Coordination compounds (complexes) are the stable structures, which contain central atom (usually metal, acceptor of electronic pairs), surrounded by ligands (neutral molecules or ions, donors of electronic pairs). So, each coordination compound contains donor-acceptor bonds. Most of the simple complexes form as a result of well known chemical reactions, for example:
3NaOH + Fe(OH)3 → Na3[Fe(OH)6].
Many ferments and biologically important compounds contain cations of metals in their active centers involving donor-acceptor mechanism of covalent bond formation. The hemoglobin in your blood, the blue dye in the ink in your ballpoint pen and in your blue jeans, chlorophyll, vitamin B12 , and s.f. belong to complexes. The position of these metals in the middle of ring structure is responsible for the certain configuration of complex and its reactivity.
To discuss the structure of simple complexes, let’s use the following example:
K3[Co(NO2)6], potassium hexanitrocobaltate (III).
This complex contains central ion, Co (+3), which forms donor-acceptor bonds with ligands, NO2– groups. Cobalt has vacant orbits and acts as an acceptor of electronic pairs from ligands.
The part [Co(NO2)6]3–, called coordination sphere or inner sphere, includes the central metal ion plus the attached ligands. K+ ions are attached to [Co(NO2)6]3– due to ionic bonds, and form outer sphere.
The coordination number of the central metal ion is the number of donor atoms bonded to it. In simplest complexes with monodentate ligands (like in our example, where each ligand attaches itself to a central ion by bonds from one donor atom) this number of bonds is equal to the “number of neighbours”. Coordination number (c/n) equals 6 in K3[Co(NO2)6]. C/n = 2 in [Ag(NH3)2]+, c/n = 4 in Na[Al(OH)4].
When a ligand attaches itself to a central ion by bonds from two or more donor atoms, it is referred to as a polydentante ligand, or a chelating ligand. The resulting complex is a metal chelate and the coordination number is not equal to the number of ligands in the coordination sphere. The complex heme in hemoglobin contains a polydentante ligand with four donor atoms.
The common geometries found in complexes are linear (with a coordination number of 2), tetrahedral and square planar (c/n = 4), octahedral (c/n = 6). Cis- and trans- configurations are possible in some octahedral and square planar complexes.
The following rules are used for naming complexes.
- If a complex is ionic, name the cation first and the anion second in accordance with usual nomenclature.
- Name the ligands first followed by the central metal.
- Name the ligands alphabetically. Negative ligands (anions) have names formed by adding –o to the stem name of the group; for example, (CN–) cyano, (NO2–) nitro, (Cl–) chloro, (ОН–) hydroxo, (О2–) oxo, (С2О42–) oxalato and so on.
- For most neutral ligands the name of the molecule is used. The four common exceptions are aqua (H2O), ammine (NH3), carbonyl (CO), and nitrosyl (NO).
- If more than one ligand of a given type is present, the number is indicated by the prefixes di- (for two), tri- (for three), tetra- (for four), penta- (for five), and hexa- (for six). Sometimes the prefixes bis- (for two), tris- (for three), and tetrakis- (for four) are used when the name of the ligand contains numbers, begins with a vowel, is for a polydentate ligand, or includes di-, tri-, or tetra-.
- When the complex is either a cation or neutral molecule, the name of the central metal atom is spelled exactly as the name of the element and is followed by a Roman numeral in parentheses to indicate its oxidation number. When the complex is an anion, the suffix -ate is added to the stem for the name of the metal (or sometimes to the stem for the Latin name of the metal) followed by the Roman numeral designation of its oxidation number.
Examples in which the complex is a cation:
[Ag(NH3)2]Cl diamminesilver (I) chloride;
[Pt(NH3)4Cl2]SO4 tetraamminedichloroplatinum(IV) sulfate.
Examples in which the complex is neutral:
[Pt(NH3)2Cl4 ] diamminetetrachloroplatinum(IV);
[Co(NH3)3(NO2)3] triamminetrinitrocobalt(III).
Examples in which the complex is an anion:
K3[Fe(OH)6] potassium hexahydroxoferrate(III);
Na2[SnCl6] sodium hexachlorostannate(IV).
Dissociation of complexes proceeds in several steps:
1) K3[Co(NO2)6] → 3K+ + [Co(NO2)6]3–; this stage proceeds completely;
2) [Co(NO2)6]3– ↔ Co3+ + 6NO2–; the process is reversible and we use the dissociation constant (or formation constant) in accordance with the rules of equilibrium (law of mass action).
Kd = [Co3+] ∙ [NO2–]6 / [Co(NO2)6]3–; Kf = 1 / Kd.
Note. Actually, six constants exist to describe the dissociation of inner sphere, but we use the summarized one, K1–6 , in order to compare the number of simple ions (numerator in formula) and complex ions (denominator). Most of complexes release a very small amount of ions, and Kd <10–3 (or Kf >103). Greater the value of Kd — lesser stability of coordination compound.