Important terminology of coordination chemistry

 Important terminology of coordination chemistry - 

Coordination Entity- Constitutes a central metal atom or ion bonded to a fixed number of ions/ molecules.it is written in square bracket. Example 

[Ni (CO)4] 

[PtCl2(NH3)2] 

[Fe (CN)6]4+ 

 b). Central atom/ion- in coordination entity, the atom/ion to which a fixed          no. of ions/group are bound in a definite geometrical arrangement around it.  Example

[Ni (CO)4]             Ni0                 

[PtCl2(NH3)2]       Pt2+

[Fe (CN)6]4+        Fe2+ 

c). Ligands – The neutral molecules or anions or cations which are directly linked with the central metal atom or ion in a complex ion are called ligands. 

The ligands are attached to the central ion or atom through coordinate bonds or dative linkage. 

Types of ligands-  

Ligands can be of following types depending on the number of donor atoms or no of donor site present in them. 

Mono- or unidentate ligands: They have one donor atom, i.e., they supply only one electron pair to central metal atom or ion. F1-, Cl1-, H2O , NH3 etc. 

Bidentate ligands: Ligands which have two donor atoms and have the ability to link with central metal ion at two positions are called bidentate ligands. 

Example- 

 H2NCH2CH2NH2  (ethane-1,2-diamine) 

 C2O42-.   oxalate 

Polydentate ligands- having donor site/atom more than two. 

Example – 

[EDTA4-]  Ethylenediaminetriacetate 

(EDTA3-)   Ethylenediaminetriacetate 

Ambidentate ligands- ligand which has two donor site but use one to bind a metal atom in a time is called ambidentate ligand

Example- NO₂^- ,CN^1- , SCN^1- etc

M←CN    ,     M→NC          

here M is metal atom /ion

 and CN^-   -ambidentate ligand     

Chelating ligands:  

A bidentate or a poly dentate ligand uses its two or more donor atoms/site to bind the same metal ion simultaneously and form one or more rings are called chelate or chelating ligands 

The formation of ring is called chelation. 

No of ligating of di or polydentate ligand is called denticity. 

Chelating ligands form more stable complexes than monodentate ligands. This is called chelating effect. All types of polydentate ligands can act as chelating ligands.

•               most stable                                           less stable

• 
  

•       chelate molecule                                          Non-Chelate molecule

•        

• d).  Coordination number – 

• The number of atoms of the ligands that are directly bound to the central metal atom or ion by coordinate bonds is known as the coordination number of the metal atom or ion. 

• Example                      coordination no.

• [Ni (CO)4]        —     4                      

• [PtCl2(NH3)2]  —     4 

• [Fe (CN)6]4+    —     6 

 

e). Coordination sphere or Coordination entity and counter ions- 

• The central metal atom or ion and the ligands that are directly attached to it are enclosed in a square bracket. It has been called coordination sphere or coordination entity. 

• The ionizable groups written outside the bracket are called counter ions. 

   4K [Fe (CN)6] -----> [Fe (CN)6]4-   +     4K+ 

                              coordination sphere      counter ions. 

f). oxidation number of central atoms is defined as the charge it would carry if all the ligands are removed along with the electron pairs that are shared with the central atom. 

                                              Oxidation no. 

 [Ni (CO)4]                      Ni0              0

[PtCl2(NH3)2]              Pt2+            0

[Fe (CN)6]4+                 Fe2+            0 

 

g). Homoleptic complex- Complex in which a metal is bound to only one kind of donor group is called homoleptic complex. Example- [Ni (CO)4]    

h). Heteroleptic complex   - Complex in which a metal is bound to only one kind of donor group is called homoleptic complex. Example-      [PtCl2(NH3)2]  

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Werner theory of coordination compound

 

Werner theory of coordination compound

·      Proposed by Alfred Werner in 1893

·       He prepared and characterised a large no of coordination compounds and studied their physical and chemical properties

In the series of compounds of cobalt(III) chloride with ammonia, he found that some of the chloride ions could be precipitate as AgCl on adding excess silver nitrate solution in cold but some remained in solution

      1mole   CoCl₃.6NH₃ (yellow) gave à 3mole AgCl

      1mole   CoCl₃.5NH₃ (purple) gave à2mole AgCl

      1mole   CoCl₃.4NH₃ (green₎ gave à1mole AgCl

      1mole   CoCl₃.4NH₃(violet) gave à1mole AgCl

The important postulates of coordination theory are as –

1). in coordination compound, metals show types of valencies/linkage.

      a). Primary valencies

      b). secondary valencies

2). The primary valencies are normally ionisable and satisfied by negative ions.

3). Secondary valencies are non-ionisable and satisfied by neutral molecules or negative    

         Ions. It represents to the coordination number and is fixed for a metal.

4. the ions/group of atoms bound by the secondary linkage to the metal have characteristics spatial arrangement corresponding to different coordination number.

