Co-ordination compounds, Nomenclature, geometries and colours

 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⁺  +    SO₄^2-

Here, [Cu(NH₃)₄]²⁺  is a complex cation and SO₄^2- 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 SO₄^2- (Sulphate), CO₃^2- (carbonate), CO₂^2- (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]SO_4,  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