It is a tricky job to name a coordination complex because a such type of complex consists of many parts i.e. a central metal ion, counter ions, ligands, where ions (may be +ve or -ve) mainly fulfill the primary valencies, which in turn counterbalance the charge of the coordination sphere (Central atom + ligands) and ligands mainly satisfy the secondary valency. Ligands can be neutral, positively charged (cationic ligand) or negatively charged (anionic ligand). So, there are some rules approved by IUPAC (International Union of Pure and Applied Chemistry) to name the central ion, ligands and ions to get an appropriate nomenclature of the coordination complex. Here we will basically learn the rules and how to apply them for such nomenclature.
Before going to the rules we can take a look to the different part of a coordination compound.
Now let’s come to the rules.
Rule 1: The naming should start with ligands maintaining alphabetic order if there are more than one type of ligands and after that central metal ion should be named.
- For anionic ligands, the name should end with “o”. For anions ending with “ide” (chloride, bromide etc.), ‘’ate’’ (nitrate, sulphate etc.) and “ite” (nitrite, sulphite etc.), the changes will be “ido” (Chlorido, bromido etc.), “ato” (nitrato, sulphato etc.) and “ito” (nitrito, sulphito) respectively.
- For neutral ligands, common name of the molecules should be used. (Some exceptions are there: water is named as aqua, ammonia is named as ammine, CO is named as carbonyl and N2 and O2 are named as dinitrogen and dioxygen respectively.)
Rule 3: When more than one ligands are connected to the central metal ion then di, tri, tetra, penta etc. prefixes are attached with the ligand name. When the ligand name already contains these prefixes then bis, tris, tetrakis, pentakis etc. are used instead of di, tri, tetra, penta respectively.
Rule 4: Cation name should be written first and then anion name should be written for an ionic coordination complex.
Rule 5: Oxidation number should be written in Roman digits with the sign in a first bracket () after the central metal ion name.
Rule 6: For cationic or neutral complex, the usual name of the central metal ion should be used. But in case of anionic complex, the “ate” suffix is attached with the usual metal name. (e.g. copper is changed cuprate, molybdenum is changed to molybdate etc.)
Rule 7: In case of ambidentate ligand (one having two donor site), the naming of the ligand depends upon which donor site is binding to the central metal ion, e.g. SCN– (donor atom S) is called as thiocyanato or S-thiocyanato and NCS- (donor atom N) is called as isothiocyanato or N-thiocyanato etc.
In GAMSAT following type of question can be asked with suitable vignette. Have a look.
- Which one of the following IUPAC nomenclature will be suitable for the compound [Cr(H2O)(en)2(SCN)2]Cl. [Given: en = ethylene diamine and the donor site of SCN– is S]
- aquadiisothiocyanatodi(ethylene diammine)chromium(III) chloride
- diisothiocyanatoaquabis(ethylene diammine)chromium(III) chloride
- aquabis(ethylene diammine)diisothiocyanatochromium(III) chloride
- aquadithiocyanatobis(ethylene diammine)chromium(III) chloride
Approach: Here among three types of ligands aqua, isothiocyanato, and ethylene diammine, the alphabetical order should be aqua, ethylene diammine and isothiocyanato (a, e, i). So the naming of ligands should be according to this sequence.
As di is already used in the name of ethylene diammine, so for two en a bis prefix should be added in place of di.
Now, here the central metal ion in the cationic portion, so the usual name of the metal ion should be used (chromium for Cr).
The oxidation number of chromium here can be calculated as +III. So, (III) should be written after metal name.
Lastly, the chloride should be used as the name of the anion satisfying primary valency.
So, the correct naming should be option C i.e. aquabis (ethylene diammine) diisothiocyanatochromium (III) chloride.