What a learner can do afterwards
- Names a complex from its formula and writes the formula back from the name
- Works out the coordination number and the oxidation state of the metal for charged and neutral ligands
- Draws cis and trans and fac and mer isomers for the appropriate geometries
- Distinguishes linkage, ionisation and coordination isomerism with an example of each
1 · Read
A complex is named in a fixed order: ligands first in alphabetical order with Greek counts, then the metal with its oxidation state in roman numerals. Salts name the cation before the anion, as in tetraamminecopper(II) sulfate. The convention runs backwards too: hexaamminecobalt(III) chloride becomes [Co(NH3)6]Cl3, with six ammonias inside the brackets and three chlorides outside.
Two numbers complete the description. The oxidation state comes from balancing charges: in [Co(NH3)4Cl2]Cl the four ammonias give 0 and the three chlorides give minus 3 in total, so cobalt must be plus 3. The coordination number counts donor atoms touching the metal: six ammonias give six, and three double-toothed ethanedioate ligands give six as well.
Geometry turns one formula into several real compounds. In a square planar complex the two matching ligands can sit side by side as cis or opposite each other as trans. In an octahedral complex, three matching ligands can cap one face as fac or ring the middle as mer, and each form has its own properties.
Three more isomerisms swap pieces rather than positions. Linkage isomers swap the donor atom of one ligand, ionisation isomers swap an ion between inside and outside the brackets, and coordination isomers swap ligands between two complex ions. Name the swapped piece and you have named the type.
Name ligands before metal with the oxidation state, count donor atoms for the coordination number, and sort isomers by what moved.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.