theory s prediction was wrong, and it started to lose favor. 

Werner literally dreamed up the modern theory of coordination compounds in 1892. He envisioned that metals had two types of valence, which we refer to today as oxidation state and coordination number. Some ligands satisfy only the coordination number, whereas others simultaneously satisfy the oxidation state. These ideas explain why some chlorides in the cobalt ammonate chlorides are ionizable and some are not. By comparing the actual number of known isomers with the number that should exist for various geometries, Werner concluded that the six ligands in the cobalt ammonates were in an octahedral arrangement. 

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. 

Sidgwick applied these ideas to coordination compounds. He noted that compounds such as the cobalt ammonates, described so ably by Alfred Werner s coordination theory, could also be classified as Lewis adducts. Equation (4.2) shows the formation of the hexaamminecobalt(III) cation from the Co " " cation and six... 

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],... 

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