IMPORTANT CONCEPTS IN COORDINATION CHEMISTRY
coordination compound is any of a class of substances with chemical structures in which a central metal atom is surrounded by nonmetal atoms or groups of atoms, called ligands, joined to it by chemical bonds. For example, in the coordination entity [CoCl3(NH3)3] Colbalt Trichloride Triammonia, the central cobalt ion is bound to three ammonia molecules and three chloride ions surrounding it. Coordination compounds include such substances as vitamin B12, hemoglobin, and chlorophyll, dyes and pigments, and catalysts used in preparing organic substances.
Other examples in coordination chemistry are [Ni (CO)4] Nickel tetracarbonyl, [PtCl2(NH3)2] dichlorodiammineplatinum(II) , [Fe (CN)6]4- Ferricyanide OR hexacyanoferrate(III), [Co(NH3)6]3+ Hexaamminecobalt(III).
A coordination entity constitutes a central metal atom or ion bonded to a fixed number of ions or molecules. For example, [CoCl3(NH3)3] (Colbalt Trichloride Triammonia) is a coordination entity in which the cobalt ion is surrounded by three ammonia molecules and three chloride ions. Other examples are [Ni(CO) ], [PtCl (NH ) ], [Fe(CN) ]4–, [Co(NH ) ]3+.
The atom/ion in a coordination entity that is bound to a fixed number of ions/groups in a definite geometrical arrangement is the central atom/ion. For example: In the coordination entities [Ni (CO)4] Nickel tetracarbonyl, [PtCl2(NH3)2] dichlorodiammineplatinum(II) , [Fe (CN)6]4- Ferricyanide OR hexacyanoferrate(III), [Co(NH3)6]3+ Hexaamminecobalt(III). These central atoms/ ions are also Lewis acids.
Ligands are the ions or molecules bound to the central atom/ion in a coordination entity. The atoms and molecules used as ligands are Usually those that are capable of functioning as the electron-pair donor in the electron-pair bond (a coordinate covalent bond) formed with the metal atom. Examples of common ligands are the neutral molecules water (H2O), ammonia (NH3), and carbon monoxide (CO) and the anions cyanide (CN–), chloride (Cl–), and hydroxide (OH–). Attachment of the ligand to the metal may be:
In a lot of cases, only one atom in the ligand is bound to the metal ion. In these cases, the ligand is unidentate. Examples of unidentate ligands are H2O, NH3 or Cl–.
Ligands that bind to the metal ion through two donor atoms are bidentate or didentate ligands. Examples are H2NCH2CH2NH2 Ethylenediamine or (ethane-1,2-diamine) and C2O42– (oxalate).
Polydentate ligands are those that can bind to the metal ion through several donor atoms. Examples are N(CH2CH2NH2)3 and Ethylenediaminetetraacetate ion (EDTA4–). EDTA is an important hexadentate ligand that can bind to a central metal ion through two nitrogen and four oxygen atoms.
When a di- or polydentate ligand uses its two or more donor atoms simultaneously to bind a single metal ion, it is said to be a chelate ligand. The number of such ligating groups is called the denticity of the ligand. Such complexes, called chelate complexes tend to be more stable than similar complexes containing unidentate ligands.
Ambidentate ligands can ligate through two different atoms. Examples are NO2– and SCN– ions where the NO2– ion can ligate with a central metal atom/ion through nitrogen or oxygen and SCN– can ligate through sulfur or nitrogen.
The number of ligand donor atoms to which the metal is directly bound is the coordination number (CN) of the metal ion in the complex. Coordination number, also called Ligancy. For example, the CN of Pt and Ni in the complex ions [PtCl6]2– Hexachloroplatinate(2-) and [Ni(NH3)4]2+ (diamagnetic or paramagnetic) are 6 and 4, respectively.
The CN for Bidentate Ligands is 2x the coordination Number you can see.
so, the CN of Fe and Co in the complex ions K[Fe(C2O4)3]3– (Potassium ferrioxalate, also known as potassium trisoxalatoferrate or potassium trisferrate(III)) and [Co(en)3]3+ (Tris(ethylenediamine)cobalt(III) chloride) is 6 because both C2O42– and en (ethane-1,2-diamine) are bidentate ligands.
Note: The CN of the central atom/ion is determined only by the number of sigma bonds between the central atom/ion and the ligand, not the Pi bonds.
The coordination sphere consists of the central atom/ion and the ligands attached to it. The ionizable groups, called counter ions are written outside the bracket. For example, in the complex K4[Fe(CN)6] 4–
( Potassium ferrocyanide), [Fe(CN)6]4– (Ferrocyanide) is the coordination sphere and K+ is the counter ion.
It refers to the geometric pattern or spatial arrangement of the ligands directly attached to the central atom/ion. The most common polyhedral are tetrahedral, octahedral and square planar. Examples: [Co (NH3)6]3+ (Hexaamminecobalt(III) chloride) is octahedral, [Ni (CO)4] (Nickel carbonyl) is tetrahedral and [PtCl4]2– (Tetrachloroplatinate(2) is square planar.
OXIDATION NUMBER OF CENTRAL ATOM
It is the charge the central atom would carry if all the ligands are removed along with the electron pairs that are shared with it. It is represented as the name of the coordination entity followed by the oxidation number written in Roman numerals in parenthesis. For example, in [Cu (CN)4]3– (Copper cyanide), the oxidation number of copper is +1 and is Cu(I).
HOMOLEPTIC AND HETEROLEPTIC COMPLEXES
Homoleptic complexes are complexes where a metal is bound to only one kind of donor group whereas in heteroleptic complexes the metal is bound to more than one kind of donor group. Examples – [Co(NH3)6]3+ (Hexaamminecobalt(III) chloride)is homoleptic and [Co(NH3)4Cl2]+( tetraammine dichloro cobalt (III) chloride) is heteroleptic.