In coordination complex compounds when does nh3 acts as strong ligand

@ kumar 
in spectrochemical series the ligands are arrenged in their increasing delta value towards a metal ion . 
as the charge on the metal ion will increase the magnitude of delta alos increase . 
there are a very few case in which nh3 is act as a weak ligands but generally its  a strong ligand . 
WHEN CHARGE ON THE METAL IS HIGHER , THEN ITS CONSIDERED AS STRONG LIGAND , 
AND WHEN CHARGE IS LOWER ITS CONSIDERED AS WEAK LIGAND . 
example – in the case of co^2+ complexes it act as a weak field ligand . 
in case of co ^3+ complexes it act as a strong field ligand . 
HOPE IT CLEARS YOUR DOUBT 

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