As $$\Delta $$₀ < P, so all ligands behaves as weak field ligands.

For octahedral

Crystal field stabilization energy (CFSE)

= (-0.4$$\Delta $$₀) $$ \times $$ n_t2g + (+0.6$$\Delta $$₀) $$ \times $$ n_eg + np

n_t2g = number of electrons in t_2g orbital

n_eg = number of electrons in e_g orbital

n = number of extra pairs

p = Pairing energy

Here n_t2g = 1

n_eg = 0

n = 0 ( For weak field ligands n always zero. Here H₂O is weak field ligand)

$$ \therefore $$ Crystal field stabilization energy (CFSE)

= (-0.4$$\Delta $$₀) $$ \times $$ 4 + (+0.6$$\Delta $$₀) $$ \times $$ 2 + 0$$ \times $$P

= -1.6$$\Delta $$₀ + 1.2$$\Delta $$₀

= -0.4$$\Delta $$₀