Metal carbonyl compounds And Bonding in metal carbonyl

Metal carbonyl compounds-

  ·       are organometallic compounds which have at least one carbon-metal bond.

Examples-

[Ni(CO)₄ ] Tetracarbonylnickel(0)

[Fe(CO)₅] Pentacarbonyliron(0)

[Cr(CO)₆] Hexacarborbonylcromium(0)

[Fe₂(CO)₉]  Tri-µ-carbonyl bis(tricarbonyl iron)

[Mn₂(CO)₁₀]   Decacarbonyldimanganese

((CO)₃ – Co – (CO)₂ – Co – (CO)₃ Di-µ-carbonylbis(tricarbonylcobalt)

[M(CO)₆] [M = Cr, Mo, W]

Bonding in metal carbonyl-

Metal -carbon bond in metal carbonyls possess both σ and π character.the formation of bonds between the metal and carbon atom of monoxide can be explained as-

·       The first overlap take place between the filled bonding π_2p orbital of the carbon monoxide with an empty metal d-orbitals resulting in a σ-bond between the metal and carbon atom of carbon monoxide. here, donation of lone pair of electrons on carbon into a vacant d-orbital of metal takes place.

·       The second overlap(is also called Pi-back bonding) takes place between the filled d-orbital of metal with an empty antibonding orbital π^*_2p of  the carbon monoxide resulting in π-bond between the metal and the same carbon monoxide molecule. here, donation of lone pair electrons  on a filled d-orbital into empty antibonding orbital π^*_2p of the carbon monoxide.

·       In other words, the M-C σ-bond is formed by donation of lone pair of electrons on the carbonyl carbon into vacant d-orbtal of the metal. The M-C π-bond formed by the donation of lone pair electrons from a filled d-orbital of the metal into vacant antibonding orbital π^*_2p of the carbon monoxide(called back bonding or Pi-back bonding). 

·       Pi-back-bonding is also known as Synergic bonding which means self-strengthening bond.

·       Synergic effect-the effect of σ-bond formation strengthens the π-bond and vice versa. It is called synergic effect.

Crystal field stabilisation energy (CFSE)

Crystal field stabilisation energy (CFSE)-

·       The splitting of the degenerate orbitals due to the presence of ligands in a definite geometry is known as crystal field splitting and the difference between two sets of degenerate orbitals as a result of crystal field splitting is known as crystal field stabilisation energy

·       It is denoted by Δ_o (the subscript o stands for octahedral) and Δ_t (the subscript t stand for tetrahedral.

·       The splitting is much smaller in tetrahedral than that in case of octahedral complexes. Thus the difference of energy, represented by Δ_t = 4/9 Δ_o.

Factors affecting the magnitude of CFSE(Δ )-

a.     Oxidation state of metal ion- higher the oxidation state of central ion, larger the value of Δ.

b.     Nature of the metal ion- in block elements, within the same group, as we move from 3d to 4d to 5d the value of Δ increases and the tendency to form low spin complexes increases. So 4d and 5d series element have high tendency to form low spin complexes than 3d series.

c.     Nature of the ligand -for the same metal ion, splitting is much smaller (Δ) in presence of weak ligand than that in case of strong ligand defined in spectrochemical series.

d.     Geometry of complex ion- in octahedral, splitting is larger that in case of tetrahedral i.e Δ will be higher in case of octahedral complexes.   

Previous topic

Ideal gas vs real gas and Ideal gas equation

 Ideal gas vs real gas

      Ideal gases obey Boyle’s law, Charles’ law and Avogadro’s law. Actually, these are based on the assumption that intermolecular forces are not present between the molecules of an ideal gases

    while real gases do not obey these laws (as Boyle’s law, Charles’ law and Avogadro’s law)

 Ideal gas equation

Ideal gas equation is derived by combining of Boyle’s law, Charles’ law and Avogadro’s law that gives the simultaneous effect of the change of pressure and temperature on the volume of gas. this is also known as combined gas law

Boyle’s law     V α 1/P   ------(1)              (at constant T and n)

Charles’ law V α T -----------(2)               (at constant P and n)

Avogadro’s law V α n   ------(3)               (at constant T and P)

By combining of these three equations (1), (2) and (3)

Then, Equation may be-

V α n T/P ----------------(4)

V = R n T/P            Or   

PV = n RT -------------(5)

Here, P= pressure of gas

           V= volume of gas

           n= number of moles of gas

           R= gas constant           

Eq (5) is known as ideal gas equation or combined gas equation for n mole gas.

