Explain the concepts of complex ions and isomerism. Illustrate the geometric isomerism of the complex [Co(NH3)4(H2O)2]3+ and determine the electron configurations of Sc3+ and Mn2+.

• $\text{Sc}$ (atomic number 21): $[\text{Ar}] 3d^1 4s^2$
• $\text{Mn}$ (atomic number 25): $[\text{Ar}] 3d^5 4s^2$

1. For $\text{Sc}^{3+}$: Three electrons are removed (two from $4s$ and one from $3d$). The electronic configuration is $\boxed{[\text{Ar}]}$.
2. For $\text{Mn}^{2+}$: Two electrons are removed from the $4s$ orbital. The electronic configuration is $\boxed{[\text{Ar}] 3d^5}$.

• $\text{Sc}^{3+}$ has a $d^0$ configuration.
• $\text{Mn}^{2+}$ has a $d^5$ configuration.

Ions of transition metals are coloured if they have partially filled d-orbitals, allowing for d-d electronic transitions when visible light is absorbed.
$\text{Sc}^{3+}$ has no d-electrons ($d^0$), so no d-d transitions are possible. Therefore, $\boxed{\text{Sc}^{3+}}$ is not coloured. $\text{Mn}^{2+}$ has a partially filled d-orbital ($d^5$), so it is coloured.