Download (Lecture(25) - MSU Chemistry

Today’s(Topics!Coordination)Chemistry)III"
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• Ligand'field'theory'
• Magnetic'properties'of'coordination'compounds'
• Electronic'absorption'spectra'of'coordination'compounds'
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Related'Problems'from'Spring'2008'Final'Exam:'33'&'Spring'2009'Final'Exam'2009:'34'
Corresponding'Connect'Problems:'1–4'
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Figures'taken'from'Chemistry,'4th'ed.'by'J.'McMurray'and'R.'C.'Fay,'PrenticeLHall,'2004,'and'Concepts.and.
Models.of.Inorganic.Chemistry,'4th'ed.'by'B.'Douglas,'D.'McDaniel,'and'J.'Alexander,'Wiley,'1994.'
All.materials.are.©.2012–2013.Lynmarie.A..Posey.and.Michigan.State.University.unless.otherwise.noted;.
all.rights.reserved..
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Learning(Objectives((Lecture(25)"
You'should:'
1. understand'how'the'd.orbitals'of'transition'metals'are'split'in'the'presence'of'a'
ligand'field'(octahedral,'tetrahedral,'and'square'planar).'
2. know'how'the'strength'of'the'metalLligand'bond'influences'the'size'of'the'splitting'
(10Dq)'between'd'orbitals'and'in'turn,'how'the'size'of'the'splitting'influences'the'
magnetic'(tendency'of'electrons'to'pair'or'not'in'the'transition'metal’s'd'orbitals)'and'
spectroscopic'(wavelength'of'light'absorbed)'properties'of'coordination'compounds.'
3. know'that'ligands'with'C'or'N'donor'atoms'are'strong/highLfield'ligands,'while'
ligands'with'O'or'X'(halide)'donor'atoms'are'weak/lowLfield'ligands.'
4. understand'that'the'observed'colors'of'coordination'complexes'arise'from'
absorption'of'visible'light,'which'causes'transitions'of'electrons'between'the'ligandL
fieldLsplit'd.orbitals.'
5. understand'the'relationship'between'the'wavelength/color'of'light'absorbed'and'the'
apparent'color'of'an'object.'
6. be'able'to'calculate'ligandLfield'stabilization'energies'(LFSEs).'
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Strength(of(Bonds(&(the$Spectrochemical-Series"
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Overview"of"the"strength"of"bonds"formed"by"ligands"based"on"donor"atom"
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strongest !!!!!! > ! > ! > X ! halide!ion !!!!!(weakest)'
Spectrochemical"Series"
• based'on'measurements'of'the'absorption'spectra'of'coordination'compounds.'
Stronger'ligands'cause'a'larger'splitting'of'the'dLorbitals.'
• the'strength'of'a'ligand'determined'from'spectrochemical'series'correlates'with'the'
strength'of'bonds'formed'by'the'ligand'with'transition'metals'
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(strongHfield.ligands)'CO'>'CN ! '>'en'>'NH3'>'|'H2O'>'C! O!!
! '>'OH '>'F '>'Cl '>'
'Br ! '>'I ! '(weakLfield'ligands)'
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Ligand'Field'Theory"
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Bethe'and'van'Vleck'developed'crystal'field'theory'in'the'1930’s'around'the'same'time'as'
the'development'of'valence'bond'theory'by'Pauling.'
• Crystal.field.theory'is'based'upon'a'completely'ionic'picture'of'bonding.'Covalent'
character'in'bonding'is'totally'neglected.'
When'valence'bond'theory'was'subsequently'applied'to'understand'bonding'in'the'
coordination'compounds'formed'by'transition'metals'and'ligands,'the'resulting'theory'
was'called'ligand.field.theory.'
• Valence'bond'theory/ligand'field'theory'is'based'on'a'covalent'model'for'bonding.'
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CrystalLfield'and'ligandLfield'theories'are'useful'for'making'predictions'about'the'
• spectroscopic'(wavelengths'of'light'absorbed/color'of'compounds)'
• magnetic'(diamagnetic'or'paramagnetic)''
properties'of'coordination'compounds.'
