3.4.1 · D5Coordination Chemistry
Question bank — Werner's theory of coordination compounds
The figure below draws that split as two concentric zones around the metal — study it before you touch the questions, because every trap is really "which zone is this species in?"
Look at the mint inner ring (locked ligands, secondary valency) versus the coral outer cloud (free counter ions, primary valency). The bracket in the written formula is literally the boundary between these two zones.
True or false — justify
Notice how moving one from outside (coral) to inside (mint) drops the particle count from 4 to 3 — that single migration is the whole story of the first question.
and ionize into the same number of particles in water.
False — the first gives 4 ions (), the second gives 3 (), because one moved inside the bracket and stopped ionizing.
Primary valency is directional and fixes the geometry of the complex.
False — it is the secondary valency that is directional and fixes geometry; primary valency is just non-directional charge balance (the modern oxidation state).
Adding more ligands to a complex can push its coordination number above 6.
False — Werner's postulate is that the secondary valency (CN) is fixed at 6 for ; extra would simply not be accommodated in the coordination sphere.
A neutral complex like still splits into ions in water.
False — its net charge is zero, there are no counter ions to release, so it stays as one neutral particle and behaves as a non-electrolyte (conductivity ≈ 0).
In , all six chlorides are equivalent and none precipitate with .
True — all six are ligands inside the bracket, held by secondary valency, so none are free and none give AgCl.
The number of isomers Werner isolated could tell him the 3-D shape of the complex.
True — an octahedron gives exactly 2 isomers of (cis, trans), while a hexagonal-planar or trigonal-prism arrangement would give 3; counting bottles distinguishes the shapes (see Isomerism in coordination compounds (cis-trans, optical)).
Werner's theory explains why is coloured.
False — Werner's theory gives shape, formula and ionization, but not colour or magnetism; those needed Crystal Field Theory and colour of complexes.
Every ligand contributes exactly 1 to the coordination number.
False — polydentate ligands (en, oxalate) bind through two or more donor atoms; one en molecule adds 2 to CN. CN counts donor atoms, not molecules (see Ligands — classification (mono/poly-dentate, chelate)).
The geometry "proof" — see why octahedral gives exactly 2
Trace it: pin the first (coral) at the top vertex. The second can go to any of the 5 remaining vertices — but 4 of them are 90° neighbours (all equivalent by rotation = cis) and only 1 is the bottom, 180° away (trans). Five choices collapse into two distinct molecules. A flat hexagon would instead allow ortho/meta/para = three, which is why nature giving only two isomers proved the octahedron.
Spot the error
" has three chlorines, so precipitates 3 AgCl."
Error — only the 2 outside the bracket are ionizable and free; the third is a ligand locked by secondary valency, so exactly 2 precipitate.
"Primary valency = the number of coordinate bonds the metal forms."
Error — coordinate bonds are counted by the secondary valency (CN); primary valency is the ionizable charge balanced by counter ions.
" dissolves to give 4 ions, since it contains 3 Cl and 1 complex."
Error — it is , so two are ligands inside; it gives only 2 ions, .
"Since is neutral, it cannot occupy a secondary valency position."
Error — secondary valencies (ligand sites) are satisfied by neutral molecules or negative ions alike; neutrality has nothing to do with it.
"Higher conductivity means the complex ion carries more ligands."
Error — conductivity tracks the number of ions released, driven by counter ions outside the bracket, not by how many ligands sit inside. More ligands can actually mean fewer free ions.
"In the metal has zero valency because nothing ionizes."
Error — the metal is still (primary valency +3); it is simply exactly cancelled inside the bracket by three ligands, giving a net-neutral, non-ionizing complex.
"Because Cl⁻ is written outside in , the chlorine inside is also just a counter ion."
Error — position matters per atom: the inside the bracket is a bonded ligand (secondary valency); only the two written outside are counter ions.
Why questions
Why can two "saturated" molecules, and , still combine?
Because Werner's second (secondary) valency provides fixed hand-hold sites beyond ordinary valency, letting the metal grab neutral molecules even when its primary valency looks used up.
Why does act as Werner's "tester" for the theory?
reacts only with free in solution; the count of AgCl precipitated directly reveals how many chlorides are outside the bracket (ionizable) versus locked inside.
Why does adding into the coordination sphere lower the compound's molar conductivity?
Each moved inside becomes a non-ionizable ligand, so fewer free ions are released, and molar conductivity (which scales with ion count, see Conductivity and ionization of electrolytes) drops — exactly the falling staircase in the bar chart above.
Why is coordination number 6 associated with an octahedral shape rather than a hexagonal ring?
Because six directional secondary valencies pointing to fixed positions give an octahedron; experiment confirmed this since shows only 2 isomers (see the cis/trans figure), matching octahedral prediction.
Why did Werner's theory need later electronic theories to complete it?
Werner explained where ligands sit (skeleton) but not why a metal chooses a given CN, nor colour or magnetism; those depend on d-electrons, handled by Valence Bond Theory (VBT) of complexes and Crystal Field Theory and colour of complexes.
Why is the charge of the complex ion computed from oxidation state plus ligand charges?
The whole compound is neutral, so the bracketed charge must be built from the metal's oxidation state and the sum of ligand charges, and then cancelled by the counter ions outside.
Edge cases
What happens to the AgCl count when all anions are ligands, as in ?
Zero precipitate — there are no free , the limiting case where the complex is neutral and behaves as a non-electrolyte (the last bar in the chart above).
What is the maximum number of that can precipitate from , and when?
Three — achieved only in (), where all six sites are and every chloride is a free counter ion.
If a complex ion has net charge zero, how many counter ions does it need?
None — a zero-charge coordination sphere requires no ions outside the bracket, so the formula is just the bracketed unit alone.
For a polydentate ligand, can the number of molecules be less than the coordination number?
Yes — this is the whole point of chelation: e.g. three oxalate ions (each bidentate) fill CN 6 with only 3 molecules, since CN counts donor atoms.
Is it possible for two different formulas to precipitate the same number of Cl⁻ yet differ in total ion count?
Yes in principle, but among each free- count maps to a unique ion count; the trap is assuming AgCl count alone fixes everything — you also need conductivity to pin down total particles.
What does it mean if a compound's molar conductivity corresponds to more ions than there are counter ions plus one complex ion?
It signals your assumed formula is wrong — you likely placed a ligand outside the bracket (or a counter ion inside); conductivity is the cross-check that catches misplaced species.