Transition metals Flashcards
Properties of transition metals
- form compounds which are often paramagnetic
- shows variable oxidation states
- formed coloured ions and compounds
- form compounds with profound catalytic activity
- form stable complexes
- have high melting and boiling points
What’s the chemistry of transition metals dominated by?
the d-orbitals (valence electrons are always 3d, not 4s) which are lower in energy than the s-orbitals
The Lanthanide Contraction
- The ionic radii of the lanthanide ions decrease with increasing atomic number (they get heavier but smaller)
- f electrons do not shield very well but more protons are added
What are ligands treated as?
point negative charges
What is Δ o ?
crystal field splitting parameter
What do the symmetry labels e and t mean?
e = doubly degenerate
t = triply degenerate
What sort of Δ do tetrahedral structures have?
Δ is always much smaller for tetrahedral complexes than square planar therefore they are almost always high spin
How does crystal field splittings vary with oxidation state?
Δ are usually larger for metals in higher oxidation states because the higher charge on the metal exerts more attraction to the ligands resulting in shorter bond lengths and more interaction between d-orbitals and ligand charge
Factors affecting Δ o
- oxidation state of metal
- position on periodic table
- type of ligand
How does crystal field splitting vary with position on periodic table?
Δ are usually smaller for the 1st row and so first row transition metals have a tendency of being high spin
Δ link to wavelength
small splitting at higher wavelengths (red)
large splitting at smaller wavelengths (purple)
What is Δ o measured between in a MO diagram?
t 2g and e g *
General trend of the spectrochemical series for crystal field splittings
⫪- donor < weak ⫪- donor < no ⫪ effects < ⫪- acceptor
(⫪- donor make Δ smaller)
What are the different light absorption and electronic transitions?
- Intra-atomic (localised) excitations
- Interatomic (charger transfer) excitations
- Molecular Orbital (HOMO to LUMO) excitations
- Band to band transitions
- Intraband excitations
- Defects and colour centres
What are the 2 key light absorption and electronic transitions for transition metals?
- Intra-atomic (localised) excitations
- Interatomic (charger transfer) excitations
What are the selection rules for transitions?
Spin selection rule, ΔS = 0
Laporte selection rule, Δl = +/- 1 because a change in parity is required for complexes with an inversion centre, i
When can the selection rules for transitions be relaxed?
- Spin forbidden transitions are ‘partially allowed’ because of spin-orbit coupling
- Laporte rule is relaxed if there’s no centre of symmetry because of orbital mixing
What is ε in the beer lambert law?
a molecules ability to absorb light at a given wavelength
What type of transition is Fe(II) → Fe(III)?
electron transition is a low energy transition therefore absorbs in the red region
The Jahn- Teller effect
any non-linear molecule with a spatially degenerate electronic ground state will undergo a geometrical distortion that removes that degeneracy because the distortion lowers the overall energy of the species
(the z repulsions are reduced)
Dynamic Jahn-Teller effect
exceptions to jahn-teller effect occurs when the time frame of the measurement does not allow the distortion to be seen because the molecule randomly undergoes movement
What can x-ray be used to see in structures?
X-ray structure obtained at varying temperatures is possible to “freeze” a molecule into a static position showing the distortions