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What'happens'to'the'atomic'orbitals'of'a'transitionLmetal'ion'when'the'ion'is'surrounded'
by'an'octahedral'arrangement'of'ligands'(negative'charges)?'
All'orbitals'(s,'p,'d)'are'raised'in'energy'due'to'repulsive'interactions'between'the'
transition'metal’s'electrons'and'the'ligand’s'lone'pairs,'which'are'donated'to'form'the'
metalLligand'bond.'
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The'!!2−!2 .and'!!2 .
orbitals'lie'along'the'x,'y,'
and'z'axes'where'the'
negative'charges'from'
the'ligand'lone'pairs'are'
found,'while'the'dxy,'dxz,'
and'dyz'orbitals'are'
oriented'between'the'x,'
y,'and'z'axes.'
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(FigureLMcMurray'&'Fay)'
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• The'five'd.orbitals'lose'their'degeneracy.'The'energies.are.no.longer.equal.for.all.five.d.
orbitals;'instead'they'are'split'into'two'groups'designated'eg'(!!2−!2 ,'!!2 )'and't2g.(dxy,'
dyz,'dxz).'(FigureLDouglas,'McDaniel,'&'Alexander)'
• The'three'p'orbitals'(!! , !! , !! )'remain'degenerate,'which'means'that'they'still'have'
the'same'energy.'
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Ligand!Field&Stabilization&Energy&(LFSE)&&&Pairing&Energy"
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Ligand'field+stabilization+energy"
• the'change'in'energy'(usually'a'decrease)'that'occurs'when'a'transition'metal’s'd.
orbitals'are'split'by'the'ligand'field.'This'change'is'relative'to'the'energy'that'the'
transition'metal’s'electrons'would'have'in'a'spherically'symmetric'field'(see'previous'
slide).'
Pairing+energy
• energy'required'to'put'two'electrons'in'the'same'orbital.'
Examples
Find'the'ligandLfield'stabilization'energies'for'the'following'configurations:'d1,'d2'and'd4.'
d1'
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d2'
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d4'
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Table'in'Lecture.Notes'gives'LFSE’s'for'all'! ! 'configurations'in'the'weakL'and'strongLfield'
cases.'
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Summary:"Electron"Configurations"in"an"Octahedral"Ligand"Field"
• d1,'d2,'d3L'only'one'possible'configuration.'Electrons'go'into'the't2g.orbitals.'
• d4,'d5,'d6,'d7Ltwo'possible'configurations.'Configuration'adopted'by'the'transition'
metal'depends'on'the'strength'of'the'ligand'field,'which'depends'on'10Dq.'10Dq'
depends'on'the'ligand'and'the'metal'and'is'a'function'of'the'strength'of'the'bonding'
interaction'between'them.'
• In'a'strong'field,'the'electrons'will'be'spinHpaired,'and'the'compound'will'be'lowH
spin.'
• In'a'weak'field,'the'electrons'will'be'spinHfree'and'the'compound'will'be'highHspin.'
• d8,'d9,'d10L'only'one'possible'configuration.'
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Effect&of&the$Ligand'on'the'Magnitude'of'10!! +
The'splitting'of'd'orbitals'increases'as'the'strength'of'the'ligand'increases'
(Spectrochemical.Series)'
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(strongHfield.ligands)'CO'>'CN ! '>'en'>'NH3'>'|'H2O'>'C! O!!
! '>'OH '>'F '>'Cl '>'
'Br ! '>'I ! '(weakLfield'ligands)'
H2O'is'a'weakLfield'ligand,'while'NH3'is'a'strongLfield'ligand.'(FigureLMcMurray'&'Fay)'
Note:'10Dq'='Δ'
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High"vs."Low"Spin"
In'general,'weakLfield'ligands'form'highLspin'compounds,'and'strongLfield'ligands'form'
lowLspin'compounds'whenever'there'are'two'possible'configurations'for'the'd'electrons.'
