Coordination Complex Formation
A chelate is a coordination complex formed between a central metal atom or ion and chelating ligands, which are ions or molecules that bond to it with more than one atom. Any bidentate or polydentate ligand is a chelating ligand.
Geometry of Coordination Complexes
Geometries of Coordination Complexes
|2||Linear: The cation and ligands form a line.|
|3||Trigonal planar: The ligands are on the same plane around the cation at 120° angles.|
|4||Tetrahedral: The ligands form a triangular pyramid. The cation is at the center of the pyramid. Square planar: The ligands are on the same plane around the cation at 90° angles.|
|5||Trigonal bipyramidal: Three ligands arrange themselves as a triangular planar shape. The remaining two ligands arrange themselves perpendicularly to this plane, forming a linear structure with the metal cation.
Square pyramidal: Four ligands arrange themselves as a square planar shape. The remaining ligand is perpendicular to this plane.
|6||Octahedral: Four ligands arrange themselves as a square planar shape. The remaining two ligands arrange themselves perpendicularly to this plane, forming a linear structure with the metal cation.|
|7||Pentagonal bipyramidal: Five ligands are on the same plane around the cation at 72° angles. The remaining two ligands arrange themselves perpendicularly to this plane, forming a linear structure with the metal cation.|
|8||Square antiprism: Four ligands form a square at one side of the cation, and the remaining four ligands form another square at the opposite side of the cation.
Dodecahedral: The ligands arrange themselves in 12 triangular planes around the cation.
|9 or more||Other configurations|
The geometry of coordination complexes can be symmetrical if all the ligands are identical. However, this is not always the case. When different ligands attach to the central metal ion or different atoms of the same ligand attach to the central metal ion, the geometries vary according to the positions of atoms relative to the central metal ion.
An isomer is one of two or more molecules that share the same chemical formula but have different arrangements of atoms. Isomers can have very different properties from each other despite having the same chemical formula.In the square planar complexes, four ligands are bound to a metal (M). M may be an ion or neutral. If three of the ligands are the same (A) and one is different (B), no isomers exist because they are related by rotation about the same axis. Rotation of the model gives the same molecule.
Square Planar Rotation
Square Planar Cis and Trans Isomers
Coordination complexes are named according to five rules outlined by Swiss chemist and Nobel Laureate Alfred Werner:
1. Name the cation first and the anion second if the coordination complex is ionic.
2. Name the ligands in alphabetical order, followed by the central metal ion. For anionic ligands, add the suffix -ido. For neutral ligands, use the name of the molecule, except for water (H2O, use aqua), ammonia, (NH3, use ammine), carbon monoxide (CO, use carbonyl), and nitrogen monoxide or nitric oxide (NO, use nitrosyl).
3. If more than one ligand of a single type is present, use Greek prefixes (di-, tri-, tetra-, and so on) to name them, unless the ligand already contains this sort of prefix (e.g., ethylenediamine). In this case or if the ligands are polydentate, use the prefixes bis-, tris-, tetrakis-, pentakis-, and so on.
4. Name the central metal. If the central metal is a cation, use the name of the element. If the central metal is part of an anion, use the suffix -ate (e.g., platinate, cuprate, ferrate).
5. After the central metal name, give its oxidation state in Roman numerals in parentheses.
For example, to name the complex [Co(NH3)6]Cl3, first name the complex cation: [Co(NH3)6]3+. The ligands are all the same, ammonia. Ammonia is one of the exceptions. When ammonia coordinates to a metal center, it is called an ammine ligand. In this case, six ammines are coordinated to the same Co(III) cation, so the Greek prefix hexa- is used to describe that six bonds are present; this is called a hexammine complex. Its oxidation is determined by examining the charge of the complex cation and the anion within it. The complex cation must be 3+, because three Cl– anions are bonded to it. The ammine ligand is neutral; therefore, the charge for the cation cobalt must be 3+. The cation of the coordination complex is therefore hexamminecobalt(III). Finally, name the anion, chloride. The coordination complex is therefore hexamminecobalt(III) chloride.
Names of Sample Coordination Complexes
|Coordination Complex Type||Chemical Formula||Name|
|Cation complexes||[Pt(NH3)5Cl]Br3||pentaamminechloridoplatinum(IV) bromide|
|Anion complexes||K4[Fe(CN)6]||potassium hexacyanidoferrate(II)|
Uses of Coordination Compounds
Many coordination compounds occur naturally and play important biological roles. For example, hemoglobin, which carries oxygen in the blood, and chlorophyll, which is the pigment responsible for photosynthesis, both rely on coordination compounds. Vitamin B12 is also a coordination compound. Because coordination compounds tend to be brightly colored, they are also frequently used as dyes and pigments, such as indigo, cochineal, and safflower, all of which are naturally occurring.
Industrially, coordination complexes with polydentate ligands are often used as chelating agents. That is, they remove heavy metals from solutions in which metals are undesirable, such as drinking water.
Further, coordination complexes can function as catalysts in both natural and industrial processes. An important pair of catalysts, titanium trichloride and triethylaluminum, catalyze the formation of polymers from organic compounds. This process is used to form many synthetic fibers, films, and plastics.
Coordination complexes also have medicinal uses. Cisplatin, cis-PtIICl2(NH3)2, is a neutral coordination complex that is used to treat many types of cancers. The nitroprusside ion, [FeII(CN)5NO]2-, is a vasodilator used to treat patients suffering from hypertension, along with nitric oxide (NO). Lanthanum carbonate (La2(CO3)3) is used to treat patients with renal failure, as a means of filtering excess phosphate. Nonsteroidal anti-inflammatory compounds, commonly referred to as NSAIDs, are ligands that can be combined with various metals for a variety of therapeutic uses. For example, when combined with copper, NSAIDs such as ibuprofen have enhanced anti-inflammatory effects and reduced side effects.