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

According to Fajan's Rules, covalent character increases with:Smaller size of cation.Larger size of anion.Among the halides ${\text{F}}^{-},{\text{Cl}}^{-},{\text{Br}}^{-},{\text{I}}^{-}$, the size increases from fluoride to iodide. Therefore, $\text{MI}$ has the maximum covalent character and $\text{MF}$ has the minimum.Order of decreasing covalent character: $\text{MI}>\text{MBr}>\text{MCl}>\text{MF}$.