SCIENCE · INVESTIGATE AND EXPLAIN

Separate formal charge, donor count and geometric possibilities

A coordination number counts bound donor atoms, not ligand names or oxidation number.

Learning outcome: Compare all four formal ligand sets for ironII/III. Reconstruct complex charge, d count and donor count, then distinguish cis/trans placement from mirror-image possibilities.

Starting recap

Refresh state-function boundaries, absolute temperature, dimensionless logarithms, written reaction/electron equivalents, conserved charge and formal orbital/identity bookkeeping.

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Formal ironIII with four neutral NH3 and two Cl− has complex charge+1, six donor atoms and d5. Three neutral bidentate en ligands also give six donor atoms but only three ligand entities.

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Predict

Can two complexes share coordination number6 while having different ligand counts and charges?

Your prediction and reason

Experiment

Compare all four formal ligand sets for ironII/III. Reconstruct complex charge, d count and donor count, then distinguish cis/trans placement from mirror-image possibilities.

Separate oxidation state, ligand count and coordination number

Worked teaching model · synthetic values · Formal octahedral iron complexes with group8 metal. NH3 and ethylenediamine(en) are neutral; Cl− is monodentate and en bidentate. Coordination number6 in each selected model, but ligand number and complex charge differ. Formal bookkeeping does not establish stability, actual synthesis or ligand-field strength.

Formal iron oxidation state: 3 oxidation number Ligand set:0 six NH3,1 six Cl,2 four NH3/two Cl,3 three en: 2 choice

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Bookkeeping counts (different meanings): solid

Axes rescale. A multiplicity distribution, inverse-temperature relation and charge-budget graph have different meanings. Read each axis and do not treat a computed curve as additional measured evidence.

All calculated plot coordinates

Bookkeeping counts (different meanings)
0 ligand entities;1 donor atoms;2 d electrons	count
0	6
1	6
2	5

Calculated under the declared model
Quantity	Value	Unit
Formal iron oxidation number	3	oxidation number
Formal d-electron count	5	electrons
Ligand entities	6	count
Donor atoms bound to metal	6	coordination number
Total ligand formal charge	-2	charge number
Complex formal charge	1	charge number

Oxidation state, complex charge, ligand number and coordination number are separate ledgers.

Group8 iron gives formal d6 forII and d5 forIII. Three neutral bidentate en ligands occupy six donor sites, while six monodentate ligands also give coordination number6. Octahedral MA4B2 permits cis/trans geometry; tris-chelate arrangements can have mirror-image forms. These formal possibilities do not prove that every named packet is stable or synthesized.

These calculated packets are not performed chemical measurements or separation procedures. Keep feasible inventories, activity conventions, fitting observations and independent checks distinct.

Octahedral donor-site accounting

Six donor directions are +x, −x, +y, −y, +z and −z. Opposite directions make a trans pair; two perpendicular directions make a cis pair. These are geometric positions, not six distinct ligand molecules when a ligand is bidentate.

cis example

Cl at +x and +y; NH3 at the other four sites. Cl–metal–Cl angle90°.

trans example

Cl at +x and −x; NH3 at the other four sites. Cl–metal–Cl angle180°.

Oxidation state, complex charge, ligand number and coordination number are separate ledgers.

Observation to attach to a capture

Captures preserve recipes and replay their calculated results. They are synthetic model evidence, not physical measurements.

Understand

Keep four ledgers separate

For a formal coordination sphere, sum metal oxidation number and ligand formal charges to obtain complex charge. Count bound donor atoms for coordination number. Count whole ligand entities separately, and use the metal group and oxidation state for a formal d-electron count. None of these is a substitute for another.

Group8 ironII is formally d6 and ironIII d5. Six neutral ammonia ligands give coordination number6 and retain the metal’s positive charge. Six chloride ligands contribute−6, so a formal ironIII hexachloride complex has charge−3. Four ammonia plus two chloride around ironIII give charge+1.

Distinguish names, denticity and geometry

Ammonia is named ammine as a coordination ligand; an anionic iron coordination entity uses ferrate in standard naming, with the oxidation state indicated. For example, the formal [Fe(NH3)6]³+ identity is hexaammineiron(III), while [FeCl6]³− is a hexachloroferrate(III) identity. These naming exercises do not claim stability under unspecified conditions.

Ethylenediamine, en, is neutral and bidentate: three molecules can provide six donor atoms. In an octahedral MA4B2 pattern, the two B donors can be adjacent cis or opposite trans. Tris-chelate arrangements can possess non-superposable mirror-image forms. Square-planar and tetrahedral coordination have different geometric possibilities; six-coordinate octahedral assumptions are not universal.

Transfer and check

A formal metalIII sphere with four neutral monodentate ligands and two chloride ligands has charge+1 and coordination number6. If the metal instead isII, charge becomes0 without changing donor count. Explain why replacing two monodentate ligands by one bidentate ligand can change ligand entity count without changing coordination number.

Source and evidence boundary

Conceptual reference checked 30 October 2026: OpenStax Chemistry 2e. All prose, cases, graphs and questions are original. These supplied thermodynamic, electrical, nuclear or orbital models are not actual measurements, apparatus instructions, synthesis protocols or practical certification. Retain conventions, independent constraints, changed-case reasoning and limits. No heating, wiring, electrolysis, source handling or chemical manipulation is requested.

Apply

Public practice checks a specific claim. It does not establish independent mastery.

Why do formal charge and geometric possibility not establish chemical stability or a synthesis route?

Your explanation and limitations

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Previous: Separate nuclear bookkeeping, remaining parents and measured activity Next: Compare splitting and pairing while preserving a degenerate boundary

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