Post on 27-Dec-2015
Ligands and electron counting in organometallic chemistry
Textbook: Chapters 1.4 – 1.6, 3.3
Ligands in organometallic chemistry
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Neutral 2e donors: PR3 (phosphines), CO (carbonyl), R2C=CR2 (alkenes), RC≡CR (alkynes, can also donate 4e), N≡CR (nitriles)
Anionic 2e donors: X- (halide), CH3- (methyl), CR3
- (alkyl), Ph- (phenyl), H- (hydride) The following can also donate 4e if needed, but initially count them as 2e donors (unless they are acting as bridging ligands): OR- (alkoxide), SR- (thiolate), NR2
- (inorganic amide), PR2- (phosphide)
Anionic 4e donors: C3H5- (allyl), O2- (oxide), S2- (sulfide), NR2-
(imide), CR22- (alkylidene)
and from the previous list: OR- (alkoxide), SR- (thiolate), NR2-
(inorganic amide), PR2- (phosphide)
Anionic 6e donors: Cp- (cyclopentadienyl), O2- (oxide) Z ligands: do not bring e to the metal: BR3, AlR3
Nomenclature
x
5-Cp 3-Cp 3-allyl 1-allyl
M
M PPh2 PPh2 1-dppe / 1-dppex
x
3
- bridging ligand
Ordering: from ACS publications In formulas with Cp (cyclopentadienyl) ligands, the Cp usually comes
first, followed by the metal center: Cp2TiCl2 Other anionic multi-electron donating ligands are also often listed in front
of the metal.
In formulas with hydride ligands, the hydride is sometimes listed first. Rule # 1, however, takes precedence over this rule: HRh(CO)(PPh3)2 and
Cp2TiH2
Bridging ligands are usually placed next to the metals in question, then followed by the other ligands Note that rules 1 & 2 take precedence: Co2(-CO)2(CO)6, Rh2(-
Cl)2(CO)4, Cp2Fe2(-CO)2(CO)2
4
Coordination geometriesCN Geometry Example
2
3, trigonal
3, T shape
4, tetrahedron
4, square planar
[NC–Ag–CN]–
Pt(PPh3)3
[Rh(PPh3)3]+
Ti(CH2Ph)4
5
M LL
L ML
L
L M
L
L
L
M
LL
L
L
ML L
L
Pt
Cl
ClCl
H
H
H
H
Coordination geometriesCN Geometry Example
5, trigonal bipyramid
5, square pyramid
6, octahedron
6, pseudo-octahedron
[Co(CNPh)5]2+
W(CO)6
FeCp2
6
L ML
L
L
L
axial
equator ial Mes TaMes
Mes
Cl
Cl
ML
L L
L
L apical
basal
ML
L L
L
L
L
M
LL
L
Coordination geometries
CN Geometry Example
6, antiprism
7, capped octahedron
7, pentagonal biprism
WMe6
[ReH(PR3)3(MeCN)3]+
[IrH5(PPh3)2]
7
ML
LL
L
L
L
ML
L L
L
L
L
L
ML
L L
L
L
L
L
Electron counting and the 18 electron rule Determine the oxidation state of the transition metal center(s) and the metal centers
resulting d-electron count. To do this one must: a) note any overall charge on the metal complex b) know the charges of the ligands bound to the metal
center (ionic ligand method) c) know the number of electrons being donated to the metal
center from each ligand (ionic ligand method)
Add up the electron counts for the metal center and ligands.
Complexes with 18e counts are referred to as saturated, because there are no empty low-lying orbitals to which another incoming ligand can coordinate. Complexes with counts lower than 18e are called unsaturated and can electronically bind additional ligands.
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A simple example
9
ReR3P CO
PR3
CH3
CO
Method
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1) There is no overall charge on the complex.
2) There is one anionic ligand (CH3-, methyl group).
3) Since there is no overall charge on the complex (it is neutral), and since there is one anionic ligand present, the Re metal atom must have a +1 charge to compensate for the one negatively charged ligand. The +1 charge on the metal is also its oxidation state. So the Re is in the +1 oxidation state.
More examples
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1.
2.
MH2C
R2P
H2C M
MH2C
R2P
H2C M
C
R2P
C
HH
H
H
C
R2P
C
HH
H
H
C
R2P
C
HH
H
H
+2e-
Metal-metal bonded example
12
MoPR2
R2P Cl
C
C
MoCl
R2P
PR2
C
C
O O
OO
Method for metal-metal complexes
13
The simple rule for M-M bonding is that if you have two metal atoms next to one another and each has an odd electron-count, you pair the odd electrons to make a M-M bond.
This example also has -Cl ligands. Bridging ligands with at least 2 lone pairs almost always donate 2e- to each metal center.
Oxidation state determination: There is a total of two anionic ligands for two metal centers (overall complex is neutral). Thus each metal center needs to have a +1 oxidation state to balance the anionic ligands.
Exceptions to the 18 electron rule
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d3 d4 d5 d6 d7 d8 d9 d10 d10s1
21
ScScandium
22
TiTitanium
23
VVanadium
24
CrChromium
25
MnManganese
26
FeIron
27
CoCobalt
28
NiNickel
29
CuCopper
39
YYttrium
40
ZrZirconium
41
NbNiobium
42
MoMolybdenum
43
TcTechnetium
44
RuRuthenum
45
RhRhodium
46
PdPalladium
47
AgSilver
57
LaLanthanum
72
HfHafnium
73
TaTantalum
74
WTungsten
75
ReRhenium
76
OsOsmium
77
IrIridium
78
PtPlatinum
79
AuGold
Group 8 Metals
Early TransitionMetals
16e and sub-16e configurations are common
Coordination geometries
higher than 6
Middle TransitionMetals
18e configurations are common
Coordination geometries
of 6 are common
Late TransitionMetals
16e and sub-16e configurations are common
Coordination geometries
of 5 or lower
Different metals: general properties From left to right, the
electronegativity increases substantially: Early TM are electropositive:
often found in the highest permissible oxidation state
d2 are very easily oxidized: very basic
Late TM are relatively electronegative: Often found in low oxidation
states Back donation is not so marked:
ligands are subject to nucleophilic attack
Electronegativity
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Types of metal-ligand interactions Sigma () donor ligands Pi () donor ligands
Pi () acceptor ligands
Sigma donor Pi donor* Pi acceptor*
CR3-
H-
RO-, R2N-
F-, Cl-
RCOO-
CO, olefin
CN-
PR3
These ligands also act as donors.
Examples of donor and acceptor ligands
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d
M LComplex
M L
emptyfull
d
M LComplex
M L
fullempty
d
M LComplex
M L
fullempty