View
218
Download
1
Embed Size (px)
Citation preview
d1 d2
d3 d4
Placing electrons in d orbitals (strong vs weak field)
Strong field Weak field Strong field Weak field
Strong field Weak field Strong field Weak field
So, what is going on here!!
d4
Strong field =Low spin
(2 unpaired)
Weak field =High spin
(4 unpaired)
< o > o
When the 4th electron is assigned it will either go into the higher energy eg orbital at an energy cost of 0 or be paired at an energy cost of , the pairing energy.
0,
Pairing Energy!!.
Strong field Weak field
Pairing Energy,
The pairing energy, , is made up of two parts.
1) c: Coulombic repulsion energy caused by having two electrons in same orbital. Destabilizing energy contribution of c for each doubly occupied orbital.
2) e: Exchange stabilizing energy for each pair of electrons having the same spin and same energy. Stabilizing contribution of e for each pair having same spin and same energy
= sum of all c and e interactions
How do we get these interactions?
Placing electrons in d orbitals
1 u.e. 5 u.e.
d5
0 u.e. 4 u.e.
d6
1 u.e. 3 u.e.
d7
2 u.e. 2 u.e.
d8
1 u.e. 1 u.e.
d9
0 u.e. 0 u.e.
d10
High Low High Low High Low
Detail working out….
1 u.e. 5 u.e.
d5
High Field Low Field (Low Spin) (High Spin)
What are the energy terms for both high spin and low spin?
High Field
Coulombic Part = 2c
Exchange part = for 3e For 1e
= 2c + 4e
Low Field
Coulombic Part = 0
Exchange part = for 3e + e
= 4e
High Field – Low Field = -20 +2e
LFSE = 5 * (-2/50) = -20
LFSE = 3*(-2/50) + 2 (3/50) = 0
When 0 is larger than e the high field arrangement (low spin) is favored.
Interpretation of Enthalpy of Hydration of hexahydrate using LFSE
d0 d1 d2 d3 d4 d5 d6 d7 d8 d9 d10
LFSE (in 0) .0 .4 .8 1.2 .6 .0 .4 .8 1.2 .6 .0
Extreme elongation: from octahedral to square planar
LM
L L
L
L
L
LM
L L
L
Less repulsions along the axeswhere ligands are missing
Magnetic properties of metal complexes
Diamagnetic complexesvery small repulsive
interaction with external magnetic field
no unpaired electrons
Paramagnetic complexesattractive interaction with
external magnetic fieldsome unpaired electrons
)2( nns
High spin Low spin M dn # u.e. (expt)
BM (calc) BM
# u.e. (expt) BM
(calc) BM
Ti3+ 1 1 1.73 1.73 V4+ 1 1 1.68-1.78 1.73 V3+ 2 2 2.75-2.85 2.83 V2+ 3 3 3.80-3.90 3.88 Cr3+ 3 3 3.70-3.90 3.88 Mn4+ 3 3 3.8-4.0 3.88 Cr2+ 4 4 4.75-4.90 4.90 2 3.20-3.30 2.83 Mn3+ 4 4 4.90-5.00 4.90 2 3.18 2.83 Mn2+ 5 5 5.65-6.10 5.92 1 1.80-2.10 1.73 Fe3+ 5 5 5.70-6.0 5.92 1 2.0-2.5 1.73 Fe2+ 6 4 5.10-5.70 4.90 0 Co3+ 6 4 5.4 4.90/5.48* 0 Co2+ 7 3 4.30-5.20 3.88/5.20* 1 1.8 1.73 Ni3+ 7 3 3.88 1 1.8-2.0 1.73 Ni2+ 8 2 2.80-3.50 2.83 Cu2+ 9 1 1.70-2.20 1.73
*total magnetic moment (S+L)
Values of magnetic moment
M
z
Metal-ligand interactions in an octahedral environment
Six ligand orbitals of symmetry approaching the metal ion along the x,y,z axes
We can build 6 group orbitals of symmetry as beforeand work out the reducible representation
6 ligands x 2e each
12 bonding e“ligand character”
“d0-d10 electrons”
non bonding
anti bonding
“metal character”
Introducing π-bonding
2 orbitals of π-symmetryon each ligand
We can build 12 group orbitalsof π-symmetry
π = T1g + T2g + T1u + T2u
The T2g will interact with the metal d t2g orbitals. The ligand pi orbitals do not interact with the metal eg orbitals.
