Among the following metal carbonyls, the C --- O bond order is lowest in :
a) [ Mn(CO)6]+ 
b) [ V(CO)6 ]- 
c) [ Cr(CO)6]
d) [ Fe(CO)5]
Correct answer is option 'D'. Can you explain this answer?

JEE Question

"Correct option would be b)
Metallic carbonyls involves both sigma and pi-bonding. pi-bonding involves the back donation of metal d-electrons into vacant orbital of CO. Larger the electrons involve in back-bonding, lesser will be the bond order.
CO is strong field ligand. The electronic configuration of metal ion in presence of CO will be as follows:
a) Mn+ = 3d5, 4s1 = 3d6​ (in presence of CO)
b)Fe0 = 3d6, 4s2 = 3d8 (in presence of CO)​
​c) Cr0 = 3d5, 4s1 = 3d6​ (in presence of CO)​
d) V- = 3d4, 4s2 = 3d6​ (in presence of CO)​​
In Fe(CO)5, 4 lone pairs of electrons are available on metals for back donation hence lowest will be bond order."

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