[Ti(H_2O)_6]^ 2+ is a colored compound where as [Zn(H_2O)_6]^ 2+ is...

The atomic number of Titanium (Ti) is $22$. Its ground state electronic configuration is $[\text{Ar}] 3d^{2} 4s^{2}$.

In the complex $[\text{Ti(H}_2\text{O)}_6]^{2+}$, Titanium is in the $+2$ oxidation state. The electronic configuration of $\text{Ti}^{2+}$ is $[\text{Ar}] 3d^{2}$. Since it has partially filled $3d$ orbitals, it can undergo $d-d$ transitions, which are responsible for its color.

The atomic number of Zinc (Zn) is $30$. Its ground state electronic configuration is $[\text{Ar}] 3d^{10} 4s^{2}$.

In the complex $[\text{Zn(H}_2\text{O)}_6]^{2+}$, Zinc is in the $+2$ oxidation state. The electronic configuration of $\text{Zn}^{2+}$ is $[\text{Ar}] 3d^{10}$. Since it has completely filled $3d$ orbitals, $d-d$ transitions are not possible, making the complex colorless.

Thus, Zinc has completely filled $3d$ orbitals and Titanium has partially filled $3d$ orbitals.

Answer: Zinc has completely filled 3d orbitals and Titanium has partially filled 3d orbitals.