Co-ordination compounds
Definition: Those compounds which contain complex molecules or complex ion capable of independent existence are called coordination compounds.
Explanation: let us mix aqueous solution of two substances that are KCN and FeCN2. When this mixture is evaporated, a new compound is obtained. When this compound is dissolved in water it ionizes into potassium K+ and [Fe(CN)6] it was concluded that you compound was K4[Fe(CN)6].
4KCN + FeCN2 ---------> K4[Fe(CN)6]
K4[Fe(CN)6] -----------> 4K+ + [Fe(CN)6]4-
Parts of complex compound
Complex compound is mostly made of two parts.
1. Positively charged ion or cation
2. Negatively charged ion or anion
For example in K4[Fe(CN)6], K+ is simple cation and [Fe(CN)6]4- a is complex anion.
Complex ion as cation:
In some complexes, positively charged part is complex ion. e.g.,
[Cu(NH3)4]SO4 ---------> [Cu(NH3)4]2+ + SO42-
Here, [Cu(NH3)4]2+ is a complex cation and SO42- is a simple anion.
Complex ion as anion:
In some complexes, the negatively recharged is a complex ion. For example
K4[Fe(CN)6] -----------> 4K+ + [Fe(CN)6]4-
In K4[Fe(CN)6], K+ is simple cation and [Fe(CN)6]4- a is complex anion.
Components of complex compound:
A complex compound consists of three components:
i. A positively charged or negatively charged ion which is not complex.
ii. Central metal atom or ion which is usually transition element.
iii. Electron pair donor specie (Ligand) which may be neutral, negatively charged or positively charged ion.
Central metal atom or ion
Central metal atom or ion is usually transition element. It is usually surrounded by a number of ligands. For example: In K4[Fe(CN)6], Fe is a central metal ion. In [Fe(CO)6], Fe is a central metal atom. In [Cu(NH3)4]SO4, Cu2+ is a central metal ion.
Ligand:
A substance which is usually a negatively charged, neutral molecule and sometime positively recharged surrounding central metal atom is called ligand.
For example in K4[Fe(CN)6], CN is an negatively charged legend with surrounds central metal ion Fe2+ In [Cu(NH3)4]SO4, ammonia as a neutral molecule which surround central metal ion Cu2+.
Types of Legends
Depending upon the number of donatable electrons, there are many types of legends.
Monodentate ligands:
The ligand which has only one donatable electron pair is called monodentate legend. For example, negative charge ligands such as Cl- (chloride), Br- (bromide), I- (iodide) etc, neutral ligands such as H2O, NH3, CO etc.
Bidentate ligands:
The ligand which has two donatable electron pairs is called bidentate legend. For example, negative charge legands such as SO42- (Sulphate), CO32- (carbonate), CO22- (oxalate) etc, neutral legands such as NH2-NH2 (hydrazine), NH2-CH2CH2-NH2 (ethylenediammine) etc.
Tridentate legend:
The ligand which has three donatable electron pairs is called tridentate ligands. For example NH2-CH2-CH2-NH2-CH2-CH2-NH2 diethylenetriamine has three donatable electron pairs.
Hexadentate legend:
The ligand which has six donatable electron pairs is called hexadentate ligands. For example
(CH2-COO)2NH-CH2-CH2-NH(CH2-COO)2 ethylenediaminetetraacetate ion has six donatable electron pairs.
Coordination number:
The total number of electron pair donated by ligand to central metal atom or iron is called coordination number of central metal atom or ion. It represents the total number of chemical bond between transition metal and ligand. For example in K4[Fe(CN)6], coordination number of Fe2+ is six.
Coordination sphere: The central metal atom or ion along with ligand is called coordination sphere. It is usually placed in square bracket. It may be positively charge, negatively charged or neutral. For example, K4[Fe(CN)6] the coordination sphere is [Fe(CN)6]4-. In [Cu(NH3)4]SO4, the coordination sphere is [Cu(NH3)4]2+
Charge on coordination sphere:
It is algebraic sum of charge present on central metal and total charge of ion.
Example:
Charge on K4[Fe(CN)6] can be calculated as follows.
Charge on a CN- = -1
Charge on 6 CN- = -6
Charge on a Fe2+ = +2
Charge on sphere = +2 - 6= -4
Nomenclature of coordination complexes
i. Order of ions
Cations are named first than anions.
For example in K4[Fe(CN)6], we write name of K first than[Fe(CN)6]. In In [Cu(NH3)4]SO4 , we will write name of [Cu(NH3)4] first and then name of SO42-
ii. Name of ligand
The ligands which are negatively charged end in o. For example
F- --------floro
Cl-----------chloro
Br- ---------bromo
CN- ---------cyano
CH3COO- ------acetateo
The Legend which are neutral are name as such. For example,
H2O-------------Aqua
NH2-NH2 --------Hhdrazine
NH3-------------Ammonia
The Legend which are positively charged end in ium.
NH2-NH3+ ----------hydrazonium
NH4+ ----------ammonium
NO+ -----------nitrosylium etc
iii. Order of ligand
All legends are arranged alphabetically. The prefixes di-, tri, tetra,.. etc are not considered as in alphabetic order.
iv. More than one same type of ligands
In order to indicate more than one ligands, use prefix as di- for 2, tri- for 3, tetrakis for 4 etc.
v. Termination of name of metal
If complex ion is negatively charged than name of metal ends in ate. For example,
K4[Fe(CN)6]
Potassium hexacyanoferrate (II).
If complex ion is positively charged or neutral than name of metal remains undchaged. For example,
[Cu(NH3)4]SO4
Tetraamminecopper(II) sulfate
Fe(CO)5
Pentacarbonyliron(O)
vi. Oxidation number of Metal
Oxidation number of metal is represented by Roman numerals in paranthesis following the name of metal.
For example
K4[Fe(CN)6]
Potassium hexacyanoferrate (II).
vii. More than one polidentate legends
If there are more than one polydentate ligands than indicate there number by by use of bis for two, tris for 3, tetrakis for 4 etc.
Shapes of complexes
Coordination number 2
The complexes with coordination number 2 are linear since this geometry provide minimum ligand-ligand repulsion.
Coordination number 4
Complexes with coordination number 4 may be tetrahedral or square planar planer depending upon the type of hybridization. For example,
tetrahedral for sp hybridized
square planar for dsp3 hybridized
Coordination number 6 (d2sp3 hybridization)
Complex with coordination 6 have octahedral geometry.
Colour of complexes
when a white light is allowed to fall on complex following process maker are complex may absorb hole of white light in this case Complex appear black Complex may reflect transfer whole white light in this case Complex beer white the complex major sum of colour of white light and may reflect the remaining light
Explanation when transition elements make compound their electron of the orbital are involve in bonding when leagan approach the central transition metal atom arrange the orbital supplied into two energy level t 2G Laurence and easy level this is called DD splitting the electron present in lower energy orbital absorb sum of photon in our promoted energy level this Photon which is involve in excitation of electron is responsible for the colour of complex the colour is base on unabsorb light transmitted light
Also read: Chemistry of Vanadium, chromium, manganese, iron and copper
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