In'a'strong'ligand'field,'the'pairing'energy'P'is'less'than'10Dq.'(FigureLMcMurray'&'Fay)'
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Effect&of&the$Metal&Ion&on&the&Magnitude&of&10Dq+
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For'first'series'(row)'transition'metals'
• ! !! ,'10Dq'~'7500–12500'cm–1''
• ! !! ,'10Dq'~'14000–25000'cm–1'
• Example:'isoelectronic'complexes'with'same'ligand'[Co(H2O)6]3+'(10Dq'=18600'cm–1)'
and'[Fe(H2O)6]2+'(10Dq'='10400'cm–1)'
Within'a'given'column,'10Dq'increases'for'the'same'metal'oxidation'state'and'ligands'
moving'down'the'column.'
[Co(NH3)6]3+,'10Dq'='23000'cm–1','3d'
[Rh(NH3)6]3+,'10Dq'='34000'cm–1','4d'
[Ir(NH3)6]3+,'10Dq'='41000'cm–1','5d'
The'Rh'and'Ir'complexes'are'more'likely'to'be'lowLspin/spinLpaired,'even'with'weak'
ligands,'because'of'the'larger'contribution'to'10Dq'from'the'metal.'
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Absorption*Spectra*&*Color"
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• Substances'absorb'light'when'the'energy'of'the'photon'E'='hν'='h(c/λ)'matches'the'
spacing'between'energy'levels'(ΔE).'Coordination'compounds'absorb'visible'and'
ultraviolet'light'when'the'photon'energy'matches'the'spacing'between'd'orbitals'split'
by'the'ligand'field.'
• The'perceived'color'of'an'object'comes'from'the'visible'light'that'is'reflected'and'not'
absorbed.'(FigureLMcMurray'&'Fay)'
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The'color'wheel'is'a'helpful'tool'for'predicting'where'in'the'visible'spectrum'a'colored'
compound'absorbs'and'what'color'a'compound'will'be'if'its'absorption'spectrum'is'
known.'(FigureLMcMurray'&'Fay)'
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Tetrahedral)Ligand)Field"
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In'a'tetrahedral'ligand'field,'the'coordination'number'for'the'transition'metal'is'4.'
Since'there'are'fewer'ligands'and'they'do'not'line'up'as'well'with'the'transition'metal'd'
orbitals,'the'magnitude'of'the'splitting'(10Dq)'is'roughly'4/9'of'the'splitting'in'an'
octahedral'ligand'field.'
In'contrast'with'the'octahedral'ligand'field,'the'energy'of'the'dxy,'dyz,'and'dxz'orbitals'(t2)'
increases'while'the'energy'of'the'!!2−!2 'and'!!2 .orbitals'(e)'decrease'in'a'tetrahedral'
ligand'field.'(FigureLDouglas,'McDaniel,'&'Alexander)'
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Tetragonal*Distortion*of*an*Octahedral*Ligand*Field"
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In'CuIIL6'complexes,'the'metalLligand'bond'lengths'are'typically'longer'for'the'axial'
ligands'(on'z'axis)'than'for'the'equatorial'ligands'(on'the'x'and'y'axes).'This'change'in'
geometry'from'the'octahedral'geometry'typically'observed'for'transition'metals'with'a'
coordination'number'of'6'is'called'tetragonal.distortion.'
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In'a'tetragonally'distorted'
complex,'the'eg'and't2g.
orbitals'split'with'the'
energies'of'!!2 ,'!!" ,'and'!!" '
decreasing'and'the'energies'
of'the'!!2−!2 .and'!!" 'orbitals'
increasing.'
(Figure-Douglas, McDaniel, &
Alexander)
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Square'Planar'Ligand'Field"
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Square'planar'complexes,'such'as'
[Ni(CN)4]2–,'represent'the'limit'of'infinite'
tetragonal'distortion'in'which'the'ligands'
on'the'zLaxis'are'moved'to'infinite'distance'
from'the'metal.'The'splitting'that'occurs'
when'a'complex'is'tetragonally'distorted'
continues'until'the'energy'of'the'!!2 .orbital'
drops'below'that'of'the'dxy'orbital.'
(FigureLDouglas,'McDaniel,'&'Alexander)'
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Problem:)Application)of)Ligand)Field)Theory"
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The'complex'ion'[Mn(H2O)6]2+'is'a'highLspin/weakLfield'complex.'What'is'the'ligand'field'
stabilization'energy'(LFSE)'for'this'complex'ion?'
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