Assertion  :    ${\mathrm{NF}}_3\mathrm{ }$is  a weaker ligand than  $\mathrm{N}{\left({\mathrm{CH}}_3\right)}_3\text{. }$

Reason     :  ${\mathrm{NF}}_3\mathrm{ }$ ionizes to give ${\mathrm{F}}^{-}$ ions in aqueous solution.  

It is correct statement that NF₃ is a weaker ligand than

$\mathrm{N}{\left({\mathrm{CH}}_3\right)}_3$  the reason is that fluorine is highly electronegative

 therefore, it withdraw electrons from nitrogen atom. Hence, the 

lone pair of nitrogen atom  cannot be ligated. While 

$\mathrm{N}{\left({\mathrm{CH}}_3\right)}_3$  is a strong ligand  because ${\mathrm{CH}}_3$ is electron releasing group .