When we draw the complex Co(H2O ) 6 we consider water as a weak field ligand whereas in Cr (H20) it is considered as a strong field ligand?

The classification of weak field and strong field ligands were given by the Crystal field theory. It says that the ligands which produces high crystal field splitting energy are considered to be the strong field ligands and the one which produces small crystal field splitting energy are considered to be weak field ligands.
H2O can act as both strong field and weak field ligand. There is no special criteria to distinguish where water acts as a strong field and weak field. 
In case of Cr(H2O)6 , the water acts as a weak field ligand. On the other hand, in case of Co(H2O)6 water will act as a strong field ligand. This is because in case of Cobalt (+3) state, the size of the cation decreases due to which the repulsion between ligand and the electrons of the metal ion increases and hence the splitting of the orbitals increases. Thus water acts as a strong field ligand.
 

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