Coordination Compounds PDF

Dr O. Erharuyi Coordination Compounds, Complexation and Chelation Coordination compounds are addition compounds of certain metallic ions in which the metal is bonded to anions or neutral molecules after the normal valency has been satisfied. Metals that forms coordination compounds in...

Dr O. Erharuyi Coordination Compounds, Complexation and Chelation Coordination compounds are addition compounds of certain metallic ions in which the metal is bonded to anions or neutral molecules after the normal valency has been satisfied. Metals that forms coordination compounds includes the transition metals e.g. Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn), Silver (Ag), lead (Pb), Manganese (Mn), e.t.c. group 2A metals e.g. Calcium (Ca), Magnesium (Mg), Chromium (Cr), metals of group 3A e.g. Aluminium (Al). Anions or neutral molecules that usually form cordination compounds with metals are compounds of the group 5A element (nitrogen), group 6A (Oxygen and Sulphur) and the halogens. These anions or molecules that are bonded to the metal in coordination compounds are called ligands. These may be simple ions such as Cl-, OH-, CN-, small molecules such as H2O, NH3, NO2, larger molecules such as H2NCH2CH2NH2 or N(CH2CH2NH2)3 or even macromolecules, such as proteins. The metal with its associated ligands is referred to as a complex ion. Examples of complex ions are; [Fe(CN)6]4–, [Ag(NH3)2]+, , [Cu(NH3)6]3+, [Co(NH3)6]3+. These complexes could be homoleptic or heteroleptic. Homoleptic complexes: complexes in which a metal is bound to only one kind of donor groups, e.g. [Co(NH3)6]3+. Heteroleptic complexes: complexes in which a metal is bound to more than one kind of donor groups, e.g., [Co(NH3)4Cl2]+. In coordination compounds metals show two types of linkages (valences); primary and secondary. 1. The primary valences are normally ionisable and are satisfied by counter ions. 2. The secondary valences are non-ionisable. These are satisfied by neutral molecules or negative ions. The secondary valence is equal to the coordination number and is fixed for a metal. The ions/groups bound by the secondary linkages to the metal have characteristic spatial arrangements corresponding to different coordination numbers. Dr O. Erharuyi Coordination number The coordination number of a metal ion in a complex can be defined as the number of ligand donor atoms to which the metal is directly bonded. For example, in the complex ions, [PtCl6]2– and [Ni(NH3)4]2+, the coordination number of Pt and Ni are 6 and 4, respectively. Coordination sphere The central atom/ion and the ligands attached to it are enclosed in square bracket and are collectively termed as the coordination sphere. The ionisable groups are written outside the bracket and are called counter ions. For example, in the complex K4[Fe(CN)6], the coordination sphere is [Fe(CN)6]4– and the counter ion is K+. Coordination polyhedron The spatial arrangement of the ligand atoms which are directly attached to the central atom/ion defines a coordination polyhedron about the central atom. The most common coordination polyhedra are octahedral, square planar and tetrahedral. Figure 1: Shapes of different coordination polyhedra. M represents the central atom/ion and L, a ligand. When a ligand is bound to a metal ion through a single donor atom, as with Cl– , H2O or NH3, the ligand is said to be unidentate. When a ligand can bind through two donor atoms as in H2NCH2CH2NH2 (ethane-1,2-diamine) or C2O42– (oxalate), the ligand is said to be bidentate and when several donor atoms are present in a single ligand as in N(CH2CH2NH2)3, the ligand is said to be polydentate. Ethylenediaminetetraacetate ion (EDTA4–) is an important hexadentate ligand. It can bind through two nitrogen and four oxygen atoms to a central metal ion. When a bi- or polydentate ligand uses its two or more donor atoms 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. Dr O. Erharuyi Such complexes, called chelate complexes tend to be more stable than similar complexes containing unidentate ligands. Ligand which can ligate through two different atoms is called ambidentate ligand. Examples of such ligands are the NO2- and SCN– ions. NO2- ion can coordinate either through nitrogen or through oxygen to a central metal atom/ion. Similarly, SCN– ion can coordinate through the sulphur or nitrogen atom. Figure 