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 ion is named ligands first, in alphabetical order with di-, tetra- or hexa- for the counts, then the metal with its oxidation state in roman numerals. Water is aqua, ammonia is ammine, and anionic ligands end in -o, as in chlorido and cyanido. A salt names the cation before the anion, as in tetraamminecopper(II) sulfate. The rule runs backwards too: diamminesilver(I) chloride is [Ag(NH3)2]Cl, with two ammonias inside the brackets and one chloride outside.
Two numbers complete the description. The oxidation state comes from balancing charges. In K3[Fe(CN)6], three K+ give +3 and six CN- give -6, so iron must be +3 for the compound to be neutral. The coordination number counts donor atoms bonded to the metal, not ligands. In [Co(en)2Cl2]+, each ethane-1,2-diamine (en) binds through two nitrogens, so 2 x 2 + 2 = 6.
One formula can give several real compounds. In a square planar complex, two matching ligands can sit side by side (cis) or across from each other (trans). In an octahedral complex with four of one ligand and two of another, such as [Co(NH3)4Cl2]+, the chlorides can be cis, 90 degrees apart, or trans, 180 degrees apart. With three of each ligand, the matching three can share one triangular face (fac) or lie in one plane through the metal with two opposite (mer).
Other isomers swap pieces rather than positions. Linkage isomers swap the donor atom of one ligand, such as NO2- binding through N (nitro) or through O (nitrito). Ionisation isomers swap an ion between inside and outside the brackets, as in [Co(NH3)5Br]SO4 and [Co(NH3)5SO4]Br. Coordination isomers swap ligands between a complex cation and a complex anion, as in [Co(NH3)6][Cr(CN)6] and [Cr(NH3)6][Co(CN)6].
Name ligands before metal with the oxidation state, count donor atoms for the coordination number, and sort isomers by what moved.
Original chemistry lesson and assessment text written for os-taxonomy.
The readings on naming and on why complexes form were written for this dataset. The readings on geometric isomers and on coordination number and oxidation state are original writing for os-taxonomy.
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.