Wk11 Coord Chem--BONDING

8/2/2019 Wk11 Coord Chem--BONDING

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complexes must be consistent with observedcomplexes must be consistent with observed..

formation) constants, magnetic susceptibility,formation) constants, magnetic susceptibility,

complexes.complexes.


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hybridization for octahedral complexes. For thehybridization for octahedral complexes. For the,,

be: dbe: d22spsp33 (using the 3d, 4s and 4p orbitals), or(using the 3d, 4s and 4p orbitals), or33 22 , ., .

The valence bond approach isnt usedThe valence bond approach isnt used

and magnetic moments of most complexes.and magnetic moments of most complexes.


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,,

the ligands are viewed as point negative chargesthe ligands are viewed as point negative charges

metal. The nature of the ligand and themetal. The nature of the ligand and the

..


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, ,, ,

corners of an octahedron affect the variouscorners of an octahedron affect the various dd..


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between ligand lonebetween ligand lone

orbitals on the metalorbitals on the metal

the energy of the dthe energy of the d

..


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__ __e

z2 x2-y2

0.6o __ __ __ __ __

Spherical field __ __ __

o

0.4ot

dxy dxz dyz


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,,

orbitals is assigned a value of 10Dq. The upperorbitals is assigned a value of 10Dq. The upper

gg ,, gg

goes down by 4Dq.goes down by 4Dq.

uu

metal and the ligands.metal and the ligands.


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metal complexes arises from the splitting of themetal complexes arises from the splitting of the..

lower tlower t2g2gset to the eset to the eggset, light in the visibleset, light in the visible

..


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The splitting dueThe splitting due

to t e nature o t eto t e nature o t eligand can beligand can be

measured using ameasured using a

spectrophotometer.spectrophotometer.ma er va ues oma er va ues o ooresult in colors in theresult in colors in thereen ran e Lar erreen ran e Lar er

gaps shift the color togaps shift the color toyellow.yellow.


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ion, the following series has been developed:ion, the following series has been developed:

II--


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cobalt (III) show thecobalt (III) show theshift in color due to theshift in color due to theligand.ligand.

(a) CN(a) CN, (b) NO, (b) NO22, (c), (c)phen, (d) en, (e) NHphen, (d) en, (e) NH33, (f), (f)

gly, (g) Hgly, (g) H22O, (h) oxO, (h) ox22, (i), (i)

33 ..


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.. oo

on the metal.on the metal.


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nature of the ligand. As a result, it cannotnature of the ligand. As a result, it cannot..

L gan e eory uses a mo ecu ar or taL gan e eory uses a mo ecu ar or ta

approach. Initially, the ligands can be viewed asapproach. Initially, the ligands can be viewed as

av ng a y r or ta or aav ng a y r or ta or app or ta po nt ng or ta po nt ng toward the metal to maketoward the metal to make bonds.bonds.


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gg

orbitals have the sameorbitals have the same

orbital on the centralorbital on the central

..


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uu

orbitals have the sameorbitals have the same

orbitals on the centralorbitals on the central

..

(T representations(T representations

..


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gg

orbitals have the sameorbitals have the samezz

and dand dxx22--yy22 orbitals onorbitals on

..

(E representations are(E representations are

..


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dont have adont have agg

symmetry, the dsymmetry, the dxyxy, d, dyzyz xyxy

metal will be nonmetal will be non--

consideringconsidering bonding.bonding.


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The molecularThe molecular

orbital diagram isorbital diagram isconsistent with theconsistent with thecrystal fieldcrystal field

approach.approach.

ote t at t eote t at t e

tt2g2gset of orbitals isset of orbitals is-- ,,

the ethe eggset of orbitalsset of orbitalsis antibondin .is antibondin .


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The electronsThe electrons

from the ligandsfrom the ligands12 electrons12 electrons

from 6 ligands infrom 6 ligands in

octahedraloctahedralcomplexes) willcomplexes) will

fill the lowerfill the lower

bonding orbitals.bonding orbitals.{


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The electronsThe electrons

from the 4s andfrom the 4s and3d orbitals of the3d orbitals of the

metal (in the firstmetal (in the first

transition rowtransition row

{

will occupy thewill occupy the

middle ortion ofmiddle ortion of

the diagram.the diagram.


