Nomenclature of Coordination Complexes
Introduction
Coordination complexes, composed of a central metal ion bonded to surrounding ligands, have unique naming conventions to standardize their representation. These complexes exhibit various chemical and physical properties, such as different isomeric forms, magnetic behavior, and colors, making proper nomenclature crucial for clear communication.
Important Terms
- Coordination Complex: A compound consisting of a metal ion and surrounding ligands.
- Complex Ion: A charged coordination complex.
- Inner Coordination Sphere: Ligands directly bonded to the metal.
- Outer Coordination Sphere: Counterions and solvent molecules surrounding the complex.
- Donor-Acceptor Concept: Ligands donate electron pairs (donors), and metals accept them (acceptors).
- Coordination Number: The number of ligand donor sites attached to the metal.
- Organometallic Complexes: Coordination compounds containing metal-carbon bonds.
Components of a Coordination Complex
A coordination complex consists of:
- Central Metal Ion – Acts as a Lewis acid by accepting electron pairs.
- Ligands – Lewis bases that donate electron pairs to the metal.
- Oxidation State – Represents the charge on the metal after accounting for the ligand contributions.
General Naming Rules
- Ligands are named first in alphabetical order.
- The metal name follows, along with its oxidation state in Roman numerals.
- Cations are named before anions in a coordination compound.
Naming Ligands
1. Anionic Ligands
- Anionic ligands ending in “-ide” are changed to “-o”
- “-ite” and “-ate” endings become “-ito” and “-ato,” respectively.
| Ligand | Common Name | IUPAC Name |
| H– | hydrido | hydrido |
| F– | fluoro | fluorido |
| Cl- | chloro | chlorido |
| Br- | bromo | bromido |
| I– | iodo | iodido |
| CN– | cyano | cyanido or cyanido-κC |
| CN– (M-NC) | isocyano | isocyanido or cyanido-κN |
| CH3NC | methylisocyanide | methylisocyanide |
| N3– | azido | azido |
| SCN- (M-SCN) | thiocyanato | thiocyanato-κS |
| NCS- (M-NCS) | isothiocyanato | thiocyanato-κN |
| CH3CO2– | acetato | ethanoato |
| N3– | nitrido | nitrido |
| NH2– | imido | azanediido |
| NH2– | amido | azanido |
| NH3 | ammine | ammine |
| RNH2, R2NH, R3N | alkylamine, dialkylamine, trialkylamine | alkylamine, dialkylamine, trialkylamine |
| P3– | phosphido | phosphido |
| PH3 | phosphine | phosphane |
| PR3 | trialkylphosphine | trialkylphosphane |
| PAr3 | triarylphosphine | triarylphosphane |
| DMSO | dimethylsulfoxide | dimethyl(oxido)sulfur |
| O2– | oxo | oxido |
| OH– | hydroxo | hydroxido |
| H2O | aqua | aqua |
| S2– | sulfo | sulfo |
| HS– | hydrosulfido | hydrosulfido |
| H2S | hydrogen sulfide | hydrogen sulfide |
| R2S | dialkyl sulfide | dialkyl sulfide |
| O2 | dioxygen | dioxygen |
| O2– (superoxide) | superoxido | dioxido(1-) or superoxido |
| O22- (peroxide) | peroxido | dioxido(2-) or peroxido |
| N2 | dinitrogen | dinitrogen |
| NO | nitrosyl | nitrosyl |
| CO | carbonyl | carbonyl |
| CS | thiocarbonyl | thiocarbonyl |
| NO2 (M-NO2) | nitryl | nitrogen dioxide-κN |
| CO32- | carbonato | carbonato |
| NO2– (M-NO2) | nitro or nitrito-N | nitrito-κN |
| NO2– (M-ONO) | nitrito or nitrito-O | nitrito-κO |
| NO3– | nitrato | nitrato |
| SO32- | sulfito | sulfito |
| SO42- | sulfato | sulfato |
| S2O32- (M-S-SO2-O-) | thiosulfato-S | thiosulfato-κS |
| S2O32- (M-O-SO2-S-) | thiosulfato-O | thiosulfato-κO |
2. Neutral Ligands
Some neutral ligands retain their molecular names, while others have special names:
| Molecular Formula | Ligand Name |
| NH₃ | Ammine |
| H₂O | Aqua |
| CO | Carbonyl |
| NO | Nitrosyl |
| CH₃NH₂ | Methylamine |
| C₅H₅N | Pyridine |
3. Polydentate Ligands
Polydentate ligands (chelating agents) are named similarly to monodentate ligands:
| Short Name | Extended Name |
| en | Ethylenediamine |
| ox | Oxalato |
| EDTA | Ethylenediaminetetraacetato |
4. Ligand Multiplicity
The number of ligands is indicated with prefixes:
| Number | Monodentate Ligands | Polydentate Ligands |
| 1 | mono | – |
| 2 | di | bis |
| 3 | tri | tris |
| 4 | tetra | tetrakis |
| 5 | penta | pentakis |
| 6 | hexa | hexakis |
Note: Prefixes do not affect the alphabetical order of ligands.
Naming the Metal
- The metal’s oxidation state is given in Roman numerals (e.g., Iron(III)).
- If the complex is an anion, the metal name ends in “-ate.” Some metals adopt Latin names:
| Metal | Latin Name |
| Iron | Ferrate |
| Copper | Cuprate |
| Silver | Argentate |
| Gold | Aurate |
| Tin | Stannate |
| Lead | Plumbate |
Naming Coordination Complexes
Example 1: [CrCl₂(H₂O)₄]⁺
- Ligands: “aqua” (4), “chloro” (2)
- Alphabetical order: Tetraaquadichlorochromium(III) ion
Example 2: [CoCl₂(en)₂]⁺
- Ligands: “chloro” (2), “ethylenediamine” (2, requiring “bis”)
- Name: Dichlorobis(ethylenediamine)cobalt(III) ion
Writing Chemical Formulas
- List ligands alphabetically (regardless of charge or quantity).
- Write the metal first followed by ligands.
- Charge balance the formula using oxidation states.
Example 3: “Amminetetraaquachromium(II) ion”
- Formula:
[Cr(NH₃)(H₂O)₄]²⁺
Example 4: “Potassium hexacyanoferrate(III)”
- Formula:
K₃[Fe(CN)₆]
The nomenclature of coordination complexes is essential for effective chemical communication. By following systematic rules for naming ligands, metals, and oxidation states, chemists can accurately describe and interpret these unique compounds.