Numerical value of R (gas constant) in different units are as-

S.No.	When unit of V	And unit of P	R value
1	L	atm	0.0821 L atm K-1 mol-1
2	ml	atm	82.1 ml atm K-1 mol-1
3	L	bar	0.08314 L bar K-1 mol-1
4	dm3	bar	0.0821 dm3 bar K-1 mol-1
5	m3	Pa or N/m2	8.314 Pa m3 K-1 mol-1   8.314 Nm K-1 mol-1 Or 8.314 J K-1 mol-1

At STP, volume of gas is taken as

In litre = 22.4 L

In ml   = 22400 ml

In m3   = 22.7 x 10^-3 m³

In dm3 = 22.7 dm³

Avogadro’s law (volume-amount relationship)

Avogadro’s law (volume-amount relationship)

Avogadro in 1811 put forward a relationship between volume of a gas to the number of molecules at constant temperature and pressure. This relationship is known as Avogadro’s law

 it states that at given temperature and pressure the volume of gas is directly proportional to the amount of gas. Mathematically, it can be expressed as

V α n -------------(1)

V= k₄ n   ---------(2)

Since n= given mas (m)/ molar mass(M)

So, V= k₄ m/M

Or   M= k₄ m/V ----------(3)        since [m/V =density(d)]

Therefore       

M= k₄ d ---------(4)

Eq. (4) gives relationship between molar mass and density of gas.

According to this principle, all gases containing equal amount of substance occupy the same volume at the same temperature and pressure. this means that one mole of each gas at standard temperature and pressure will have same volume. this is known as molar volume.

Gas law - Charles’ Law and Significance of Charles law

 Charles’ Law (V—T relationship)- 

•       In 1787 J Charles proposed a relationship between volume and temperature. It is called Charles’ Law 

• It states that 

               at constant pressure, the volume of a fixed mass of gas is directly proportional to the Kelvin temperature of absolute temperature. If the absolute temperature is doubled, the volume is doubled. Charles’ Law may be expressed mathematically as 

                 V ∝ T  (at constant P and n) -----------------(1) 

            or V =  k2 T                                             ----------- (2) 

where k2 is a constant. 

or  V/T = k2 

If V1, T1 are the initial volume and temperature of a given mass of gas at constant pressure and V2, T2 be the new values, we can write  

 V1/T1 =   K2  = V2/T2  

In other word  

                         At constant pressure the volume of a given mass of a gas increase or decreases by 1/273 of its volume at 00C, for each one degree rise or fall in temperature. 

Let V0 be ----------> the volume of a given mass of a gas at 00C 

     Vt    ------------------------>is its volume at any temperature t0C 

Then the volume, Vt may be written in term of Charles ‘law (at constant pressure) 

Significance of Charles law

·       It's interesting application is the use of hot air balloons in sports and for Meteorological observations. according to Charles’ law gases expand on heating, the larger volume at higher temperature will have lower mass per unit volume and therefore lower density thus hot air is less than cool air. this causes a hot air balloon to rise by displacing the cool air of the atmosphere

on this theory, hydrogen balloons rise higher because of lower density of hydrogen it was developed as a means of transportation across the Atlantic.

·       However, such airships are not being used because hydrogen is inflammable and that transportation will be risky, therefore, hot air balloons are preferred in place of hydrogen balloon

Significance of crystal field theory, Limitations of crystal field theory

Significance of crystal field theory 

This theory is useful to explain the following properties of complexes. 

a) High spin states and low spin states- 

    Strong field ligand     →   high CFSE value → low spin complexes

    Weal field ligand      →   low CFSE value → high spin complexes

b) Magnetic properties-

c) Colour- 

d) Geometry of complexes 

Limitations of crystal field theory

It was successful to explain the colour, magnetic properties, the effects of weak and strong field ligands etc in the coordination compound. However, it has the following limitations-

·       As ligands are considered as a points charges, the anionic ligands should exert greater splitting effect however, actually the anionic ligands are present at low end of the effect of spectrochemical series

·       It treats the metal-ligand bond as purely ionic and does not take into account the covalent character of the bond.

 These weaknesses have been explained by ligand field theory and Molecular orbital theory which are out of the CBSE syllabus of class 12.