Why transition metal compounds are colourful

Their valence electrons absorb certain frequencies of visible light causing the remaining light to be reflected showing the complimentary colour of the wavelengths absorbed

Why transition metal compounds are colourful but the metals aren’t

Presence of ligands causes splitting of d-subshell, ligands repel some orbitals more than others in the cation causing some electrons to gain energy

2 groups of orbitals, 3 d-orbitals being slightly lower energy (ground state) & 2 being slightly higher (excited state)

Rules for promotion of an electron

There must be at least 1 grounded d-electron & 1 space in excited orbital, an electron in the grounded state will absorb either visible light or UV light and become promoted to an empty space in an excited orbital. This is called d-d transition

Difference in energy between grounded & excited electrons in a complex ion (ΔE)

ΔE is the energy from light absorbed by the compound & corresponds with the wavelength of the colour complementary to colour of the compound

Links frequency of visible light absorbed by an ion to splitting of the d energy level ΔE = hv ΔE = change in energy/J h = Planck constant/6.63 x 10^-34Js^-1 v = frequency of light absorbed/Hz

Alternate Planck constant equation

ΔE = hc/λ c = speed of light/3.00 x 10^8ms^-1 λ = wavelength of light absorbed/m

What affects the value of ΔE

Central metal ion & its oxidation state, type of ligand, coordination number, metal-ligand bond strength, energy transfer, & complex shape

How large & small energy transfers affect colour of complex ion

Large energy transfer = high frequency & short wavelength radiation, blue end of spectrum (e.g., copper sulphate) Small energy transfer = low frequency & long wavelength radiation, red end of spectrum (e.g., iron oxide)

How shapes affect splitting

Generally the more ligands the more splitting (i.e., linear<tetrahedral<square planar<octahedral) but square planar causes more splitting than tetrahedral

Conditions where complex ions appear white

Complexes where d-subshell is full or empty, d-orbitals must be split, presence of ligands. Anhydrous complexes generally don’t have split d-orbitals & thus don’t absorb light

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Trans. metal comp.

AQA A-level chemistry transition metal compounds year 13

Term	Definition
Why transition metal compounds are colourful	Their valence electrons absorb certain frequencies of visible light causing the remaining light to be reflected showing the complimentary colour of the wavelengths absorbed
Why transition metal compounds are colourful but the metals aren’t	Presence of ligands causes splitting of d-subshell, ligands repel some orbitals more than others in the cation causing some electrons to gain energy
How d-orbitals split	2 groups of orbitals, 3 d-orbitals being slightly lower energy (ground state) & 2 being slightly higher (excited state)
Rules for promotion of an electron	There must be at least 1 grounded d-electron & 1 space in excited orbital, an electron in the grounded state will absorb either visible light or UV light and become promoted to an empty space in an excited orbital. This is called d-d transition
Difference in energy between grounded & excited electrons in a complex ion (ΔE)	ΔE is the energy from light absorbed by the compound & corresponds with the wavelength of the colour complementary to colour of the compound
Planck constant	Links frequency of visible light absorbed by an ion to splitting of the d energy level ΔE = hv ΔE = change in energy/J h = Planck constant/6.63 x 10^-34Js^-1 v = frequency of light absorbed/Hz
Alternate Planck constant equation	ΔE = hc/λ c = speed of light/3.00 x 10^8ms^-1 λ = wavelength of light absorbed/m
What affects the value of ΔE	Central metal ion & its oxidation state, type of ligand, coordination number, metal-ligand bond strength, energy transfer, & complex shape
How large & small energy transfers affect colour of complex ion	Large energy transfer = high frequency & short wavelength radiation, blue end of spectrum (e.g., copper sulphate) Small energy transfer = low frequency & long wavelength radiation, red end of spectrum (e.g., iron oxide)
Order of strength of d-subshell splitting for unidentate ligands	Cl-<OH-<H2O<NH3<CN-
How shapes affect splitting	Generally the more ligands the more splitting (i.e., linear<tetrahedral<square planar<octahedral) but square planar causes more splitting than tetrahedral
Conditions where complex ions appear white	Complexes where d-subshell is full or empty, d-orbitals must be split, presence of ligands. Anhydrous complexes generally don’t have split d-orbitals & thus don’t absorb light

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