Werner's Theory of Coordination Compounds

Setting the Stage

Transition metals readily bind to a number of anions or neutral molecules through shared electron pairs, producing what are now called coordination compounds. Long before electronic structure was understood, the Swiss chemist Alfred Werner (1866–1919) worked out how these compounds must be put together, purely from their chemical and physical behaviour. His theory, proposed in 1898, is the historical foundation on which the modern picture of coordination compounds rests, and it earned him the Nobel Prize in 1913.

Two Kinds of Valence

Werner's central insight was that a metal in one of these compounds exercises two distinct kinds of valence (linkage) rather than one.

• Primary valence is the ordinary, ionisable valence — the one satisfied by negative ions and reflected in a simple binary salt. For instance, $CrCl_3$, $CoCl_2$ and $PdCl_2$ show primary valences of 3, 2 and 2 respectively. Because it is ionisable, this valence corresponds to what is now called the metal's oxidation state.
• Secondary valence, by contrast, is non-ionisable. It is satisfied by neutral molecules or by negative ions that stay firmly attached to the metal, and — unlike the primary valence — it is a fixed number characteristic of the metal, corresponding to what is now called the coordination number.

Evidence from the Cobalt(III) Chloride–Ammonia Series

Werner's case rested on a set of adducts formed between cobalt(III) chloride and ammonia. When these compounds were dissolved and treated with excess silver nitrate in the cold, only part of the chlorine they contained precipitated as $AgCl$ — and the fraction that precipitated depended on how much ammonia the compound carried:

NCERT records the observations as a four-line relation:

• 1 mol $CoCl_3\!\cdot\!6NH_3$ (Yellow) gave 3 mol $AgCl$

• 1 mol $CoCl_3\!\cdot\!5NH_3$ (Purple) gave 2 mol $AgCl$

• 1 mol $CoCl_3\!\cdot\!4NH_3$ (Green) gave 1 mol $AgCl$

• 1 mol $CoCl_3\!\cdot\!4NH_3$ (Violet) gave 1 mol $AgCl$

• The compound with six ammonia molecules per cobalt (yellow) released all three of its chlorides as $AgCl$.

• The one with five ammonia molecules (purple) released only two of its three chlorides.

• The ones with four ammonia molecules (green and violet) each released only one chloride.

The consistent pattern is that the chloride ions not precipitated by silver nitrate are not "missing" — they are simply unavailable for that reaction because they are held inside the same unit as the cobalt, while the chlorides that do precipitate are held outside that unit as free ions in solution. Conductivity measurements told the same story: the yellow compound behaves as a 1:3 electrolyte, the purple one as a 1:2 electrolyte, and both four-ammonia compounds as 1:1 electrolytes.

Werner explained this by proposing that in every one of these compounds, six groups in total — chloride ions, ammonia molecules, or a mixture of the two — stay bonded directly to the cobalt ion throughout the reaction, forming a single non-dissociating unit that he wrote inside square brackets. Any chloride not needed to fill this unit of six is left over as a simple counter ion outside the brackets, and it is only this "outside" chloride that reacts freely with silver nitrate. So the six-membered inner unit is written as, for example, $[Co(NH_3)_6]^{3+}$ or $[CoCl(NH_3)_5]^{2+}$, with the remaining chlorides as separate ions completing the formula. In every one of these cobalt–ammonia compounds the count of directly bonded groups works out to six, so Werner assigned all of them a secondary valence of six.

Werner's Postulates

Werner's 1898 theory of coordination compounds can be stated as four main postulates:

1. In a coordination compound, the metal shows two kinds of linkages (valences) — primary and secondary.
2. The primary valences are normally ionisable, and they are satisfied by negative ions.
3. The secondary valences are non-ionisable. They are satisfied either by neutral molecules or by negative ions, and this secondary valence is fixed for a given metal — it is equal to what is now called the coordination number.
4. The ions or groups attached through the secondary linkages have a characteristic spatial arrangement corresponding to the particular coordination number involved — that is, the geometry around the metal is fixed by how many groups are secondarily bonded to it.

Coordination Entities, Counter Ions, and Shape

In the language used today, the spatial arrangement referred to in the fourth postulate is called a coordination polyhedron, the group of atoms held inside the square bracket is called a coordination entity (or complex), and any ion left outside the bracket is called a counter ion. …