Post on 08-Oct-2020
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Transition Metal ComplexesBonding
Bonding Theory
•Crystal Field TheoryCoulomb Interactions–attraction between metal ion and ligand electrons–repulsion between metal electrons and ligand electrons
•Ligand Field TheoryMO TheoryOverlap of orbitals between ligands and metal
Crystal Field Theory
n+L M
repulsion
attractione-
e-
Crystal Field Theory
Free M ion andLigands
M andLigands
attraction
repulsion
E
n+L Mrepulsion
attractione-
e-
Splitting of d Orbitals in Oh Field
o: Ligand Field Splitting Parameter (10 Dq)
More repulsion
Less repulsion
Factors Affecting the MagnitudeCrystal Field Splitting
•Identity of the ligand•Charge on the metal•Position in a group•Geometry and coordination number
Absorption Spectrum of [Ti(OH2)6]3+
eg t2g
Variation of o in Octahedral Ti(III)Complexes
•Ti(III) is a d1 complex and exhibits one absorption inits electronic spectrum due to transition of theelectron from the t2g orbitals to the eg orbitals. Theenergy of the absorption corresponds to o.
Spectrochemical Series:Arrangement of ligandsin order of increasingligand field strength
Spectrochemical Series•Order of ligand field strength with decreasing o:
acceptor > no effect > weak donor > donorstrong field weak field
CO > CN–> PPh3 > phen ~ NO2–> bipy > en > NH3 ~ py
> CH3CN > NCS- > H2O > C2O42–> OH–> F–> N3
- >NO3
–> Cl–> SCN–> S2- > Br–> I–
•Order of metal with increasing o:Increases with increasing oxidation numberIncreases down a groupMn(II) < Ni(II) < Co(II) < Fe(II) < V(II) < Fe(III) < Co(III) <Mn(IV) < Mo(III) < Rh(III) < Ru(III) < Pd(IV) < Ir(III) <Pt(IV) weak field strong field
Ligand Field StabilizationEnergy (LFSE)
LFSE = (- 0.4 x + 0.6 y) o
# of electrons
# of electrons
Strong Field and Weak Field
0.8 o 1.2 o
1.6 o 0.6 o
Pairing Energy
1.6 o 0.6 o
Pairing Energy:Coulombic repulsion between electrons in the sameorbital
1.6 o –P (pairing energy) > 0.6 o Strong Field1.6 o –P (pairing energy) < 0.6 o Weak Field
Strong FieldLow spin, 2 unpaired electrons
Weak FieldHigh spin, 4 unpaired electrons
LFSE o and t
0000Zn2+Cu+10
0.410.61Cu2+9
0.821.22Ni2+8
1.230.831.81Co2+7
0.640.442.40Co3+Fe2+6
05052.01Fe3+Mn2+5
0.440.641.62Mn3+Cr2+4
Weak FieldStrong Field
0.831.23V2+Cr3+3
1.220.82V3+2
0.610.41Ti3+1
0000Sc3+Ca2+0
LFSENLFSEN
TetrahedralOctahedralExampledn
Magnetic Measurements
•Paramagnetic:unpaired electrons
•Diamagnetic:all electrons paired
Origins of Magnetism•Self spinning
•Spinning about nucleus
Z+e
Origin of OrbitalAngularMomentum
Spin-only Paramagnetism•Magnetic Moment
= 2{S(S + 1)}1/2 B S = si
= {N(N + 2)}1/2 B
B : Bohr magneton, 9.274 X 10-24 J/T
5.95.925/25Fe3+
4.8 –4.94.9024Mn3+
3.83.873/23Cr3+
2.7 –2.92.8312V3+
1.7 –1.81.731/21Ti3+
Exp.Calc.
/BSNIon
Thermochemical Correlations
Without LFSE
With LFSE
More Stable
Tetrahedral Complexes•Only 4 ligands, all weak field
LFSE o and t
0000Zn2+Cu+10
0.410.61Cu2+9
0.821.22Ni2+8
1.230.831.81Co2+7
0.640.442.40Co3+Fe2+6
05052.01Fe3+Mn2+5
0.440.641.62Mn3+Cr2+4
Weak FieldStrong Field
0.831.23V2+Cr3+3
1.220.82V3+2
0.610.41Ti3+1
0000Sc3+Ca2+0
LFSENLFSEN
TetrahedralOctahedralExampledn
Tetragonal Complexes
t < 1/2 o
Square Planar Complexessp > o
sp ~ 1.3 o
d8 Square Planar stable
Jahn-Teller Effect
•If the ground electronic configuration ofa linear complex is orbitally degenerate,the complex will distort so as to removethe degeneracy and achieve a lowerenergy.
Ligand FieldTheory
a1g
t1u
eg
t2g
MO Energy Level Diagram
CFT
M-L -Bonding
-donor (-base) and -acceptor (-acid)
Effect of -Interaction on o
0 decreasedWeak Field
0 increasedStrong Field
-donor -acceptor
M-L -Bonding•Order of ligand field strength with decreasing o:acceptor > no effect > weak donor > donorstrong field weak field
CO > CN–> PPh3 > phen ~ NO2–> bipy > en >
NH3 ~ py > CH3CN > NCS- > H2O > C2O42–> OH–
> F–> N3- > NO3
–> Cl–> SCN–> S2- > Br–> I–
•Low Oxidation State Metal (electron rich)–Acceptor Ligands
•High Oxidation State Metal (electron poor)–Donor Ligands
PES ofMo(CO)6