Transition Metals (COLOURS)

Starting colour: green solution (if pure, its meant to be violet) A few drops of NaOH / NH3: green precipitate - Cr(H2O)3(OH)3 Excess NaOH: green precipitate dissolves to form a deep green solution - [Cr(OH)6]3- Excess NH3: pale green precipitate dissolves into a green solution (purple solution if pure)- [Cr(NH3)6]3+

Starting colour: pink A few drops of NaOH: blue precipitate turning pink on standing - Co(H2O)4(OH)2 NH3: Blue precipitate - Co(H2O)4(OH)2 Excess NaOH: no reaction Excess NH3: blue precipitate dissolves to a brown solution - [Co(NH3)6]2+

Starting colour: pale green precipitate A few drops of NaOH / NH3: dirty green precipitate, turning brown on exposure to air as Fe2+ is oxidised to Fe3+ by air- Fe(H2O)4(OH)2 Excess NaOH: no reaction Excess NH3: no reaction

Starting colour: yellow A few drops of NaOH / NH3: red brown precipitate - Fe(H2O)3(OH)3 Excess NaOH: no reaction Excess NH3: no reaction

Starting colour: pale blue A few drops of NaOH / NH3: pale blue precipitate - Cu(H2O)4(OH)2 Excess NaOH: no reaction Excess NH3: pale blue precipitate dissolves to a deep blue solution - [Cu(NH3)4(H2O)2]2+

Different complexes have different metal ions / ligands When a ligand binds/ approaches a metal ion, the d orbitals split into groups that are at a higher and lower energy than the original energy (CANNOT just say d orbital splits, must say orbitals) The energy gap between these groups of d orbitals depends on the metal ion, it’s oxidation number and the ligand When white light is passed through a solution of this complex, light with a frequency equivalent to the energy gap is absorbed and an electron in the lower energy group of the d orbitals is promoted to the higher energy orbital The remaining light passes through the solution (transmitted) and this is what we see, and is known as the complimentary colour

Heat energy is absorbed in flame tests whereas light energy is absorbed in transition metal complexes Electrons are excited from ground state to a higher state in flame tests whereas electrons from the d orbitals are excited from a lower d orbital to a higher d orbital in transition metals As electrons fall back to ground state they emit energy in the form of light. This light corresponds to a particular wavelength so we see a particular colour in flame tests. However, in transition metal complexes, the wavelength of light absorbed depends on the energy change and that determines the complimentary colour. The remaining transmitted or reflected light, which is complementary to the absorbed color, is seen.

Because there are no available electrons to be excited, so energy cannot be absorbed

3C2H5OH + 2Cr2O72- + 16H+ → 3CH3COOH + 4Cr3+ + 11H2O

It acts as an acid or base

The precipitate dissolving then reforming

Cr2+ = blue Cr3+ = green Cr2O7 2- = orange CrO42- = yellow

2CrO42- 2H+ —> Cr2O72- + H2O

Cr(H₂O)₃(OH)₃

There are different energy gaps between d orbitals due to different ligands; different coordination numbers; different oxidation states of the metal combined Therefore different wavelengths if light are absorbed during d-d transitions so different wavelengths of light are then transmitted and seen as the new complementary colour

Amphoteric behaviour is when a species can act as both an acid (proton donor) and a base (proton acceptor). The complex Cr(H₂O)₃(OH)₃ shows amphoteric behaviour. Acting as an acid (proton donor) It donates protons to hydroxide ions: Cr(OH)3(H2O)3 + 3OH−(aq) → [Cr(OH)6]3−(aq) + 3H2O(l) Acting as a base (proton acceptor) It accepts protons from acid: Cr(OH)3(H2O)3 + 3H+(aq) → [Cr(H2O)6]3+(aq)