Transcript Slide 1

Lecture 35
Isolobal analogy
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The isolobal analogy allows to relate and compare organic, inorganic and organometallic
compounds on one uniform basis.
What in common have
CH3, NH2, OH, F, Cl, Co(CO)4 and CpMo(CO)3 ?
These “fragments” or “building blocks” can replace each other in more complex structures.
For example, all the groups above can combine to form an ordinary bond:
CH3-CH3, NH2-NH2, HO-OH, F-F, Cl-Cl, (CO)4Co-Co(CO)4, CpMo(CO)3-Mo(CO)3Cp
CH3-NH2, CH3-OH, CH3-F, CH3-Cl, CH3-Co(CO)4, CH3-Mo(CO)3Cp
NH2-OH, NH2-F, NH2-Cl, NH2-Co(CO)4, NH2-Mo(CO)3Cp etc.
“Fragments” or “building blocks” that can replace each other in complex structures are
isolobal.
The definition: Two “fragments” are called isolobal if their frontier orbitals: i) are same in
number, ii) have similar symmetry, iii) are of approximately the same energy, and iv) have the
same number of electrons on them.
All examples above represent the case of one singly occupied (SO) “working” orbital:
HOMO
CH3
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Co(CO)4
a1
The definition above is not always easy to use. Simpler ways to identify isolobal groups exist.
2) Groups isolobal with CH3 (one SO working frontier orbital)
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Electron-equivalent groups miss the same number of electrons necessary for the central
atom to reach the stable electronic configuration (8, 16, or 18 e’s, etc.). Electron-equivalent
groups are always isolobal.
Isolobal
groups
# of e’s in the
valence shell
Stable electron
configuration
# of missing e’s
(“holes”)
CH3
4+3=7
8
1
NH2
5+2=7
8
1
OH
6+1=7
8
1
F
7
8
1
Co(CO)4
9+4(2)=17
18
1
CpMoI(CO)3
5+6+3(2)=17
18
1
HOMO
a1
Transition metal derived fragments MLn isolobal with CH3
d1-ML8
d3-ML7
d5-ML6
d7-ML5
d9-ML4
(C8H8)2La
Cp2V(CNR)
CpMn(CO)3+
[Co(NH3)5]2+
Co(CO)4
3) Groups isolobal with triplet CH2 (two working SO frontier orbitals)
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Groups that are electron-equivalent with the triplet methylene of (a1)1(b2)1 configuration
have two electrons missing in the valence shell of the central atom:
Isolobal
groups
# of e’s in the
valence shell
Stable electron
configuration
# of missing e’s
(“holes”)
CH2
4+2=6
8
2
NH
5+1=6
8
2
O
6
8
2
Ni(CO)3
10+3(2)=16
18
2
Os(CO)4
8+4(2)=16
18
2
CpRh(CO)
6+8+2=16
18
2
b2
a1
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Combination of two fragments isolobal with CH2 gives “dimers” with double bond between the
fragments like in CH2=CH2: CH2=NH, CH2=O, CH2=Os(CO)4, CpRh(CO)(=CH2) etc.
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“Trimers”:
(OC)CpRh
NH
Fe(CO)4
(Fe ethylene complex)
RhCp(CO)
(OC)CpRh
4) Groups isolobal with CH (three working SO frontier orbitals)
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CH of (s)1(p)2 configuration is isolobal with N, O+, Co(CO)3, Re(CO)4, NiCp, CpW(CO)2 (and
W(OMe)3!) etc.
Group
# of e’s in the
valence shell
Stable electron
configuration
# of missing e’s
CH
4+1=5
8
3
N
5
8
3
O+
6-1=5
8
3
Co(CO)3
9+3(2)=15
18
3
Re(CO)4
7+4(2)=15
18
3
CpWI(CO)2
5+6+ 2(2)=15
18
3
p
s
Some combinations of the fragments above: CH≡N, CH≡O+, N≡O+, Cp(CO)2W≡WCp(CO)2
(CO)3
Ir
P
Ir(CO)3
(OC)3Ir
P
(OC)4Co
P
Ir(CO)3
P
Co(CO)4
- 2CO
(OC)3Co
Co(CO)3
5) Non-electron-equivalent isolobal groups
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CH3+ is isolobal with singlet CH2, Cr(CO)5, CpMn(CO)2, PtCl3-, Au(PPh3)+, (all have
one empty “working” orbital)
All these species can react in the similar manner with donors of an electron pair:
CH3+ + NH3  CH3-NH3+
Cr(CO)5 + NH3  H3N-Cr(CO)5
PtCl3- + NH3  H3N-PtCl3Au(PPh3)+ + NH3  H3N-Au(PPh3)+
More applications of the isolobal analogy:
In boron cages isolobal groups can replace each other: BH, Si, Ge, Sn, CH+, NH2+,
H
H
Os(CO)3 etc.
2Sn
C
B
2HB
BH
BH
B
H
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HB
BH
BH
C
H
Sn
Sn
Sn
Sn
In organometallic chemistry:
(OC)4Fe
(OC)4Fe
Sn
Fe(CO)4
Fe(CO)4
C
Fe(CO)4 Fe(CO)4
6) Isolobal ligands
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Ligands can also be isolobal analogues of each other and thus can replace each other
in metal complexes. The bonding modes between the metal and any of the isolobal
ligands will remain the same:
PH3
y
dyz
dxz
dz2
z
M
M
M
x
P
P
(-10.3 eV)
P
(4.6 eV)
LUMO
(4.6 eV)
(-14.8 eV)
C
CO
O
M
M
s-bonding
p-backbonding
(5.0 eV)
(-10.2 eV)
CH2=CH2
M
M
p-backbonding
s-bonding
(-16.3 eV)
H2
(6.6 eV)
H
H
M
M
H
s-bonding
H
p-backbonding
HOMO