Coordination Compounds & Complexation (Lecture Notes) PDF

COORDINTION COMPOUNDS AND COMPLEXATION The metallic cation is able to bond with additional anions or neutral molecules after the normal valence requirements have been satisfied. The additional bonding species are termed ligands, and appear to bond directly to the metal cation in accordance with maxi...

COORDINTION COMPOUNDS AND COMPLEXATION The metallic cation is able to bond with additional anions or neutral molecules after the normal valence requirements have been satisfied. The additional bonding species are termed ligands, and appear to bond directly to the metal cation in accordance with maximum coordination numbers (the secondary valence )(Table below ) :the maximum number of ligands that can be accommodated by a metal ion, and is a property of the metal and its charge. - The metal + associated ligands = coordination compound. Some complexes are stable in crystalline form and decompose in solution, while others are stable only in solution. Ex. : FeCl3, a simple compound of trivalent iron III and chlorine. When FeCl3 dissolved in water and/or HCL the following coordination compounds are formed: - The ligands are arranged around the metallic ion in certain characteristic geometry. PROPERTIES OF LIGANDS 1.– ligand species are generally anions or neutral molecules and not neutral atoms. 2.- all ligands have in common the possession of at least one nonbonded pair of electrons which is used to form a coordinate covalent bond with the metal ion. 3.- The more stable complexes are formed with anionic or molecular ligands involving the elements of Groups VA, VIA, or VIIA. Generally speaking, the order of stability of a ligand in a complex follows the order of basicity of the ligand (lewis base). 4.- ligand species may be classified according to the number of positions on the molecules capable of coordinating with a metal , exe , monodentate , bidentate , ect , …. (table 1-9)  - When polydentate ligands complex a metal ion a ring structure is produced , composed of the metal and the ligand molecule. These ring structures have special significance, and are termed chelates. - The more stable chelates are those where the total number of atoms in the ring including the metal are five, six, or seven. Four and eight-membered rings are usually unstable. - The process of chelation is employed in pharmaceuticals and in drug therapy. The polydentate ligands used for chelate formation are generally referred to as chelating agents. The term sequestering agent is usually applied when a polydentate ligand is used to improve the solubility and/or to stabilize a metal ion by chelation (sequestration).  BONDING IN COMPLEX -It is generally accepted that the (n — 1)d, ns, and np orbitals are close enough in energy to become hybridized into six bonding orbitals which are directed alone the same axes occupied by the ligands. These hybrid orbitals are designated d2sp3 hybrids and are equivalent as long as the six ligands are equivalent. -In the case of the trication of Cr(III) complexing with six CN- , Cr(III) is d3 ion (3 electron occupy d xy , d yz , d xz ), leaving two d orbitals (d x2-y2 , d z2) , one s ,three p orbitals empty to bonding with six cyanato groups to form [ Cr(CN)6] -3 -In the case of metal ions have four or more electrons in their d orbitals, the complexes must alter the ground state configuration and that can be measured experimentally by measuring the magnetic moment -MM-of the complex which is the unpairedness of electrons. -The complex of Fe(III) – 3d5 element – with 6 water units ,the MM is 6 which indicative of 5 unpaired electrons so finding six empty atomic orbitals to overlap with the donor water molecules can be accomplished by assuming that the 4d orbital to hybridize with the 4s and 4p , this type of hybridization-HP- termed outer orbital HP. If water replaced by six CN- the MM is about 2 which indicative of one unpaired electron and that because a cyanato anion has a negative electronic field of sufficient strength to repel the electrons in the 3d orbital (dx2-y2 and dz2) that directly oppose the approaching ligand and force them to paired in the other d orbitals as below :  COMPLEXES AND CHELATING AGENTS. -Complexation plays an important role in analytical chemistry where ,for example, concentrations of metals can be determined by titration with complexing agents. -In some analytical solutions containing metal ions, chelating agents are used to solubilize the metal and to stabilize its oxidation state. Two classical examples are found in solutions employed in the identification of reducing substances (e.g., sugars), Benedict’s solution and Fehling’s solution. Both contain copper(II) ions which are chelated by citric acid in Benedict’s and by tartaric acid in Fehling’s solution. - Chelating agents are also used as preservatives in preparations subject to decomposition due to trace quantities of metals, such as preparations containing hydrogen peroxide  COMPLEXES AND CHELATING AGENTS. - Chelating agents occupy unique place in drug therapy. They essentially the only compounds which have shown much efficacy in the treatment of heavy metal poisonings from such elements as lead, mercury, iron, etc. - They are also being used to treat certain metabolic disorders where metals such as iron and copper are accumulated in abnormal amounts in various tissues. The particular chelating agents that will be discussed include calcium disodium edetate (EDTA), dimercaprol (BAL), penicillamine, and deferoxamnine.  