1. Consequences OF Crystal Field Splitting PDF

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TRANSITION METAL CHEMISTRY CONSEQUENCES OF CRYSTAL FIELD SPLITTING

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Octahedral crystal field split    

As a result of application of an octahedral crystal field, the 5 free d-orbitals will no longer be at the same energy (no longer degenerate) Degenerate = all orbitals of equal energy Overall energy cannot change – the 3 going down should drop down 2/5ths and the 2 going up 3/5ths Difference in energy between the t 2g and eg orbitals is known as the crystal field splitting parameter (Δo)

Tetrahedral Crystal Field  

Energy level scheme is the reversed of the octahedral’s But no. of ligands has been reduced  Δt = 4/9 Δo

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In tetrahedral eg and t2g change to e and t 2

High and low spin configurations in octahedral fields 

If Δo > P  low spin configuration (+vice versa) o P = electron spin pairing energy o Thus, electron configuration (in cases where possible) depends on magnitude of Δo

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Unpaired electron = paramagnetic No unpaired electrons = diamagnetic

High and low spin configurations in tetrahedral fields 

As t is only about 4/9 o in almost all cases  P will be greater than t o Only high spin configs

Magnitude of crystal field parameter Δ (Δo)   

o for corresponding complex of the same group of the periodic table and same oxidation state increases 25-50% on going from 3d to 4d elements and again from 4d to 5d. t values are about 40-50% those for o (t4/9o) for complexes differing as little as possible, except in geometry.  values depend on nature and identity of ligands which exert the crystal field, following a regular order known as the Spectrochemical Series

Spectrochemical Series I-  Br-  Cl-  F-  OH-  C2O42-  H2O  -NCS  pyridine  NH3  1,2-diaminoethane  bipyridine  ortho-phenanthroline  NO2-  PR3  CO  CN     

Magnitude of crystal field exerted on the metal d-orbitals varies from one ligand to another Stronger ligands are weak field ligands (+ vice versa) CN is a strong field ligand – gives rise to low spin complexes X’s are weak field ligands – give rise to high spin complexes NH3 – low spin H2O – high spin

Variation of Ionic Radii w/ atomic number 

+slides 16-17 Ionic radius decrease w/ increasing atomic no. bc d-orbital points away from ligand/metal & higher shielding [dc]

Thermodynamic effects of CFS (crystal field splitting) 

For a dn ion, there will be a net stabilisation of the system as a result of the cfs, depending on the arrangement of e-‘s

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This crystal field stabilisation energy (CFSE), has a magnitude similar to that of chemical changes  expected to have an effect on the thermodynamic properties of these complexes

CFSE for octahedral crystal fields

Octahedral vs Tetrahedral – effects of CFSE 

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Several factors influence adoption of tetra/octahedral structures: o From bonding pov, 6 ligands favoured over 4  octahedral favoured o Ligand-ligand repulsion favours tetrahedral (especially if ligand is bulky) o High cationic charge gives large Δo  complex low spin  CFSE is high for octahedral complex CFSE is v. small for tetrahedral complexes (all high spin) and doesn’t favour tetrahedral Octahedral site stabilisation energy (OSSE): o OSSE = CFSEoctahedral - CFSEtetrahedral  If it’s small/zero  no thermodynamic preference for octahedral o OSSE small for d1-2, d5-7  no strong preference for an octahedral config o OSSE large for d3, d8  strong preference for octahedral config

Jahn – Teller Effect 

Any non-linear molecule system in which e- can be arranged in more than one way, to provide the same lowest energy state will be unstable and will distort to lower its symmetry and separate the 2 energy states.

o

Eg. 

According to the Jahn-Teller theorem, a regular octahedral symmetry for the d9 will be unstable and the structure must distort in some way.

Splitting pattern:

(a) o >> 1 or 2 (b)  12 (c) centre of gravity rule obeyed Energy changes on distortion:

+slide 33

Electronic Spectroscopy and Colour 

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Visible light (λ=800-380 nm) may interact w/ complexes of trans. elements to give: o Ligand Spectra  Spectrum of ligand (usually in UV region) remains on complexation – sometimes shifted from uncoordinated posn o Counter-Ion Spectra  May have an absorption spectrum which overlaps that of the metal complex  Eg. NO2-/MnO42-/CrO42-/Cr2O72o Charge-transfer Spectra  From electronic transitions b/t molecular orbitals which are predominantly metal and ligand – very intense (frequently tail into visible region) o Ligand-field or d-d Spectra  From electronic transitions b/t d-orbitals of the metal which have been split by the crystal/ligand field – d-d transitions are responsible for the colour of most trans metal complexes Selection Rules: o Electron spin cannot change during a transition – spin conserved o Transitions involving redistribution of e-‘s within the same quantum shell are forbidden (sp or pd are ok but no dd)  Laporte Rule  Δl must be = +/-1  if l=2 d-orbital o Adsorption of radiation is usually governed by the Beer-Lambert Law – A=εcl  Where ε=molar absorptivity (dm3mol-1cm-1)  λmax=measure of energy of transition  ε=measure of probability of transition (thus of intensity of adsorption) o charge-transfer bands result from allowed transitions  ε is high (>1000) – “allowed” bands and spectrum is intense o d-d transitions are forbidden by the Laporte Rule, but the rule is relaxed (bc of crystal field splitting)  ε is low  They occur due to 3d orbitals interacting w/ ligand orbitals and mixing, but w/ low probability  These forbidden bands have values ε around 0.1-100 (for octahedral) and 400 (for tetrahedral – more relaxed than octahedral bc less symmetrical)

Example of Electronic Spectrum {[Ti(H2O)6]3+}   

intense absorption at low λ: charge-transfer transition d-d band weak: bc of restriction (Laporte) At 570 nm: excited state has an e- config t2g 0eg1 – Jahn-Teller distortion of this state will occur2 overlapping peaks will be observed

Relaxation of Laporte Rule (Nephelauxetic Effect) 

Achieved by mixing metal-ligand orbitals (covalency) o Pairing energies are lower in complexes than in (g) metal ions  electronic repulsions are less o Effective size of metal orbitals is greater in a complex

Magnetic Properties  

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Experimental distinction b/t low and high spin is based on determination of their magnetic properties Diamagnetic complexes are repelled by a magnetic field and tend to move out of it o Arises from the closed electron shells of atoms  all matter is diamagnetic to some extent o All electron spins paired o Much weaker than paramagnetism – only observed in absence of other types of magnetism Paramagnetic complexes are attracted by a magnetic field and are drawn into it o Arises from the presence of unpaired e-‘s  molecules contain 1/+ unpaired e-‘s o Electron spins align w/ applied magnetic field Gouy balance can distinguish paramagnetic/diamagnetic complexes

Gouy Balance     

Involves weighing complex in presence of a magnetic field Diamagnetic species will appear to weigh less in a magnetic field than in its absence Paramagnetic species will appear to weigh more Measurements yield value for a parameter (“magnetic susceptibility”) – must be corrected for diamagnetism Parameter is then related to the spin-only magnetic moment of the complex (which is related to the no. of unpaired e-‘s (n)): o μ=[n(n+2)]1/2μB  μB = Bohr Magneton...