Download Schedule • Last Week: Electronic spectroscopy • Last Week

Schedule
• Last Week: Electronic spectroscopy
Interelectron repulsion, covalency and spin-orbit coupling
• Lecture 4: Re-cap
• Lecture 5: π-Acceptor
π Acceptor Ligands and Biology
N2, CO, N2 and O2 complexes
• Lecture 6: M-M bonding
Multiple bonds and metal clusters
Slide 2/12
Summary of the Last Lecture
Selection Rules and Band Intensity
Spin selection rule
• The spin cannot change during an electronic transition
• ‘Relaxed’ by spin-orbit coupling for heavy elements
O bit l selection
Orbital
l ti rule
l
• ‘d-d’ transitions cannot occurs
• ‘Relaxed’
Relaxed’ by d
d-p
p mixing in complexes without centre of
inversion (e.g. tetrahedron)
Laporte selection rule
• No ‘d-p’ mixing possible in complexes with a centre of
inversion (e.g. octahedron or square planar complex
• ‘Relaxed’ due to molecular vibrations
Charge transfer transitions
• LMCT, MLCT and IVT – cover up ‘d-d’ if in visible region
Slide 3/12
•
The height of the band in the spectrum is called the ‘molar
extinction cofficient’
cofficient – symbol ε:
ε (mol-1 cm-1)
very
pale colours
10-3
-1
1 – 10
10 – 102
10 – 103
intense
colours
> 103
type of transition
spin forbidden
orbitally forbidden,
L
Laporte
t fforbidden
bidd
spin forbidden
orbitally forbidden
forbidden,
spin allowed,
orbitally forbidden
Laporte forbidden
spin allowed,
orbitally forbidden
LMCT, MLCT, IVT
type of complex
octahedral d5 complexes
(e.g. [Mn(H2O)6]2+)
tetrahedral d5 complexes
(e.g. [MnCl4]2-+)
octahedral and square
planar complexes
tetrahedral complexes
Slide 4/12
Oxidation States of Manganese: +7
•
Oxidation States of Manganese: VI
[MnO4]−: the permanganate ion
•
[MnO4]2−: the manganate ion
2MnO4−(aq) + C6H10(l) + 2OH-(aq) Æ 2MnO42−(aq) + C6H10(OH)2(aq)
deep purple colour
(absorbs green/yellow ~ 18000
¾ high metal charge (+6) makes it highly oxidizing (and easily reduced)
cm-1)
¾ O Æ M charge transfer occurs at relatively low energy (in the visible
region but at higher energy than for permanganate
¾ LMCT – orbitally allowed and spin allowed so highly intense
¾ d1 – ligand-field transition lost under LMCT bands
¾ high metal charge (+7) makes it highly oxidizing (and easily reduced)
¾ O Æ M charge transfer occurs at relatively low energy (in the visible
region)
deep green colour
¾ LMCT – orbitally allowed and spin allowed so highly intense
(absorbs purple ~ 25000 cm-1)
Slide 5/12
Slide 6/12
Oxidation States of Manganese: IV and II
•
MnO2: manganese dioxide
•
brown
MnO42−(aq) + 3H+(aq) + C6H10(l) Æ 2MnO2(s) + C6H10(OH)2(aq)
• Mn2+((aq):
q) manganese
g
ion
Manganese(II)
pale pink
p
p
Mn2+ d5: all transitions are spin forbidden:
¾ become possible through spin-orbit coupling
¾ spin-forbidden transitions are extremely weak for 3d metal complexes
¾ bands due to spin-forbidden transitions are normally hidden under the
spin allowed bands
¾ for d5, there are no spin-allowed bands allowing spin-forbidden bands to
be seen
MnO2(s) + 4H+(aq) + 2Fe2+(aq) Æ Mn2+(aq) + 2Fe3+(aq) + 2H2O(l)
¾
eg
low metal charge (+2) so O Æ M charge transfer in ultraviolet
¾ high spin d5 – all transitions are spin forbidden
t2g
¾ similar colour to permanganate but very different intensity
ground state
Slide 7/12
AJB lecture 3
Slide 8/12
Octahedral and Tetrahedral Cu(II)
Manganese(II)
Mn2+ d5: all transitions are spin forbidden
¾ turning a spin over requires energy even if the orbital is not changed
¾ called a “spin flip transition”
¾ as the orbital occupation does not change, there is very little change in
the M
M-L
L bond lengths
•
eg
•
Cu2+
[Cu(H2O)6]2+(aq) + 4Cl-(aq) ↔ [CuCl4]2−(aq) + 6H2O(l)
pale blue
green
absorbs ~13000 cm-1
absorbs ~10000 cm-1
eg
•
d9: one d-d
d d transition with frequency = Δoct or Δtet
¾ Δoct > Δtet (fewer ligands in a tetrahedron)
t2g
ground state
g
•
t2g
Higher intensity for tetrahedral complex
¾ d-p mixing possible in tetrahedron
spin-flip
spin
flip
excited state
¾ d-p
d p mixing only due to vibrations for octahedron
Slide 9/12
Octahedral and Tetrahedral Co(II)
•
Summary
Co2+
By now you should be able to
• explain the number of bands
• obtain Δoct from spectrum for d1, d3, d4, d6, d7, d8 and d9
• predict relative intensity of spin-allowed vs spin
forbidden octahedral vs tetrahedral and ligand
forbidden,
ligand-field
field vs
charge-transfer transitions
[Co(H2O)6]2+(aq) + 4Cl-(aq) ↔ [CoCl4]2−(aq) + 6H2O(l)
•
Slide 10/12
pale pink
blue
absorbs at 8680,
18400 and 19200 cm-11
absorbs at 4780,
15700 and
d 16230 cm-11
d7: three d-d
d d transitions
¾ Δoct = v2 – v1 = (18400 – 8680) = 9720 cm-1
•
Higher intensity for tetrahedral complex
¾ d-p mixing possible in tetrahedron
¾ d-p
d p mixing only due to vibrations for octahedron
Slide 11/12
Slide 12/12