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splitting of the tsplitting of the t2g2gand eand eggmolecular orbitals.molecular orbitals.

complexes arises from electronic transitions ascomplexes arises from electronic transitions as

x .x .

Additional information is gained fromAdditional information is gained from

measur ng t e magnet c moments o t emeasur ng t e magnet c moments o t ecomplexes.complexes.


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Magnetic susceptibilityMagnetic susceptibility

easu e en s can eeasu e en s can e

used to calculate theused to calculate thenu e o un a enu e o un a e

electrons in a compound.electrons in a compound.

ParamagneticParamagnetic

substances are attractedsubstances are attracted

to a magnet c e .to a magnet c e .


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determine the magnetic moment of a substance.determine the magnetic moment of a substance.,,

paramagneticparamagnetic, and attracted to a magnetic field., and attracted to a magnetic field.

u ,u , --

only magnetic moment,only magnetic moment, ss, can be used to, can be used to

..= n n+2= n n+2 1/21/2


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,,

Bohr magnetons, can be related to the numberBohr magnetons, can be related to the number..

ss

= [n(n+2)]= [n(n+2)]1/21/2

W ere n s t e num er o unpa re e ectronsW ere n s t e num er o unpa re e ectrons


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--

orbitals have two possibilities for theorbitals have two possibilities for the..

low spinlow spin, in which the electrons occupy the lower, in which the electrons occupy the lower

gg ,, ..these complexes, the electrons will fill the upperthese complexes, the electrons will fill the upper

gg

..


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thethe ddorbitals is large,orbitals is large,

fill the lower (tfill the lower (t2g2g) set of) set of

occupying the eoccupying the eggset ofset of

..

are calledare called low spinlow spin..


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complexes, the sizecomplexes, the sizeoo

the pairing energy ofthe pairing energy of

..


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thethe ddorbitals is small,orbitals is small,

eeggset of orbitals beforeset of orbitals before

lower (tlower (t2g2g) set of orbitals) set of orbitals

..

are calledare called high spinhigh spin..


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complexes, the sizecomplexes, the sizeoo

pairing energy of thepairing energy of the

..


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all weak field, high spin cases.all weak field, high spin cases.

ddoo dd11 dd22 dd33 dd44 dd55 dd66 dd77 dd88 dd99 dd1010

.. oo .. .. .. .. .. .. ..


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transition metals should increase across the periodtransition metals should increase across the period..

MM2+2+ + 6 H+ 6 H22O(l)O(l)M(M(HH22O)O)662+2+


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hydration show twohydration show two

with the expected LFSEwith the expected LFSE

..values for dvalues for d55 and dand d1010 areare

with a LFSE equal to 0.with a LFSE equal to 0.


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ddoo dd11 dd22 dd33 dd44 dd55 dd66 dd77 dd88 dd99 dd1010

LFSELFSE 00 .4.4oo .8.8 1.21.2 .6.6 00 .4.4 .8.8 1.21.2 .6.6 00


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with very strong ligands. CNwith very strong ligands. CN-- forms low spinforms low spin3+3+,, ..

& meta s& meta s genera y ave a arger va ue ogenera y ave a arger va ue o oothan for 3d metals. As a result, complexes arethan for 3d metals. As a result, complexes are

typ ca y ow sp ntyp ca y ow sp n..


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differ in that LFT considers the nature of thediffer in that LFT considers the nature of the. ,. ,

ligands as electron pairs used for makingligands as electron pairs used for making

..formform bonds with the metal. Group theorybonds with the metal. Group theory

orbital diagrams.orbital diagrams.


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rere ,,

cartesian coordinates is established for each ofcartesian coordinates is established for each of..

arbitrarily set as thearbitrarily set as theyyaxis (or the paxis (or the pyyorbitals).orbitals).

xx zz ..


