Ligands, coordination numbers and complex nomenclature

Start with the coordination entity

A coordination entity contains a central atom or atoms and surrounding bound ligands. In the simple mononuclear examples here, the centre is a metal and the ligands are molecules or ions directly attached through specified donor atoms. A Lewis acid–base picture treats a ligand pair as available for interaction with the centre, while the resulting bond belongs to the electronic structure of the whole complex. The pair’s original source does not create a permanently separate category of electrons.

Square brackets identify the entity whose composition and charge are being given. Counterions outside those brackets balance a salt’s overall charge. They need not be donors to the same metal in the specified entity. A solution can contain other species, equilibria and solvent coordination; the written formula is the starting identity rather than a complete speciation measurement.

Count donor attachments and ligand entities separately

For these ordinary localized donor examples, coordination number counts atoms directly bound to the centre. A monodentate ligand contributes one such donor; a bidentate ligand contributes two when both sites bind that centre. Denticity describes the attachment of one ligand to one centre. Coordination number therefore need not equal the number of ligand entities. Later metal–carbon and delocalized ligand cases require additional bonding conventions.

Ethane-1,2-diamine, abbreviated en here, has formula C₂H₈N₂ and two N donor sites. In [Co(en)₂Cl₂]⁺, two bidentate en ligands and two monodentate chlorido ligands give six donors but four ligand entities. In [Co(en)₃]³⁺, three bidentate en ligands give six donors but three molecules. Chelation means attachment at two or more separate binding sites of one ligand to the same centre; the connecting ligand framework matters.

An ambidentate ligand has alternative donor choices. Nitrite in the selected linkage isomers binds through N or O, one donor at a time. That alternative does not make it bidentate in either entity. A bridging ligand joins more than one centre and needs a separate connectivity description.

Use the ligand assignments before counting d electrons

For [Co(NH₃)₄Cl₂]⁺, neutral NH₃ and two assigned Cl⁻ ligands give x−2=+1, so Co is formally +3. Co is a Group 9 metal, giving the selected ionic d count 9−3=6. There are six bound donor atoms and six ligand entities in this particular monodentate entity. The oxidation state and d count are formal assignments, not measured partial charge or a complete metal population.

Similarly, assigning six CN⁻ ligands gives Fe +2 in [Fe(CN)₆]⁴⁻ and +3 in [Fe(CN)₆]³⁻. Their selected d counts are 6 and 5. Neutral potassium salts require four and three K⁺ counterions respectively. Both entities have six directly bound donors; a one-electron formal redox change does not by itself specify the rate, spin state or reaction pathway.

The IUPAC transition-element definition allows an incomplete d subshell in the atom or in a cation. A selected d¹⁰ complex does not exclude its element from that definition, nor does the definition require every compound of a transition element to have the same d occupancy.

A name can communicate more than composition

In the additive convention used here, name the ligands before the centre and alphabetize ligand names while disregarding multiplicative prefixes. Bound NH₃ is ammine, with two m letters; bound water is aqua. Modern ligand forms include chlorido and cyanido. The ion [Co(NH₃)₄Cl₂]⁺ can be described as tetraamminedichloridocobalt(III), with its stereochemistry still unspecified.

The κ label identifies a bound donor. For [Co(NH₃)₅(NO₂)]²⁺ and the O-bound connectivity written [Co(NH₃)₅(ONO)]²⁺, nitrito-κN and nitrito-κO distinguish linkage while preserving Co(III), entity charge +2 and six bound donors. A charge-based additive name can instead append cobalt(2+) for the whole entity; that charge descriptor must not be mistaken for Co oxidation state +2.

Names and formulae may need cis/trans, fac/mer, bridging μ labels or other descriptors to specify the intended connectivity and stereochemistry. Use the adopted convention consistently. A composition-only formula and a coordination count cannot settle every geometry or actual species present.

Worked examples

Try it yourself

1. In [Co(en)₂Cl₂]⁺, both N atoms of each neutral en ligand bind and each chlorido contributes one donor. Find ligand-entity count and coordination number.

   Show the answer and reasoning

   Four ligand entities; coordination number six.

   Two en molecules and two chlorido ligands give four ligand entities; 2× 2+2× 1 gives six bound donors.

2. With each CN assigned −1, find the Fe oxidation state and formal d count in [Fe(CN)₆]³⁻, and the number of K⁺ counterions for a neutral salt.

   Show the answer and reasoning

   Fe+3, formal d⁵; three K⁺.

   x−6=−3 gives +3; Group 8 minus 3 gives 5; three positive unit charges balance the entity charge.

3. Which spelling denotes bound NH₃ in this additive convention?

   Show the answer and reasoning

   Ammine.

   It uses two m letters; amine names describe a different ligand naming context.

4. Does cobalt(2+) in the selected charge-based nitrito entity name mean Co oxidation state +2?

   Show the answer and reasoning

   No.

   The whole complex has charge +2; the stated ligand assignments give Co +3.

What to carry forward

• Read the bracketed entity and its external counterions separately.
• Count direct donors separately from ligand entities.
• State denticity and the actual bound atom.
• Use ligand assignments before a formal d count.
• Retain connectivity and stereochemistry in names when needed.