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Today’s(Topics!Coordination)Chemistry)III" " • Ligand'field'theory' • Magnetic'properties'of'coordination'compounds' • Electronic'absorption'spectra'of'coordination'compounds' ' ' ' ' ' Related'Problems'from'Spring'2008'Final'Exam:'33'&'Spring'2009'Final'Exam'2009:'34' Corresponding'Connect'Problems:'1–4' ' ' 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.. ' 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).' ' ' 2'' ' Strength(of(Bonds(&(the$Spectrochemical-Series" " Overview"of"the"strength"of"bonds"formed"by"ligands"based"on"donor"atom" ' ' 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' ! ! ! (strongHfield.ligands)'CO'>'CN ! '>'en'>'NH3'>'|'H2O'>'C! O!! ! '>'OH '>'F '>'Cl '>' 'Br ! '>'I ! '(weakLfield'ligands)' ' ' ' 3'' ' Ligand'Field'Theory" " 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.' ' 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.' ' ' ' ' 4'' ' 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.' ' 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.' ' (FigureLMcMurray'&'Fay)' ' ' ' ' ' 5'' • 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.' ' ' ' 6'' ' Ligand!Field&Stabilization&Energy&(LFSE)&&&Pairing&Energy" " 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' ' ' ' ' ' ' ' 7'' ' d2' ' ' ' ' ' d4' ' ' ' ' ' ' ' Table'in'Lecture.Notes'gives'LFSE’s'for'all'! ! 'configurations'in'the'weakL'and'strongLfield' cases.' ' ' ' 8'' ' 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.' ' ' 9'' ' Effect&of&the$Ligand'on'the'Magnitude'of'10!! + The'splitting'of'd'orbitals'increases'as'the'strength'of'the'ligand'increases' (Spectrochemical.Series)' ! ! ! (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' ' ' ' ' 10'' ' 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)' ' ' 11'' ' Effect&of&the$Metal&Ion&on&the&Magnitude&of&10Dq+ " 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.' ' ' 12'' ' Absorption*Spectra*&*Color" " • 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)' ' ' ' ' 13'' ' 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)' ' ' 14'' ' Tetrahedral)Ligand)Field" " 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)' ' ' 15'' ' Tetragonal*Distortion*of*an*Octahedral*Ligand*Field" " 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.' ' 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) ' ' 16'' ' Square'Planar'Ligand'Field" " 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)' ' ' 17'' ' Problem:)Application)of)Ligand)Field)Theory" " The'complex'ion'[Mn(H2O)6]2+'is'a'highLspin/weakLfield'complex.'What'is'the'ligand'field' stabilization'energy'(LFSE)'for'this'complex'ion?' ' ' 18''