 

In modern formulation –

·      Spatial arrangement of ions/ groups present around to central metal ion is called coordination polyhedral.

·      The species within the square bracket are coordination entities or complex

·      Ions outside the square bracket are called counter ions.

·      Primary valencies usually indicates to the oxidation no. and Secondary valencies refers to the coordination no. of central metal atom/ions.

Coordination chemistry- Molecular or Addition Compounds,Double salt or lattice compound

 

Coordination chemistry

    ·      Is branch of inorganic chemistry dealing with the study of coordination compounds.

    ·    Coordination compound are the compounds in which the central atom is linked to a number of ions or neutral molecules by coordination bonds(this type of bond is formed between electron pair acceptor species and electron donor species)

    ·      These compounds play a vital role in our lives. Haemoglobin of blood and chlorophyll of plants, vitamin B-12 are also coordination compounds of Fe, Mg, Co respectively.

Molecular or Addition Compounds

·      When two or more simple salts are chemically combined together in definite of fixed  proportion by weight. the compounds formed are called molecular or addition compound.

Example K₂SO₄ +Al₂(SO₄)₃ + 24H₂O ------>  K₂S0₄·AI₂(SO₄)₃ ·24H₂O

                                                                                Alum

4KCN + Fe (CN)₂ ------ > Fe (CN)₂·4KCN

molecular or addition compound may be further classified into two types on  their behavior in an aqueous solution

a).Double salt or lattice compound

The addition compounds which are stable in solid state only but are broken down into individual constituents when dissolved in water (i.e. in aqueous, these are not stable)are called double salts or lattice compounds are called double salts.

KCl.MgCl₂ .6H₂O -------> K⁺ + 2Cl^-     + Mg²⁺ + 6H₂O

Example -

K₂SO₄. Al₂(SO₄ )₃ .24H2O— Potash Alum

FeSO₄(NH₄) ₂ SO₄ .6H2O— Mohr's salt

KCl.MgCl₂ .6H2O —Carnallite

b). Coordination or complex compounds   

Those molecular or addition compounds which retain their identity in aq. solution are called complex salt or coordination compound.

CuS0₄·4NH₃ -------->     [Cu (NH₃)₄]²⁺ + SO₄^2-

in other words, a coordination or complex compound may be defined. as a. molecular compound that results from the combination of two or more simple stable molecular compounds and retains its identity in the solid as well as in dissolved state.

Note- but the constituents in complex looses their identity in solid as well as in aqueous.

A complex ion may be defined as an electrically charged radical which consists of a central metal atom or ion surrounded by a group of ions or neutral molecules or both.

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The Inner Transition f-Block Elements

 The Inner Transition f-Block Elements

   ·      28 elements from atomic number 58(Ce) to 71(Lu) —14 elements and from atomic number   90(Th)  to 103(Lr)  —14 elements . these have been arranged in two horizontal rows below the periodic table. These elements are collectively called f- block elements

     Since the last electron enters one of the f-orbitals

    ·      These are also known as inner transition series

    ·      f-block consists of two series of elements known as Lanthanides or Lanthanons and Actinides or Actinons.

     ·      The lanthanide series follows lanthanum (At. No. 57), because lanthanum closely resembles the lanthanoids

·            Position in periodic table -the 14 members of lanthanide series have been placed along with lanthanum in the third group and sixth period and similarly 14 members of the actinide series have been placed with actinium in the third group and seventh period

    ·        The justification for assigning one place to these elements has been given on the basis of their similar properties. The properties are so similar that the fifteen elements from La to Lu can be considered as equivalent to one element. The same explanation can be given in the case of actinides.

    ·      In case, these elements are assigned different positions in order of their increasing atomic numbers, the symmetry of the whole arrangement would be disrupted. Due to this reason, the two series of elements, i.e., lanthanides and actinides are placed at the bottom of the periodic table

    ·      4f- series( lanthanide series) - There are fourteen elements from cerium (At. No. 58) to lutetium (At. No. 71) in this series. 4f-orbitals are gradually filled up. In past, elements were called rare earths.

    ·      5f-series (Actinides series): There are fourteen elements from thorium (At. No. 90) to lawrencium (At. No. 103) in this series. 5f-orbitals are gradually filled up.

   The elements from atomic number of 93 onwards are called transuranic or trans-uranium elements

 

GENERAL CHARACTERISTICS OF LANTHANIDES

 Electronic configuration –

     ·      General Electronic configuration -       (n-2)f^{1 -14}(n-1)d^0-1ns²     or   4f^1-14 5d^0-1 6s² 

     ·      The energies of 5d- and 4f-orbitals are nearly similar and thus their fillings show certain irregularities.

    

·        All tri-positive ions (the most stable oxidation state of all lanthanoids) have 4f^1-14 electronic configuration  

Atomic size and ionic size

     ·      There is decrease in atomic and ionic radii from lanthanum to lutetium due to lanthanoid contraction. However, the shielding of one 4f electron by another is less than a d-electron by another with the increase in nuclear charge along the series.