2: Pictorial representation of a metal-ligand chelate complex. Chelating agents and their Pharmaceutical/Medicinal importance The term chelation (derived from the Greek chele, lobster’s claw) relates to the interaction between a metal atom or ion and another species, known as the ligand, by which a heteroatomic ring is formed. Chelation changes the physical and chemical characteristics of the metal ion, and the ligand. It is simplest to consider the ligand as the electron pair donor and the metal as the electron pair acceptor, with the donation establishing a coordinate bond. Dr O. Erharuyi Many chelating agents act in the form of anions which coordinate to a metal ion. For chelation to occur there must be at least two donor atoms capable of binding to the same metal ion, and ring formation must be sterically possible. For example, ethylenediamine (1,2 diaminoethane, NH2CH2CH2NH2) has two donor nitrogens and acts as a bidentate (two-toothed) ligand. When a drug forms a metal chelate the solubility and absorption of both drug and metal ion may be affected, and drug chelation can lead to either increased or decreased absorption. Selected Examples 1. Tetracyclines Tetracyclines readily form complexes with divalent metals such as calcium, magnesium, but they have a greater affinity for the trivalent metals such as iron and aluminium with which they form 3:1 drug-metal chelates. Tetracycline chelation with metal ions is a common example of complex formation leading to decreased drug absorption. Figure 3: Structure of tetracycline 2. Ethylenediaminetetraacetic acid (EDTA) EDTA is an aminopolycarboxylic acid with the formula [CH2N(CH2CO2H)2]2. It is a white, water-soluble solid. It is commercially available in several salts form notably disodium EDTA, sodium calcium edetate, and tetrasodium EDTA. Figure 4: Chemical structure of EDTA Dr O. Erharuyi Uses: i. In complexometric titration, EDTA is used to find the total calcium and magnesium content of milk, sea water and various organic materials including drug products. It binds these metal ions as a hexadentate ("six-toothed") chelating agent. Figure 5: EDTA-Calcium ion complex ii. Formation of EDTA calcium complex is employed in the reduction of calcium concentration in serum in conditions of hypercalcaemia. iii. Sodium calcium edetate, an EDTA derivative, is used to bind metal ions in chelation therapy, for treating mercury and lead poisoning. iv. Dentists and endodontists use EDTA solutions to remove inorganic debris (smear layer) and lubricate the root canals in endodontics. v. It serves as a preservative (usually to enhance the action of another preservative such as benzalkonium chloride or thiomersal) in ocular preparations and eyedrops. vi. EDTA is used as an anticoagulant for blood samples, where the EDTA chelates the calcium present in the blood specimen, arresting the coagulation process and preserving blood cell morphology. vii. EDTA is a slime dispersant, and has been found to be highly effective in reducing bacterial growth during implantation of intraocular lenses Dr O. Erharuyi 3. Deferoxamine (Desferal®) Deferoxamine, also known as desferrioxamine is a chelating agent that binds iron and aluminium. It is specifically used in iron overdose, hemochromatosis either due to multiple blood transfusions or an underlying genetic condition, and aluminium toxicity in patients on dialysis. Figure 6: Structure of Deferoxamine 4. Penicillamine (Cuprimine®) Penicillamine is a medication primarily used for the treatment of Wilson's disease. It is also used for people with kidney stones who have high urine cystine levels, rheumatoid arthritis, and various heavy metal poisonings. A B Figure 7: Structure of Penicillamine (A) and Penicillamine-copper complex (B) 5. Dimercaprol Dimercaprol, also called British anti-Lewisite (BAL), is a medication used to treat acute poisoning by arsenic, mercury, gold, and lead. H2C S A B As R HC S H2C OH Figure 8: Structure of Dimercaprol (A) and Arsenic-Dimercaprol Complex (B) Dr O. Erharuyi 6. 8-Hydroxyquinoline 8-Hydroxyquinoline (8HQ) also known as oxine is a chelating agent which has been used for the quantitative determination of metal ions. Figure 9: Structure of 8-hydroxyquinoline 8HQ is capable of forming complexes with divalent metal ions through chelation. N O M M = Fe, Mn, Cu, Ni Figure 10: Structure of 8-hydroxyquinoline-metal complexe