Why the CO stretching frequency of metal complexes V(CO)6 -1, Cr(CO)6 and Mn(CO) 6 +1 are reduced from the expected value ?

Why the CO stretching frequency of metal complexes V(CO)^-1, Cr(CO)6 and Mn(CO) ₆ ⁺¹ are reduced from the expected value  ?

All the three given metal carbonyl  complexes are iso electronic and hexa coordinated . Thus iso electronic complexes forming metal species are V ^-1 , Cr⁰ and Mn ⁺¹ respectively .

Each complex is octahedral in shape and the ligand CO is a π-acid ligand.In each case there is bonding and back-bonding takes place between the metal and the ligand .

Since each complex species is a d ⁶- system ,therefore there is a possibility of formation of such π-bonding with three CO groups  at a time in each case and by resonance each M-CO  bond occupy some multiple bond character .

By calculation it can be found that each M – C bond order 1.6  . During back-bonding the back-donated electron pairs from metal is taken up by C-atom of CO ligand in its vacant π*orbital, as a result the bond order of CO molecule in the complex is reduced to some extent from the real value of 3 which is found in free CO molecules.

Thus stretching frequency  is reduced in complex from the value of 2143 cm ^-1 , which is necessary for free CO molecule.

The more the degree of back-bonding the more is the lowering in such frequency . In the complexes V(CO)₆ ^-1, Cr(CO)6 and Mn(CO) ₆ ⁺¹ the metal ions are are V ^-1 ,Cr⁰ and Mn ⁺¹ respectively .

Back bonding ability of metal ion depends on its oxidation state and it is clear that ability decreases in the order of V ^-1 > Cr⁰ > Mn ⁺¹.

For the above reason the CO stretching frequency  of metal complexes V(CO)₆ ^-1, Cr(CO)6 and Mn(CO) ₆ ⁺¹ are reduced  from  the expected value  .

Practice problem:

Why the CO stretching frequency  of metal complexes V(CO)6 -1, Cr(CO)6 and Mn(CO) 6 +1 are reduced  from  the expected value  ?

Why the CO stretching frequency  of metal complexes V(CO)6 -1 is less than expected value ? 

Why the CO stretching frequency  of metal complexes Cr(CO)6 is less than expected value ? 

What is pericyclic reaction ? Click here