Crystal field theory Flashcards
what makes a metal a ‘d block element’ ?
partially filed d-orbital
Coinage metals
Cu, Ag, Au
Platinum group elements
Ru, Os, Rh, Ir, Pd, Pt
organometallic compounds
ligands that are C based
e.g. CO, CH3
5-bond compound?
quintuple bond
metathesis
swapping reaction
inner sphere ligands
bonded directly to metal
outer sphere ligands
electrostatically associated with inner sphere complex
primary valency
value that never changes
oxidation number
satisfied by -ve ions
secondary valency
no. of groups covalently bonded to M
coordination number
directed to fixed positions in space about central M ion
trend in oxidation state across group
increasingly less favoured L->R
Zeff increases
IE increases
harder to move away from nucleus
trend in oxidation ability across group
becomes increasingly oxidising - wants to be removed
trend down group in oxidation state
increasingly favoured
RDF more diffuse
less electron repulsion
M easier to oxidise
crystal field theory
[simple]
based on electrostatics
treats ligands as point charges
ignores possibility of metal d-electrons being removed in bonding
ligand field theory
[more complex]
allows for covalent bonding
3d orbital degeneracy
as ligands approach more closely along x/y/z axis, 3d orbitals lose degeneracy
dxy, dxz, dyz = t2g (less electron repulsion; stabilised as they lie between x/y/z axis)
dx2-y2 + dz2 = eg (destabilised)
Δoct
[crystal field stabilisation energy]
represents stabilisation relative to spherical field
= 0 (stabilisation must offset destabilisation) -> centre of gravity/barycentre
t2g e- value
-2/5
eg e- value
3/5
Δoct in Dq
Δoct = 10Dq
evidence for CF theory
[double humped plot of hydration for octahedral M2+ ions]
hydration = M2+(g) + H2O -> [M(H2O)6]2+
expected trend = increased favourable (exothermic) L->R as Zeff increases; ion size decreases ; M-L interaction increases
measured trend = Ca2+, Mn2+, Zn2+ sit on line - others vary significantly
related to Δoct (makes favourable contribution to hydration energy of system - more exothermic)
filling orbitals
d1-d3 = no choice
d4 = either keep filling or double one of t2g sets
-although t2g gives -2/5 Δoct, filling doubly filled orbital increases e- repulsion
high spin
[weak field arrangement]
energy to put e- in eg set < e- repulsion
low spin
[strong field arrangement]
energy to put e- in eg set > e- repulsion
effects on radii metals - iron
Fe2+/Fe3+ = high spin (choice)
Fe2+ = low spin
low spin = smaller
-more e- repulsion in HS arrangement (2e- in eg orbital)
-ion = bigger
-closer ligand approach in LS - smaller
barycentre
energy before split
where does colour arise from?
t2g -> eg orbital transition
= direct measure of Δoct
only works with d1 systems
∈
measure of the probability of a transition taking place
bigger ∈ = more likely for transition to take place
selection rules
- spin can’t change
- change in parity required (i.e. symmetry)
- implies ΔI (OAM quantum number) = +/- 1
LMCT
[ligand -> metal charge transfer band]
transfer of e- of oxygen lone pair to M-centred orbital
MLCT
[metal -> ligand charge transfer band]
Lenz’s Law
in absence of any magnetic moment (i.e. any unpaired e-), a magnetic field is induced that opposes the main field
= MAGNETIC MOMENT/MOLE from diamagnetic effect MD
what does ueff relate to?
no. of unpaired e-
= √n
temperature dependent paramagnetism
e.g.[Ni(Et3)2Cl2
8 d electrons - √8 = 2.83BM
Ueff (80K) = 3.2BM
Ueff (300K) = 3.8BM
higher than expected = effect from orbital angular momentum
as temperature increases, so does effect of orbital angular momentum
