It is really a coordination compound. It has got a central metal atom or ion, normally a cation, bounded by a number of negatively charged ions or neutral molecules having lone pairs. Counteracting (opposite in sign) ions often encircle the metal complex ion thus the compound will have no net charge.
The ions or molecules adjacent the metal is known as ligands. Ligands are normally attached to a metal ion by a coordinate covalent bond. Coordination numbers (which are the number of bonds formed between the metal ions and ligands here) are by and large between 2 and 8 but can broaden higher. The number of bonds formed largely depends on the size, charge, and electron configuration of the metal ion.
It is interesting to note that some metal ions may have more than one coordination number. Different types and modes of ligand structural arrangements effect from the coordination number. A coordination number of 2 normally have a linear geometry , a coordination number of 4 with either a tetrahedral or square planar molecular geometry (depending on the orbitals involved) and a coordination number of 6 is associated with an octahedral geometry. Transition metals make good central ions for complexes.
Table 1 (Courtesy – Washington University in St. Louis )
C 2O 4 2-
CO 3 2-
C 5H 5N
CH 3COO –
H 2NCH 2CH 2NH 2
The basic procedure for naming a complex:
- The ligands are named before the metal ion.
- The names of the ligands are written in alphabetical order. (It should be noted that numerical prefixes do not affect the order.)
- Numerously (more than one) occurring monodentate ligands obtain a prefix according to the number of occurrences: di-, tri-, tetra-, penta-, or hexa. Polydentate ligands (e.g., ethylenediamine, oxalate) receive bis-, tris-, tetrakis-, etc. (normally)
- Anions end in o. (This replaces the final ‘e’ when the anion ends with ‘-ate’, e.g. sulfate becomes sulfato. It replaces ‘ide’: cyanide becomes cyano). (normally)
- Neutral ligands are given their usual name, with some exceptions: NH 3 becomes ammine; H 2O becomes aqua; CO becomes carbonyl; NO becomes nitrosyl. (normally pattern)
- Write the name of the central atom/ion. If the complex is an anion, the central atom’s name will end in -ate, and its Latin name will be used if available (except for mercury).
- If we need to mention the oxidation state central atom’s (when it having one of several probable values or zero state), write it as a Roman numeral (or 0) in parentheses.
- Name the cation then anion as separate words
Consider the following examples, and then we could make out well:
[Co(NH 3) 5Cl]SO 4 → pentaamminechlorocobalt(III) sulfate
[CuNH 3Cl 5] 3- → amminepentachlorocuprate(II) ion
[NiCl 4] 2- → tetrachloronickelate(II) ion
[Cd(en) 2(CN) 2] → dicyanobis(ethylenediamine)cadmium(II)
Table 2 (Courtesy – Washington University in St. Louis )
Another aspect concerned with this is that simple ligands like water, chlorine, hydroxide, nitrite, and thiocyanate etc make only one link with the central atom. They are known as monodentate. While there are also some ligands which are able to form multiple bonds to the same metal atom (known as bidentate, tridentate etc as the case may be). We can mention Oxalate and ethylenediamine as examples of bidentate ligands type, while diethylenetriamine is a good example for tridentate ligand.
Table 3 (Courtesy – Washington University in St. Louis )
Name of Metal
Name in an Anionic Complex
Characteristically the mechanism of complexes is subjected to the interactions between s and p orbitals of the ligands and the d (or f – in higher ones) orbitals of the metal ions. Due of this the octet statute fails in the aspect concerned of complexes.
To understand this we could resort to rules like ‘electron counting’, or ‘the rule of 18’, Crystal field theory (which was proposed by Hans Bethe in 1929), etc. But crystal field theory (which is a more quantum mechanically dependent effort to understand complexes) considers all interactions in a complex as ionic. Ligand field theory, (which was introduced in 1935) is built from molecular orbital theory. It can hold a broader variety of complexes and can elucidate complexes where the interactions are covalent in nature.
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