Exercise 5.1
Explain the bonding in coordination compounds in terms of Werner’s postulates.
Not cross-checked
This solution has not been cross-checked against the answer printed in NCERT.
Werner's central idea is that a metal ion in a complex satisfies two different kinds of valency at the same time — one that ions can neutralize, and one that only fixed positions in space can satisfy. Once you separate those two valencies, every observation about coordination compounds — why some chloride is precipitated by \(\displaystyle \text{AgNO}_3 \) and some is not, why the compounds have definite geometric shapes — falls into place.Postulate $\displaystyle 1$ — Primary and secondary valency are distinct.
Every metal ion has:
A primary valency, which is ionizable. It is satisfied only by negative ions and corresponds to what we now call the oxidation state of the metal.
A secondary valency, which is non-ionizable (does not dissociate into ions in solution). It is satisfied by neutral molecules or negative ions, called ligands, and corresponds to what we now call the coordination number.
Postulate $\displaystyle 2$ — Every metal has a fixed secondary valency.
The secondary valency (coordination number) is a fixed characteristic of the metal ion — for example \(\displaystyle \text{Co}^{3+} \) almost always shows a secondary valency of $\displaystyle 6$, \(\displaystyle \text{Pt}^{2+} \) shows $\displaystyle 4$ — and the metal tries to satisfy both its primary and secondary valencies.Postulate $\displaystyle 3$ — Secondary valencies point in fixed directions in space.
Because the secondary valencies are directional, the groups attached by secondary valency (ligands) occupy fixed positions around the metal, giving the complex a definite geometry: a coordination number of $\displaystyle 6$ gives an octahedral shape, $\displaystyle 4$ gives tetrahedral or square planar. Primary valencies, by contrast, are non-directional.Applying this to a real series of compounds.
Werner explained the puzzling behaviour of the compounds of \(\displaystyle \text{CoCl}_3 \) with \(\displaystyle \text{NH}_3 \) using exactly this idea. Experimentally, when treated with excess \(\displaystyle \text{AgNO}_3 \), the number of \(\displaystyle \text{Cl}^- \) ions precipitated as \(\displaystyle \text{AgCl} \) per formula unit was found to be:\[\text{CoCl}_3\cdot 6\text{NH}_3 \;\to\; 3\ \text{Cl}^-\text{ precipitated}
\]
\[\text{CoCl}_3\cdot 5\text{NH}_3 \;\to\; 2\ \text{Cl}^-\text{ precipitated}
\]
\[\text{CoCl}_3\cdot 4\text{NH}_3 \;\to\; 1\ \text{Cl}^-\text{ precipitated}
\]This is exactly the observation that a simple ionic formula cannot explain — all three compounds contain the same \(\displaystyle \text{Co}^{3+} \) and the same total \(\displaystyle \text{Cl}^- \), yet different fractions of the chloride behave as free ions. Werner resolved this by saying \(\displaystyle \text{Co}^{3+} \) has a primary valency of $\displaystyle 3$ and a secondary valency of $\displaystyle 6$, and that \(\displaystyle \text{NH}_3 \) molecules preferentially occupy the six secondary-valency positions, with \(\displaystyle \text{Cl}^- \) ions filling any secondary positions left over. A \(\displaystyle \text{Cl}^- \) held by secondary valency sits directly on the metal and is not free to ionize, while a \(\displaystyle \text{Cl}^- \) held only by primary valency is outside this fixed arrangement and ionizes in solution.This gives the constitutions
\[[\text{Co(NH}_3)_6]\text{Cl}_3,\qquad [\text{Co(NH}_3)_5\text{Cl}]\text{Cl}_2,\qquad [\text{Co(NH}_3)_4\text{Cl}_2]\text{Cl}
\]The species inside the square brackets is the coordination entity, held together by the six secondary valencies of cobalt (a fixed octahedral arrangement, per Postulate $\displaystyle 3$) — these ligands do not dissociate. The \(\displaystyle \text{Cl}^- \) ions written outside the bracket are held only by primary valency, remain as free ions in solution, and are the ones that precipitate with \(\displaystyle \text{AgNO}_3 \). This is why the count of precipitable chloride drops from $\displaystyle 3$ to $\displaystyle 2$ to $\displaystyle 1$ as more \(\displaystyle \text{NH}_3 \) molecules (up to the fixed secondary valency of $\displaystyle 6$) take up positions directly on the cobalt and push chloride ions out of the primary-valency role into the secondary-valency role inside the bracket.Answer: Werner's postulates explain bonding in coordination compounds by proposing that a metal has a fixed primary (ionizable) valency equal to its oxidation state and a fixed, spatially directed secondary (non-ionizable) valency equal to its coordination number; ligands occupying the secondary-valency positions form a fixed geometric coordination entity (e.g., octahedral for \(\displaystyle [\text{Co(NH}_3)_6]^{3+} \)) and do not ionize, while only groups satisfying primary valency outside this entity dissociate as free ions — as confirmed by the graded \(\displaystyle \text{AgCl} \) precipitation from \(\displaystyle \text{CoCl}_3\cdot 6\text{NH}_3 \), \(\displaystyle \text{CoCl}_3\cdot 5\text{NH}_3 \), and \(\displaystyle \text{CoCl}_3\cdot 4\text{NH}_3 \).