Electronic spectra of transition metal complexes

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Electronic spectra of transition metal complexes
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Characteristics of electronic spectra

• Wavelength Energy of electronic transition
• Shape. Gaussian Band Shape - coupling of
  electronic and vibrational states
• 
  
• Intensity. Molar absorptivity, ? (M?1cm?1) due
  to probability of electronic transitions.
• d) Number of bands Transitions between States of
  given dn configuration.

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Band intensity in electronic spectra (e)
Electronic transitions are controlled by quantum
mechanical selection rules which determine the
probability (intensity) of the transition. Transi
tion emax (M?1cm?1) Spin and
Symmetry forbidden "d-d" bands 0.02 - 1
Spin allowed and Symmetry forbidden "d-d"
bands (Oh) 1 - 10 (Td)
10 103 Spin and Symmetry allowed LMCT and
MLCT bands 103 - 5 x 104
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Spin Selection Rule There must be no change in
the spin multiplicity (2S 1) during the
transition. i.e. the spin of the electron must
not change during the transition. Symmetry
(Laporte) Selection Rule There must be a change
in parity (g ? u) during the transition Since s
and d orbitals are g (gerade) and p orbitals are
u(ungerade), only s ? p and p ? d transitions
are allowed and d ? d transition are formally
forbidden. i.e. only transitions for which ?l
1 are allowed. d ? d bands are allowed to
the extent that the starting or terminal level of
the transition is not a pure d-orbital. (i.e. it
is a molecular orbital of the complex with both
metal and ligand character).
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States for dn configurations

• Russel-Saunders Coupling
• Angular momentum of individual electrons
  couple to give total angular
• momentum for dn configuration ML ?ml
• Spin momentum of individual electron spins
  couple together to give total
• spin, S ?s
• Inter-electronic repulsions between the
  electrons in the d orbitals give rise to
• ground state and excited states for dn
  configurations.
• States are labeled with Tern Symbols
• Electonic transitions between ground and
  excited states are summarized in
• Orgel and Tanabe-Sugano diagrams .
• Term Symbols (labels for states) contain
  information about L and S for state Hunds
  Rules. i) Ground state has maximum spin, S
  ii) For states of same
  spin, ground state has maximum L.

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Number of d-d bands in electronic
spectrum Excitation from ground state to excited
stated of dn configuration
Triple degeneracy of a d2 ions 3T2g ground
state due to three possible sites for hole in t2g
level
Singly degenerate 3T2g ground state. Only one
possible arrangement for three electrons in t2g
level
Triple degenerate ground state for d7 Three
possible sites for hole in t2g level
Singly degenerate 3T2g ground state. Only one
possible arrangement for six t2g electrons.
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• Labeling of d-d bands in electronic spectrum.
• Consider states of dn configuration
• Determine free ion ground state Term Symbol
  (labels for states)
• Assign splitting of states in ligand field
• Spectroscopic labeling of bands.
• Orgel diagrams (high-spin)
• Tanabe-Sugano diagrams (high-spin and low-spin)

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Individual electron l 2, ml 2, 1, 0,
-1, -2 Maximum ml l l 0, 1,
2, 3, Orbital s, p, d , f
_______________________________ dn
configuration, L 0, 1, 2, 3, 4 Term
Symbol S, P, D, F, G ML
S ml, maximum ML L Spin Multiplicity
2 S 1
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Free ion ground state Term Symbols
for dn configurationsTerm Symbols (labels for
states) contain information about L and S for
ground stateHunds Rules. i) Ground state has
maximum spin, S ii) For
states of same spin, ground state has maximum L
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Splitting of the weak field dn ground state terms
in an octahedral ligand field
Ground state determined by inspection of
degeneracy of terms for given dn
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Orgel Diagrams
Mn3
Ti3
Cr3
V2
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(a) Ni(H20)62 (b) Ni(NH3)62
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The Tanabe-Sugano diagram for the d2 ion
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Evidence for covalent bonding in metal-ligand
interactionsThe Nephelauxetic Effect (cloud
expansion)

• Reduction in electron-electron repulsion upon
  complex formation
• Racah Parameter, B electron-elctronic repulsion
  parameter
• Bo is the inter- electronic repulsion in the
  gaseous Mn ion.
• B is the inter- electronic repulsion in the
  complexed MLxn ion.
• The smaller values for B in the complex compared
  to free gaseous ion is taken as evidence of
  smaller inter-electronic repulsion in the complex
  due to a larger molecular orbital on account of
  overlap
• of ligand and metal orbital, i.e. evidence
  of covalency (cloud expansion).
• Nephelauxetic Ratio, ß B
• Bo

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Nephelauxetic Effect

• Nephelauxetic Ligand Series
• I lt Br lt CN lt Cl lt NCS lt C2O42- lt en lt
  NH3 lt H2O lt F
• Small ß Large ß
• Covalent Ionic
• Nephelauxetic Metal Series
• Pt4 lt Co3 lt Rh3Ir3 lt Fe3 lt Cr3 lt Ni2
  lt V4lt Pt2 Mn2
• Small ß
  Large ß
• Large overlap
  Small overlap
• Covalent
  Ionic

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Empirical Racah parameters, h, kß 1
h(ligand) x k(metal)

• Cr(NH3)63 ß 1 hk
• ß 1 (1.4)(0.21)
• 0.706
• Cr(CN)63- ß 1 hk
• ß 1 (2.0)(0.21)
• 0.580
• Bo - B hligands x kmetal ion
• Bo

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Typical ?o and ?max values for octahedral (ML6)
d-block metal complexes __________________________
________________________________________ Complex
?o cm-1 ?max (nm)
Complex ?o cm-1 ?max (nm) _________________
__________________________________________________
________________ Ti(H2O)63 20,300
493 Fe(H2O)62 9,400 1064 V(H2O)63
20,300 493 Fe(H2O)63 13,700
730 V(H2O)62 12,400 806 Fe(CN)63-
35,000 286 CrF63- 15,000
667 Fe(CN)64- 33,800 296 Co(H2O)63,
l.s. 20,700 483 Fe(C2O4)33- 14,100
709 Cr(H2O)62 14,100
709 Co(CN)63- l.s. 34,800
287 Cr(H2O)63 17,400
575 Co(NH3)63 l.s. 22,900
437 Cr(NH3)63 21,600
463 Ni(H2O)62 8,500
1176 Cr(en)33 21,900
457 Ni(NH3)62 10,800 926 Cr(CN)63-
26,600 376 Ni(en)32 11,500
870 ______________________________________________
_____________________________________
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• 1. Assign the metal oxidation state in the
  following compounds.
• a. K2PtCl6
• b. Na2Fe(CO)4
• c. Mn(CH3)(CO)5
• 2. Account for the following
• The manganous ion, Mn(H2O)62, reacts with CN-
  to form Mn(CN)64- which has
• m 1.95 B.M., but with I- to give MnI42- which
  has m 5.93 B. M.
• Co(NH3)6Cl3 is diamagnetic, whereas Na3CoF6
  is paramagnetic (? 5.02 B.M).
• PtBr2Cl22? is diamagnetic and exists in two
  isomeric forms, whereas NiBr2Cl22?
• has a magnetic moment, ? 3.95 B.M., and does
  not exhibit isomerism.
• Copper(II) complexes are typically blue with one
  visible absorption band in their
• electronic spectra whereas copper(I) complexes
  are generally colorless.
• Assign a spectroscopic label to the Cu2
  transition.