We now look at things more closely.
Some schematic diagrams showing how p bonding occurs with a ligand having a d orbital (such as in P), or a * orbital, or a vacant p orbital.
6 ligands x 2e each
12 bonding e“ligand character”
“d0-d10 electrons”
non bonding
anti bonding
“metal character”
ML6 -only bonding
The bonding orbitals, essentially the ligand lone pairs, will not be worked with further.
t2g
eg
t2g
ML6
-onlyML6
+ π
Stabilization
(empty π-orbitals on ligands)
o
’oo has increased
π-bonding may be introducedas a perturbation of the t2g/eg set:
Case 1 (CN-, CO, C2H4)empty π-orbitals on the ligands
ML π-bonding (π-back bonding)
t2g (π)
t2g (π*)
eg
These are the SALC formed from the p
orbitals of the ligands that can interac with the d on the metal.
t2g
eg
t2g
ML6
-onlyML6
+ π
π-bonding may be introducedas a perturbation of the t2g/eg set.
Case 2 (Cl-, F-) filled π-orbitals on the ligands
LM π-bonding
(filled π-orbitals)
Stabilization
Destabilization
t2g (π)
t2g (π*)
eg’o
o
o has decreased
Spectrochemical Series
Purely ligands:
en > NH3 (order of proton basicity)
donating which decreases splitting and causes high spin:: H2O > F > RCO2 > OH > Cl > Br > I (also proton basicity)
accepting ligands increase splitting and may be low spin
: CO, CN-, > phenanthroline > NO2- > NCS-
Merging to get spectrochemical series
CO, CN- > phen > en > NH3 > NCS- > H2O > F- > RCO2- > OH- > Cl- > Br- > I-
Strong field, acceptors large low spin
onlyWeak field, donors small high spin
Turning to Square Planar Complexes
y
x
zMost convenient to use a local coordinate system on each ligand with
y pointing in towards the metal. py to be used for bonding.
z being perpendicular to the molecular plane. pz to be used for bonding perpendicular to the plane, .
x lying in the molecular plane. px to be used for bonding in the molecular plane, |.
ML4 square planar complexesligand group orbitals and matching metal orbitals
bonding
bonding (in)
bonding (perp)
Angular Overlap Method
An attempt to systematize the interactions for all geometries.
M
1
65
4 2
3
M
109
78
M 2
6
1
12
11
The various complexes may be fashioned out of the ligands above
Linear: 1,6
Trigonal: 2,11,12
T-shape: 1,3,5
Tetrahedral: 7,8,9,10
Square planar: 2,3,4,5
Trigonal bipyramid: 1,2,6,11,12
Square pyramid: 1,2,3,4,5
Octahedral: 1,2,3,4,5,6
Cont’d
All interactions with the ligands are stabilizing to the ligands and destabilizing to the d orbitals. The interaction of a ligand with a d orbital depends on their orientation with respect to each other, estimated by their overlap which can be calculated.
The total destabilization of a d orbital comes from all the interactions with the set of ligands.
For any particular complex geometry we can obtain the overlaps of a particular d orbital with all the various ligands and thus the destabilization.
ligand dz2 dx2-y2dxy dxz dyz
1 1 e 0 0 0 0
2 ¼ ¾ 0 0 0
3 ¼ ¾ 0 0 0
4 ¼ ¾ 0 0 0
5 ¼ ¾ 0 0 0
6 1 0 0 0 0
7 0 0 1/3 1/3 1/3
8 0 0 1/3 1/3 1/3
9 0 0 1/3 1/3 1/3
10 0 0 1/3 1/3 1/3
11 ¼ 3/16 9/16 0 0
12 1/4 3/16 9/16 0 0
Thus, for example a dx2-y2 orbital is destabilized by (3/4 +6/16) e
= 18/16 e in a trigonal bipyramid complex due to interaction. The dxy, equivalent by symmetry, is destabilized by the same
amount. The dz2 is destabililzed by 11/4 e.