COMPLEXES and CHELATING AGENTS
COORDINATION COMPOUNDS are specialized compounds involving metal atoms that appear to bond with additional anions or molecules after normal valence requirements have been satisfied by anions; the additional bonding species are usually termed ligands. typically consist of a complex ion and counter ion.
LIGANDS- occupy the coordination sphere.
-may be displaced by other ligands, but do not normally dissociate from the metal atom.
LIGANDS-possesses at least one non–bonded pair of electrons which is used to form a coordinate covalent bond with metal atoms.
-surround and bind directly to the central metal atom in a complex through its donor atom(s) or position(s) in the ion or molecule with unshared pair of valence electrons that can be donated.
-are classified depending on the number of donor atoms capable of coordinating with a metal atom.
WERNER’s THEORY -is the most widely accepted theory of complex ion formation.
-postulated that metals exhibited a secondary valence (non–ionizable valence) that corresponds to the coordination number, which is the maximum number of donor atoms that can surround the central metal atom in a complex.
-also postulated that ligands surround the central metal atom in certain characteristic geometries.
COORDINATION NUMBER of 2 LINEAR or ANGULAR
COORDINATION NUMBER of 3 TRIGONAL–PLANAR or TRIGONAL–PYRAMIDAL
COORDINATION NUMBER of 4 TETRAHEDRAL or SQUARE–PLANAR
COORDINATION NUMBER of 5 TRIGONAL–BIPYRAMIDAL or SQUARE–PYRAMIDAL
COORDINATION NUMBER of 6 OCTAHEDRAL or TRIGONAL–PRISMATIC
NAMING COORDINATION COMPOUNDS
The cation is named before the anion, as in other ionic compounds. The rule holds regardless of whether the complex ion bears a net positive or a net negative charge.
Within a complex ion, the ligands are named first, in alphabetical order, and the metal ion is named last.
When several ligands of a particular kind are present, we use the Greek prefixes di–, tri–, tetra–, penta–, and hexa– to name them.
Prefixes are ignored when alphabetizing ligands.
if the ligand itself contains a Greek prefix, we use the prefixes bis (2), tris (3), and tetrakis (4) to indicate the number of ligands present. The ligand ethylenediamine already contains di; therefore, if two such ligands are present the name is bis(ethylenediamine).
The oxidation number of the metal is written in Roman numerals following the name of the metal. Thus, the [Co(NH3 )4Cl2 ] +1 cation is called tetraamminedichlorocobalt (III) ion. If the complex is an anion, the metal name ends in –ate
In Ni(CO)4 , the CO ligands are neutral species, and therefore the Ni atom bears no net charge. The compound is called tetracarbonylnickel(0)
SODIUM NITROPRUSSIDE: Na2 [Fe(CN)5NO] is intravenously infused in cases of acute hypertensive crises.
- breaks down in the systemic circulation to release nitric oxide (NO).
-common side effects include cyanide toxicity.
GEOMETRIC ISOMERS usually come in pairs.
- are distinguished from one another using the terms “cis” (meaning adjacent to each other) and “trans” (meaning on opposite sides to each other).
CISPLATIN: [Pt(NH3 )2Cl2 ] is one of the synthesized platinum compounds containing ammonia and chlorine, useful as an anti– cancer agent.
-can cause serious side effects, including severe kidney damage.
CISPLATIN: [Pt(NH3 )2Cl2 ] binds to DNA by forming cross–links in which the two chlorides on cisplatin are replaced by nitrogen atoms in the adjacent guanine bases on the same strand of the DNA.
CARBOPLATIN differs from cisplatin in that it has a bidentate dicarboxylate in place of the two chloride ligands.
-main drawback is its myelosuppressive effect, but its greatest benefit is the elimination of nephrotoxic effects.
CHELATE came from the Greek word chele, meaning claw.
-consists of a central metal atom attached to a ligand, in a cyclic or ring structure.
-more stable when the total number of atoms in the ring, including the metal, are five, six, or seven.
CALCIUM DISODIUM EDETATE is actually the calcium complex of the disodium salt of ethylenediaminetetraacetic acid (EDTA).
-is primarily used in the treatment of lead poisoning (plumbism).
DIMERCAPROL or British anti–lewesite (BAL), forms stable mercaptides of metals which are excreted in the urine.
-is an effective neutralizing agent for arsenical war gases, such as lewisite.
PENICILLAMINE use has been reserved for the improvement of copper excretion in patients with hepatolenticular degeneration, also known as Wilson’s disease.
-has the ability to reduce the copper (II) in the tissues to copper (I).
DEFEROXAMINE is produced naturally by Streptomyces pilosus as a ferric complex; after chemical removal of the iron, it is purified as the methylsulfonate (mesylate) salt.
-is used in the treatment of acute iron toxicity.
CYANOCOBALAMIN is a commercially– prepared form of vitamin B12 from bacterial fermentation by Propionibacterium species.
-is used to treat pernicious anemia.
SEQUESTERING AGENTSnare polydentate ligands used to improve the solubility and/or to stabilize a metal atom by chelation (sequestration).
-classical examples are found in solutions employed in the identification of reducing substances (e.g., sugars),
Benedict’s reagent and Fehling’s reagent is a complex mixture of sodium carbonate, copper (II) sulfate, and sodium citrate.
-copper (II) ions are chelated by citrate ions.
FEHLING’s REAGENT is a complex mixture of sodium hydroxide, copper (II) sulfate, and sodium potassium tartrate.
copper (II) ions are chelated by tartrate ions.