CALCIUM DISODIUM EDETATE, Calcium Disodium Ethylenediaminetetra acetate; Mol. Wt. (anhydrous 374.28), exists as a white crystalline granule. It is odorless, slightly hygroscopic, and has a faint saline taste. It is stable in air, freely soluble in water, and the pH of an aqueous solution is between 6.5 and 8. It is used in the treatment of heavy metal poisoning, primarily that caused by lead (plumbism) by forming insoluble complex excreted by the kidney. EDTA preparations have a strong affinity for calcium; therefore, the disodium calcium form is used to avoid inducing hypocalcemic states (low serum calcium). CALCIUM DISODIUM EDETATE This chelating agent may also be employed in poisonings due to copper, nickel, cadmium, zinc, chromium, and manganese, but it is of no value in the treatment of toxicities produced by mercury, arsenic, or gold. The compound is poorly absorbed from the gastrointestinal tract, so given by I.V. route. Intramuscular (I.M.) administration is employed in diagnosis of metal poisonings. An increase in the excretion of the metal in the urine (500 pg/liter/24 hours or greater for lead) is indicative of toxicity.  DISODIUM EDETATE -This compound is a white crystalline powder which is soluble in water, providing an aqueous solution of pH between 4.0 and 6. It will chelate the same metals as the disodium calcium form. -Its limitation is The chance of hypocalcemia during such therapy. Its primary use of is in conditions related to hypercalcemic states (high serum calcium). The compound may be useful in treating such problems as occlusive vascular disease and cardiac arrhythmias when associated with high blood levels of calcium. -It is apparently of no value in aiding dissolution of urinary calculi (calcium-containing stones in the urinary tract.The usual route of administration is by intravenous injection. The usual route of administration is by intravenous injection. DIMERCAPROL 2,3-Dimercapto- 1-propanol; BAL ,is a colorless or almost colorless liquid having a disagreeable odor. It is soluble in water, alcohol, and benzyl benzoate. Certain heavy metals, such as trivalen arsenic, owe their cellular toxicity to the sulfhydryl (—SH) groups present in enzymes which are responsible for oxidation-reduction reactions in tissues. Presumably, this inactivation involves covalent bond formation between the metal and the sulfhydryl groups.  DIMERCAPROL The introduction of BAL as a competitor with the enzymes for these metals as an effective neutralizing agent in arsenic war gases and other poisoning conditions from other source, poisoning from other sources, and has been extended to the treatment of mercury and gold poisoning. It is contraindicated in poisonings due to iron, cadmium, or selenium because the resulting complexes have greater renal toxicities than do the free metals. Dimercaprol-metal chelates tend to dissociate in acid media; therefore in therapy the urine should be alkalinized (e.g., with sodium bicarbonate) to prevent the release of free metal, producing renal toxicity. The usual route of administration is by intramuscular injection.  PENICILLAMINE 3-mercaptovaline is a white or off-white crystalline powder, having a alight characteristic odor. It is freely soluble in water, and slightly soluble in alcohol. The pH of an aqueous solution is between 4.5 and 5. It is a chelating agent capable of forming soluble complexes with copper, iron, mercury, lead, gold, and other metals. Its use has been reserved for the improvement of copper excretion in patients with Wilson’s disease (degenerative changes in the brain associated with increased levels of copper in the tissues and degeneration of the liver).  PENICILLAMINE -The effectiveness of penicillamine is related to its resistance to metabolic inactivation by amino acid oxydase since it lacks a hydrogen on the beta-carbon atom. -It is used in the treatment of gold dermatitis in patients on chronic gold therapy. -Unlike the other chelating agents Discussed the usual route of administration of penicillamine is oral. Penicillamine Capsules are official in the U.S.P.  DEFEROXAMINE MESYLATE usually available as a white, crystalline, lyophilized powder. It is soluble in water and the aqueous solution is stable at room temperature for two weeks. Deferoxamine is produced naturally by Streptomyces pilosus as a ferric [Fe(III)]complex. After chemical removal of the iron, the chelating agent purified as the methylsulfonate (mesylate) salt. It is a polydentate ligand with a particular affinity for ferric ions with which it forms stable, water soluble, octahedral complexes. It does not have a very strong affinity for ferrous or other divalent metal ions. Deferoxamine is used with other indicated drugs and procedures for the treatment of acute iron toxicity. DEFEROXAMINE MESYLATE The usual route of administration is by intramuscular or intravenous injection. The compound is poorly absorbed from the gastrointestinal tract. Intravenous administration is generally done by slow infusion with isotonic sodium chloride or other electrolyte solution.