In
[Mn(CO)6]+ :
Mn+ =
3d54s1. In the presence of a strong field ligand, CO, the electrons get paired up.
3d6 So there are three lone pairs available for back bonding with the vacant orbital of C in CO.
In
[Fe(CO)5] :
Fe0 =
3d64s2. In the presence of a strong field ligand, CO, the electrons get paired up.
3d8 So, there are four lone pairs available for back bonding with the vacant orbital of C in CO.
In
[Cr(CO)6] :
Cr0 =
3d54s1. In the presence of a strong field ligand, CO, the electrons get paired up.
3d6 So, there are three lone pairs available for back bonding with the vacant orbital of C in CO.
In
[V(CO)6]− :
V− =
3d44s2. In presence of strong field ligand, CO, the electrons get paired up.
3d6 So, there are three lone pairs available for back bonding with the vacant orbital of C in CO.
Maximum back bonding is present in
[Fe(CO)5] ; therefore, its CO bond order is the lowest.