Question

Why is $Br$ a weaker field ligand than $CO$ .

Answer 1

Ligands are electron donors to metal atoms. Ligands are of two types which are strong field ligands and weak field ligands. Carbonyl is a ligand with the molecular formula of $CO$ has pi-electrons and easily donates the electrons to metal atoms and is known as a strong field ligand.

Complete answer:
Complex compounds consist of a central metal atom and ligands inside the coordination sphere.
Halogens are the elements that are non-metallic and have high electronegativity values. The halogens are fluorine, chlorine, bromine and iodine. Bromine is also a halogen and has high electronegativity value.
Electronegativity is defined as the tendency of the attraction of a shared pair of electrons towards itself. Thus, halogens will not easily donate electrons. Carbonyl ligands have carbon and oxygen atoms in which the oxygen atom is donor and has pi-bonds. These pi-bonds are loosely held and can be readily available to donate.
Pi-bonds are formed by the sideways overlap of orbitals and is a weak bond, whereas sigma bonds are formed by the lateral overlap of orbitals and is a strong bond.
Due to the presence of pi-bonds carbonyl is a strong field ligand than $Br$

Note:
Carbonyl is a neutral ligand as it does not consist of any charge and a stronger field ligand due to the presence of pi-bonds. Bromine can exist as $B{r^ - }$ it can be considered as a negatively charged ligand. Due to the electronegativity of bromine, bromine does not easily donate electrons and is considered as a weak field ligand.