In the case of alkali metals, the covalent character decreases in the order

As the size of halide ion decreases ${\mathrm{I}}^{-}>{\mathrm{Br}}^{-}>{\mathrm{Cl}}^{-}>{\mathrm{F}}^{-}$, tendency to get polarised
by cation M+ decreases and hence covalent character decreases.