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reduces to: Treduces to: T1g1g+ T+ T2g2g+ T+ T1u1u + T+ T2u2u..

The TThe T1g1gand Tand T2u2u group orbitals for the ligands dont matchgroup orbitals for the ligands dont matchy y y .y y y .

The TThe T1u1u

set has the same symmetry as the pset has the same symmetry as the pxx

, p, pyy

and pand pzz..

make themake the bonds to the ligands.bonds to the ligands.

The TThe T set has the same s mmetr as the dset has the same s mmetr as the d dd and dand d

orbitals on the metal.orbitals on the metal.


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the dthe dxyxy, d, dyzyz and dand dxzxz orbitals on the metal and theorbitals on the metal and the**,, ..


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**

and serve as aand serve as a acceptoracceptorligand, or full p or dligand, or full p or d..


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accept electron density from a filledaccept electron density from a filled ddorbital onorbital on.. ..

*orbital

e

orbital


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..

pp oror ddorbitals may also serve as pi donor ligands.orbitals may also serve as pi donor ligands.-- -- 22--,, ,, ..

The filled p or d orbitals on these ions interactThe filled p or d orbitals on these ions interact

gg xyxy,, yzyz xzxz the metal to form bonding and antibondingthe metal to form bonding and antibonding

..


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,,

which are lower in energy,which are lower in energy,

electrons from the ligand,electrons from the ligand,

molecular orbitals aremolecular orbitals are

electrons from the metal.electrons from the metal.


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The size ofThe size ofoodecreases, since it is nowdecreases, since it is now

between an antibonding tbetween an antibonding t2g2gorbital and the eorbital and the egg* orbital.* orbital.

This is confirmed bThis is confirmed bthe spectrochemical series.the spectrochemical series.

Weak field li ands are alsoWeak field li ands are also

pi donor ligands.pi donor ligands.


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,,

NN22 and CO have emptyand CO have empty

of the proper symmetryof the proper symmetry

with filledwith filled ddorbitals onorbitals on

..


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tt2g2gset of orbitals (dset of orbitals (dxyxy,,yzyz xzxz

pi bonding with thepi bonding with the

**..on the ligand are usuallyon the ligand are usually

the d orbitals on thethe d orbitals on the..


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tt2g2gset of orbitals (dset of orbitals (dxyxy,,yzyz xzxz

pi bonding with thepi bonding with the

**..on the ligand are usuallyon the ligand are usually

the d orbitals on thethe d orbitals on the..


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causes the energy of thecauses the energy of thegg

drop slightly, thusdrop slightly, thus

oo..


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.. . ,. ,

engage solely inengage solely in bonding are in the middle ofbonding are in the middle of..

donors, such as CHdonors, such as CH33-- and Hand H-- are found high inare found high in

..2.2. LigandsLigands with filledwith filledpp oror ddorbitalsorbitals can also servecan also serve

..oo..


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**.. ,,

serve asserve as acceptors. This results in a largeracceptors. This results in a largeroo..

II--< r< r--


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quite common for certain transition metals. Thequite common for certain transition metals. The

will differ depending on the geometry of thewill differ depending on the geometry of the

..


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zz xx --yy

point directly between thepoint directly between the

arrangement. As a result, thesearrangement. As a result, these

,,the point groupthe point group TTdd, are lower in, are lower in

..

T t h d l C lT t h d l C l


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Tetrahedral Com lexesTetrahedral Com lexes

TheThe tt22set of orbitals,set of orbitals,

cons st ng o t econs st ng o t e xyxy,, yzyz, an, an

ddxzxz orbitals, are directed moreorbitals, are directed moren t e rect on o t e gan s.n t e rect on o t e gan s.

These orbitals will beThese orbitals will behigher in energy in ahigher in energy in a

tetrahedral field due totetrahedral field due to

repulsion with the electronsrepulsion with the electronson the ligands.on the ligands.

