The paper "Heavy Metal Complexes" is an outstanding example of an essay on chemistry. In coordination chemistry, a complex or a coordination compound consists of a central metal ion with surplus atoms or ions attached (Gharpure) for electrovalent or covalent binding (Gilreath 225). This complex results from the union of an electron-deficient metal with an electron donor (Gilreath 225). The uncharged molecule coordinated to the central metal ion is called a ligand. It is a Lewis base where the atom with which it is directly bonded with is called the donor atom (Gharpure). Electronegative atoms such as carbon, oxygen, nitrogen, sulfur, and halogens (fluorine, chlorine, bromine, and iodine) are donor atoms (Gilreath 238). To elaborate, in the compound Ag (NH3)2, the two molecules of NH3 are ligands with nitrogen as the donor atom to silver (Ebbing and Gammon 715). Carbon forms cyanide and carbonyl complexes; nitrogen forms ammine, nitroso, nitrito complexes; oxygen forms aquo, hydroxide, oxalate, carhonato complexes; sulfur forms thiocyanate, thiosulfate, sulfide complexes; and halogens form fluoride, chloride, bromide, or iodide complexes. Organic molecules could have at least two coordination positions (Gilreath 239).
Below is a discussion of the common behavior of ammoniate, hydroxide, and chelate complexes with heavy metal ions.
Ammoniates. Ammonia ligands are present in ammoniate complexes. Ammoniates complexes are formed when cations combine with inorganic molecules. Heavy metal ions such as copper, zinc, and cobalt from such complexes with ammonia. When one ammonia molecule coordinates with a proton, it results in an ammonium ion NH4. In a complex formation of ammonia and copper (II) sulfate, an ammonium atom combines with a water molecule and results to an aqueous solution with hydroxide ion (OH-) and an ammonium ion (NH4 +) while a precipitate forms when an hexaaquacopper (II) ions ([Cu(H2O)6]2) combines with two hydroxide ions (2OH-). The protons of the water ligands are removed. When the copper (II) hydroxide precipitate reacts with ammonia molecules, a ligand exchange happens to result to tetraamminediaquacopper(II) ions ([Cu(H2O)6]2++ 4NH3 ⇌ [Cu(NH3)4(H2O)2)]2+ + 4H2O) (“An Equilibrium”). Only a few ammonium and hydroxide ions are created by ammonia because it is by nature a weak base. Hydroxide ions may initiate the first reaction with the cation because of the low ammonia concentrations. With higher concentrations of ammonia, on the other hand, precipitates dissolve (Gilreath 241).
The stability of the complex and the ammonia concentration are two indispensable factors to which an ammonia complex or hydroxide coordinates with a cation (Gilreath 241). Ammonia complexes with aluminum or iron or certain anions are deficient instability and couldn’t dissolve hydroxide. In turn, they precipitate as hydroxides with ammonia (Gilreath 241).
Hydroxide Complexes (Amphoteric Hydroxides). Hydroxide complexes result from the union of cations to inorganic anions. ydroxideSome insoluble hydroxides form salts upon reacting with acids and bases (Gilreath 244). They are likewise called amphoteric hydroxides. Zinc is one of those metals which form insoluble hydroxides. Those of aluminum, chromium (III), lead (II), tin (II) are common amphoteric hydroxides (Ebbing and Gammon 717). Also called zincate ion, the zinc hydroxide complex (Zn(OH) 4 ) is formed with an excess strong base. Example of a zincate ion is zinc sulfate heptahydrate (ZnSO4 +7H2O) which consists of barium sulfate (BaS) and zinc sulfide (ZnSo4) (Pauling 714).
In another example ([M(H2O)6]2+ = [M(H2O)5 (OH)] + + H+), the hexaaqua ions donate hydrogen ions to the water molecules and thus, act as acids. If hydroxide ions are added to that equilibrium, the hydroxide ions either react with the hydrogen ion or with the hexaaqua ion – forming the same new complex ions. Either way, the same [M(H2O)5(OH)]+ ions are created in the solution. If hydroxide ions are added again this equilibrium, a neutral complex is formed which is insoluble in water, forming the precipitate, M(OH)2 (Clark).
Chelate Complexes. Chelate compounds form when cations and organic anions or molecules combine. There are also cases where inorganic cations combine with organic ions or molecules. When the cations unite with organic ions, the anion becomes soluble in water (Gilreath 244). This complex often results in a chelate compound which has a ring-like structure where at least two ions from an organic or inorganic molecule are attached to the central metal ion (Ebbing and Gammon 962). An example is when a nickel ion (Ni) reacts with two covalent and two coordinate bonds of two molecules of dimethylglyoxime (CH3C(NOH)C(NOH) (Gilreath 244).
There are three general types of metal complexes: 1.) complexes formed when a cation combines with inorganic molecules; 2.) those formed by the coordination of a cation with inorganic anions; 3.) complexes formed when a cation combines with inorganic anion or molecules. Each of which has entirely different complexing behaviors.