Question 1

Are all compounds of transition elements coloured?

Accepted Answer

No, this is a common misconception. A compound will be colourless if the central metal ion does not have a partially filled d-subshell. For example, compounds containing scandium(III) (Sc³⁺, 3d⁰ configuration), titanium(IV) (Ti⁴⁺, 3d⁰), copper(I) (Cu⁺, 3d¹⁰), and zinc(II) (Zn²⁺, 3d¹⁰) are typically white or colourless because d-d transitions are not possible.

Question 2

Why can iron form stable Fe²⁺ and Fe³⁺ ions, while a Group 2 metal like calcium only forms Ca²⁺?

Accepted Answer

For iron, the first two ionisation energies to remove the 4s electrons are relatively low, forming Fe²⁺. The third ionisation energy to remove a 3d electron is not prohibitively large because the 3d and 4s orbitals are close in energy, allowing stable Fe³⁺ to form. For calcium, after removing the two 4s electrons to form Ca²⁺, the third ionisation energy is extremely high as it would involve removing a core electron from the stable 3p sub-shell.

Question 3

In a ligand substitution reaction, does the coordination number of the central metal ion always change?

Accepted Answer

Not necessarily. While some substitutions do involve a change in coordination number (e.g., octahedral [Cu(H₂O)₆]²⁺ changing to tetrahedral [CuCl₄]²⁻), many do not. For example, when aqueous cobalt(II) ions react with excess ammonia, the six water ligands are replaced by six ammonia ligands, and the coordination number remains six: [Co(H₂O)₆]²⁺ + 6NH₃ → [Co(NH₃)₆]²⁺ + 6H₂O.

General characteristic chemical properties of the first set of transition elements, titanium to copper

What you need to know

Transition Metals: A Chemical Chameleon Act