Why CO is a poor Lewis base towards H + but it is an excellent Lewis base towards Ni ?
CO is a neutral polar molecule . According to M.O theory it’s bond order is 3 , of the two π
bond one is dative π type. The bonding of CO molecule are as
follows,
Due to this dative pi bond C-atom in CO molecule becomes
slightly negatively charged.
The polarity of the molecule acts in opposite direction to what is expected. As a result polarity is very low and also the dipole moment.
Both C and O atom possess lone pair of electrons. Again the molecule possesses vacant anti bonding π*-orbitals.
Lone pair of electrons on C –atom is in higher energy level than that in O-atom , thus the molecule can behave as a Lewis base during which it can donate the lone pair on carbon atom .
But due to unexpectedly low polarity basic is very feeble and it can not form adduct with strong Lewis acid like H + , BF3 etc.
Although CO has a poor σ donor ability but it has a strong π accepting property due to presence of the above mentioned vacant π* molecular orbital, in which it can easily receive back donated electron from suitable filled π orbital of metal atom forming a stable π bond.
Such donation and back donation mutually reinforce each other and makes the so called weak base. CO a strong π acid ligand toward a lower oxidation state ( 0,-1,-2 ) . This is known as synergic effect.
Thus although CO is a poor Lewis base towards H + , since there no scope of back donation but it is an excellent Lewis base towards Ni(0) .
The polarity of the molecule acts in opposite direction to what is expected. As a result polarity is very low and also the dipole moment.
Both C and O atom possess lone pair of electrons. Again the molecule possesses vacant anti bonding π*-orbitals.
Lone pair of electrons on C –atom is in higher energy level than that in O-atom , thus the molecule can behave as a Lewis base during which it can donate the lone pair on carbon atom .
But due to unexpectedly low polarity basic is very feeble and it can not form adduct with strong Lewis acid like H + , BF3 etc.
Although CO has a poor σ donor ability but it has a strong π accepting property due to presence of the above mentioned vacant π* molecular orbital, in which it can easily receive back donated electron from suitable filled π orbital of metal atom forming a stable π bond.
Such donation and back donation mutually reinforce each other and makes the so called weak base. CO a strong π acid ligand toward a lower oxidation state ( 0,-1,-2 ) . This is known as synergic effect.
Thus although CO is a poor Lewis base towards H + , since there no scope of back donation but it is an excellent Lewis base towards Ni(0) .
What do you mean by synergic effect ?
When two or more elements or substances ( metal and ligand ) are combined together in
the purpose of additional π
bond formation, they produces a greater effect than the sum of the individual
effect , which is known as synergic effect.
For example : A carbonyl group donates it’s lone pair of
electrons to an empty metal d orbital to
form an π-
back bonding.
Significance of synergic effect :
This synergic effect strengthens the metal carbonyl
interaction or metal –carbonyl bond order. It also weakens the CO triple bond ,
that is, CO bond order.
Summary:
What do you mean by synergic effect ?
What is significance of synergic effect ?
Why CO is a poor Lewis base towards H + but it is
an excellent Lewis base towards Ni ?
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