T tr h dr l C m lT tr h dr l C m l


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The size of the splitting,The size of the splitting,

TT, s cons era e sma er, s cons era e sma er

than with comparablethan with comparableocta e ra comp exes. s socta e ra comp exes. s s

because only 4 bonds arebecause only 4 bonds are

orme , an t e meta or ta sorme , an t e meta or ta s

used in bonding dont pointused in bonding dont point

r g t at t e gan s as t ey or g t at t e gan s as t ey oin octahedral complexes.in octahedral complexes.



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In general, In general, TT 4/9 4/9

oo. nce t e sp tt ng . nce t e sp tt ng

is smaller, all tetrahedralis smaller, all tetrahedralcomp exes are weacomp exes are wea --

field, highfield, high--spin cases.spin cases.


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,,

CuCu2+2+

, distort from octahedral geometry. One, distort from octahedral geometry. One,,

which the bonds along one axis elongate, withwhich the bonds along one axis elongate, with

other two axes.other two axes.


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The elongationThe elongation

along thealong the zzaxis causesaxis causes

thethe ddorbitals withorbitals with

density along the axis todensity along the axis to

drop in energy. As adrop in energy. As aresult, the dresult, the dxzxz and dand dyzyzorbitals lower in energy.orbitals lower in energy.


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The compressionThe compression

along thealong the xxandandyyaxisaxis

causes orbitals withcauses orbitals with

density along these axesdensity along these axes

to increase in energy.to increase in energy...


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For com lexes withFor com lexes with

11--3 electrons in the e3 electrons in the eggsetsetof orbitals this t e ofof orbitals this t e of

tetragonal distortion maytetragonal distortion may

lower the ener of thelower the ener of thecomplex.complex.


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For complexes with 2For complexes with 2

electrons in the eelectrons in the egg

set ofset of

orbitals, a dorbitals, a d88 configuration,configuration,

a severe distortion maya severe distortion may

occur, resulting in a 4occur, resulting in a 4--coordinate square planarcoordinate square planar

shape, with the ligandsshape, with the ligands

along thealong the zzaxis no longeraxis no longerbonded to the metal.bonded to the metal.


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Square planarSquare planar

complexes are quitecomplexes are quite

common for the dcommon for the d88 metalsmetalsn t en t e anan per o s:per o s:Rh(I), IR(I), Pt(II), Pd(II)Rh(I), IR(I), Pt(II), Pd(II)

..transition metals have largetransition metals have largeli and field stabalizationli and field stabalization

energies, favoring fourenergies, favoring four--coordinate complexes.coordinate complexes.


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Square planarSquare planar

complexes are rare for thecomplexes are rare for the

33rdrd period metals. Ni(II)period metals. Ni(II)

generally forms tetrahedralgenerally forms tetrahedral

complexes. Only with verycomplexes. Only with verystrong ligands such as CNstrong ligands such as CN--,,

is square planar geometryis square planar geometry

seen with Ni(II).seen with Ni(II).


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The value ofThe value ofss for afor a

given metal, ligands andgiven metal, ligands and

bond length isbond length is

approximately 1.3(approximately 1.3(oo).).


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--

If the ground electronic configuration of a nonIf the ground electronic configuration of a non--linearlineara a ,a a ,

so as to remove the degeneracy and achieve a lower energy.so as to remove the degeneracy and achieve a lower energy.


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----

structures will distort. It does not predict thestructures will distort. It does not predict the..

most often seen when the orbital degneracy is inmost often seen when the orbital degneracy is in

ligands.ligands.


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--,,

pronounced in high spin dpronounced in high spin d44

, low spin d, low spin d77

and dand d99

,,

eeggset of orbitals.set of orbitals.

d4 d7 d9

e

t2g


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----

tabulated below: (w=weak, s=strong)tabulated below: (w=weak, s=strong)# e# e-- 11 22 33 44 55 66 77 88 99 1010

HighHigh ** ** ** ** ** **spinspin --

LowLow-- -- -- --

spinspin*There is only 1 possible ground state configuration.

- No Jahn-Teller distortion is expected.

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Wk11 Coord Chem--BONDING