d-block - coordination compounds Flashcards
(17 cards)
what is the visible region of the EM spectrum?
400-700 nm
why do we see colour?
objects can absorb/transmit light - if a colour of light is transmitted that is the colour the object will appear, if a colour is absorbed the object will appear the opposite colour
colour arises due to absorption/transmission of visible light
how do photons interact with molecules?
if photons are absorbed by a molecule they can promote electrons from one d-orbital to another, or lead to transitions involving d-orbitals and ligand based orbitals
what is molar extinction coefficient, ε ?
provides a measure of the probability of a transition taking place, helps to characterise transitions of electrons caused by photons, alongside λ or λmax
what 3 selection rules must electronic transitions obey?
- spin cannot change
- there must be a change in parity e.g. g-> u or u-> g but not g->g
- laporte selection rule: d->d symmetrical transitions are not allowed
why are selection rules not always 100% obeyed in electron transitions?
lots of factors can relax the rules
e.g. orbitals can mix, so d->d may not be strictly d->d
also electronic transitions can mix with vibration/vibronic coupling, causing transitions to momentarily not be strictly g->g or u->u as vibrations distort molecule geometry making it not perfectly octahedral
how does ε provide evidence for charge transfer transitions?
some metals have very high ε values for transitions, which cannot be accounted for by d->d orbital transitions
what is a ligand to metal charge transfer band (LMCT)?
a very intense band of colour seen on UV-Vis spectra, which is the result of an electron moving from ligand to metal
these occur specifically between easily reduced metals and easily oxidised ligands
what is a metal to ligand charge transfer band (MLCT) ?
an intense band of colour seen on UV-Vis spectra, which is the result of an electron moving from metal to ligand
these occur specifically between easily reduced ligands and easily oxidised metals
how can electron transitions cause distortions + are these distortions observable?
a d->d transition from t2g -> eg results in a degenerate electronic state undergoing a distortion to remove degeneracy leading to a lower energy state = jahn-teller distortion
however the jahn-teller distortion will not be seen, as there are no eg electrons, so electrons don’t point towards ligands on axis there will be no observable elongation, although the distortion does take place
what type of complex does the laporte rule not apply to?
tetrahedral complexes have no centre of symmetry, so laporte selection rule doesn’t have to be contravened - this means they can have stronger colours
how do complexes interact with magnetic field?
a complex placed in a magnetic field undergoes perturbation of circulation of electron density within it
diamagnetism definition
in the absence of any unpaired electrons, electrons oppose main field/induce opposing field
paramagnetism definition
when unpaired non-interacting electrons are present, they try to align with the field - this affect is much stronger
how are d-block metals magnetic?
unpaired electrons lead to a magnetic moment, μ
any d-block metal with electrons has a magnetic moment
effective magnetic moment, μeff definition
this simply relates to the number of unpaired electrons by √n(n+2), units = μB, bohr magnetrons
this is the spin only formula, it reflects how paramagnetism arises from spin angular momentum of the electron
what is orbital angular momentum + how does it arise?
the rotational inertia + momentum of an object, particularly in relation to its motion around a central point or axis
this is generated by electrons moving from orbital to orbital, this generates magnetism, adding to it such that μeff < μspin only
