Which of the following ligands causes the maximum splitting of $d$-orbitals?

In which of the following complexes will $H_2O$ act as a weak field ligand?

Identify the correct decreasing order of wavelength of light absorbed for the following complex ions:
$I. [Co(H_2O)_6]^{3+}$
$II. [Co(CN)_6]^{3-}$
$III. [CoI_6]^{3-}$
$IV. [Co(en)_3]^{3+}$

The wavelengths of light absorbed by the complexes $[Ni(H_2O)_6]^{2+}$,$[Ni(en)_3]^{2+}$,and $[Ni(H_2O)_4en]^{2+}$ are $\lambda_1$,$\lambda_2$,and $\lambda_3$ respectively. The correct order of wavelengths is:

The configurations of the complexes of manganese $A$ and $B$ respectively are $t_{2g}^3 e_g^1$ and $t_{2g}^4 e_g^0$. Then $A$ and $B$ are:

Explain crystal field splitting in octahedral complexes.