     ·      The decrease in atomic radii is quite not regular as it is regular in M³⁺

    Lanthanoid contraction –

     is a regular decrease in atomic radii by filling of 4f before 5d orbital. Since 4f orbital causes poor shielding effect(screening) than 5d.

    Consequences of lanthanoid contraction –

    ·      Similarity in atomic size of 2^nd and 3^rd  transition series.

    ·      Variation in basic strength of hydroxide

       Basic strength decreases from La(OH)₃ to Lu(OH)₃ due to lanthanoid contraction ,size of M³⁺ decreases its result is the increase in covalent nature

    Oxidation states

    ·      common oxidation à +3

    ·      In   La, Gd, Lu are especially stable because M³⁺, ions have stable electronic configuration as 4f⁰, 4f⁷ ,4f¹⁴ respectively

    ·      Ce(4f⁰ )and Tb(4f⁷ )exhibit à +4  , also show+3

    ·      Lanthanoids have a tendency to attain the oxidation state +3 due to most stable.

    ·      Ce⁴⁺ is good oxidising agent  while Sm⁺² is good reducing agent

    Ce⁴⁺  +  Fe²⁺------------> Ce³⁺  +  Fe³⁺

    2 Sm²⁺  +  H₂O -------> 2Sm³⁺  +  2OH^-  + H₂

    ^·              Eu²⁺ is strong reducing reagent while Tb⁺⁴ is an oxidant

    ·      The E° Value for Ce4+/Ce3+ is +1.74 V which suggests that it can oxidise water. However, the reaction rate is very slow and hence Ce(IV) is a good analytical reagent.       

    Colour

     The colour is due to f-f transitions since they have partly filled f-orbitals

        Mischmetal consists of a lanthanoid metal (nearly 95%) and iron (nearly 5%) and traces of S,C, Ca, and Al . these are used in Mg-based alloy to produce bullets, shell and light flint.

Gas laws - Boyle's law

 

The gaseous state

·      General characteristics are

a.     Shape and volume- gases neither has definite shape nor definite volume

b.     Highly compressible

c.      Gases exert pressure

d.     Have much lower density than the solid and liquids

e.     Diffusion- gases intermix completely in all proportions without any mechanical aid

f.       Liquefaction – can be liquified by cooling and by applying pressure.

This simplicity of gases is due to weak or  negligible force of interaction between gaseous particles

The gas laws

·      From the study of the behaviour of gases, certain generalisation were made. i.e the behaviour of gases are governed by some general laws, these generalisation are called gas laws.

·      These laws are related to measurable properties(pressure, volume, temperature and mass) of gases

These laws are as follows

a.     Boyle’s law(pressure-volume relationship)

b.     Charles’law (Temperature-volume relationship)

c.      Gay lussac’s law(pressure-Temperature relationship)

d.     Avogadro law(volume-Amount relationship)

a.     Boyle’s law (pressure-volume relationship)

·      In 1662 Robert Boyle proposed a relationship between Pressure and temperature. It is called Boyle’s law

·      The Pressure of a fixed amount of a gas are inversely related to its volume at constant temperature and.

                          P α 1/V   ---------(1)

           PV = K₁    ----------(2)      

     

 Let us consider  initial pressure and volume as P_1, V₁    After applying external pressure on gas, the final pressure and volume are as      P_2, V₂

   At constant amount of substance and Temperature

Then, according to Boyle’s law-

 P₁V₁ =K₁     ---------(1)

 P₂V₂ = K₁ ---------(2)

i.e.

    P₁V₁ = P₂V₂ =K₁     --------(3)

Graphical representation of Boyle’s law-

    ·      If graph is plotted between P and V we get curve called rectangular hyperbola)

   The curve clearly shows that when volume is increased, pressure decreases and vice versa.

   Similar curves are obtained at other temperature. Higher curve corresponds to higher temperature.

   Each curve corresponds to a different constant temperature, is known as isotherm (constant temperature curve) 

    ·      If the graph is plotted between P and 1/V, a straight-line graph is obtained passing through origin.

    ·      At high pressure, gasses deviate from Boyle’s law and so at high pressure, a straight-line graph cannot be obtained.

`            relationship between pressure and density

·      If density(d) = m/V    or V = m/d     Here m=mass of gas and V=volume of gas

Then we can give relationship between pressure and density by putting the value of volume (V) in Boyle’s equation as-

                              P m/d = K₁                                     (Boyle’s law  PV= K₁ )

                              So, P α d

Significance of Boyle’s law-

     ·      Gases are compressible      

     ·      Low pressure at high altitude, cause altitude sickness(anoxia) symptoms-uneasiness, sluggish feelings, headache

       Jet and aeroplanes fly at very high altitude with emergency oxygen supply in case of pressure falls