Title: Outline
1Outline
Chapter 1. General properties of organometallic
complexes
- 1. Review on coordination chemistry
- 2. The 18-electron rule
- 3. Limitations of 18-electron rule
- 4. Oxidation number
- 5. Coordination number and geometry
- 6. Effect of complexation
- 7. Differences between metals
References
1. The Organometallic Chemistry of the Transition
Metals, Robert H. Crabtree, 3rd Edition, 2001,
Chapter 1-2. 2. Organotransition Metal Chemistry,
Akio Yamamoto, 1986. Chapter 1-4. 3. Organometalli
c Chemistry, G. O. Spessard, G. L. Miessler,
Prentice-Hall New Jersey, 1997, Chapter 1 3.
21. Review on coordination chemistry
- Complexes or coordination compounds are compounds
composed - of a metal and ligands which donate electrons to
the metal, e.g.
Ligands a molecule or ion that has at least one
electron pair that can be donated.
The electron pair can be lone pair, p-bonding
electron pair, or s-bonding electron pair.
3Classification of ligands A). Based on nature
of the donating electron pairs, ligands may be
classified as Lone pair donor, p-bonding electron
pair donors, s-bonding electron pair donors.
41) Lone pair donors
MO description of Metal-Ligand interactions
- Note
- Ligands that donate electron pairs to form M-L s
bond are also called s donors.
5Some lone pair donor ligands may have orbitals to
form M-L p bonds. gt p donor, p acceptor.
e.g. OR-
Note although ligands such as OR- can form p
bond with a metal, we usually don't indicate such
interaction in writing the structures.
Ligands that donate electrons to metal to form p
bond are called p donor
How many p bonds can an OR form with a transition
metal ion?
6Depending on the hybridization of O
7CO is not only a s donor, but also a p acceptor.
accept e- to form p bond (d??p? back bonding)
Ligands that accepting electrons from metal to
form p bond are called p acceptor.
How many p bonds can a CO form with a transition
metal ion?
8TWO
Can CO function as a p donor?
9TWO
Can CO function as a p donor? In principle yes,
but the p-accepting is dominant.
10Types of lone pair donor ligands
11Evidences for dp-pp interactions. Take M-CO
complexes as an example for p accepter
Explanation?
The lone pair on C atom has a little anti-bonding
character.
12Take M-OR complexes as an example for p donor.
In H2O, O atom has sp3 hybridization, leading to
the A-O-B angle being about 104.5?. However, in
the above two complexes the angles are 120? and
180?, respectively. Why ?
13Take M-OR complexes as an example for p donor.
If no p-bonding
14e
10e
With p-bonding
18e
18e
In H2O, O atom has sp3 hybridization, leading to
the A-O-B angle being about 104.5?. However, in
the above two complexes the angles are 120? and
180?, respectively. Why ?
142). p-bonding electron pair donors
How can these ligands interact with M ? Take
CH2CH2 as an example.
Is CH2CH2 a p donor or a p acceptor?
152). p-bonding electron pair donors
How can these ligands interact with M ? Take
CH2CH2 as an example.
Is CH2CH2 a p donor or a p acceptor?
p acceptor
16Hapticity of ligands A ligand may have more than
one way to bond to a metal center, e.g.
In describing the number of atoms (n) attached to
a metal, a short hand hn is used. e.g.
17Exercise. Give the hapticity of ligands in
following complex
18Exercise. Give the hapticity of ligands in
following complex
Ti(h5-C5H5)2(h1-C5H5)2
Fe(h4-C6H8O)(CO)3
Fe(h4-C8H9O)(CO)3
Ru(h6-C8H10)(h4-C8H12)
19Notes Notation of bridging ligand ?n
203). s-bonding electron pair donors
Relatively fewer stable complexes are known.
Typical examples
21How can these ligands interact with M? Take H2 as
an example.
Further Notes Relative basicity of electron
pairs lone pairs gt p bonding electron pairs
gt s bonding electron pairs Therefore usual
order of binding ability lone pair donor gt p
bonding electron pair donor gt s bonding electron
pair donor. Consequence
22 Some ligands have several types of electron
pairs. The types of e- pairs to be used to
bind metal will depend on metals. e.g.
23- B) based on the nature of bonding interaction,
ligands may be classified as soft or hard. -
- Hard ligands have low polarizability,
especially those containing period 2 donor atoms
N, O, F, e.g. O2H, NH3, F- - Soft ligands have high polarizability, they
include - . Those with period three or subsequent donor
atoms, - e.g. Cl, Br, S, P.
- b). ?-acceptors, e.g. CO, CS (carbon sulfide),
H2, CN- (cyanide) - c). Those containing p-electrons, e.g.
24Importance of the concepts of soft and hard
ligands Hard ligands tend to form stable
complexes with hard metal ions, (usually
those at high oxidation state, e.g. Al3, Fe3,
Cu2, Ti4, Pt(IV)). Soft ligands tend to
form stable complexes with soft metal ions,
(usually those at low oxidation state, e.g.
Mn(I), Co(I), Fe(II), Pt(II), Pt(0)
.......) Examples AlF63-, Ti(OR)4 are very
stable complexes, but Pt(0)--F, Pt(0)--OR,
W(0)--F are very rare.
(Olefins are soft base Pt(II) is soft but Pt(IV)
is hard)
25Exercise. explain the following facts based on
the concept of Soft-Hard Acid-Base.
1) W(CO)6 is air stable, but W(NH3)6 has never
been observed. W(0),soft acid CO, soft base
NH3,hard base. 2).
Os(II), soft Os(III), hard NH2, hard arene, soft
3). Low oxidation state complexes (most
organometallic compounds) are often
air-sensitive, but are rarely water
sensitive. Low oxidation state M gt soft
acid air O2, N2, CO2 etc. O2 is a soft base.
H2O is a hard base.
26Exercise. explain the following facts based on
the concept of Soft-Hard Acid-Base.
- 1) W(CO)6 is air stable, but W(NH3)6 has never
been observed. - W(0),soft acid CO, soft base NH3,hard base.
- 2).
Os(II), soft Os(III), hard NH2, hard arene, soft
3). Low oxidation state complexes (most
organometallic compounds) are often
air-sensitive, but are rarely water
sensitive. Low oxidation state M gt soft
acid air O2, N2, CO2 etc. O2 is a soft base.
H2O is a hard base.
27Further notes on ligands
- General presentation of Ligands
- L form coordination bond with a metal (e.g.,
CO, PR3) - X form covalent bond with a metal (i.e., H, Cl,
CH3)
MX2(L2)
MXL(LX)
MXL5
Types of ligand coordination Terminal Ligand
is bound to only one metal center (L-M or
X-M) Bridging (µ) Ligand is attached to
different metal centers Hapticity (?) Ligand
attached to a metal center through more than one
atoms
28Further notes on ligands
Chelation Ligand attached through more than one
atom usually separated by one or more atoms.
Chelating ligands are sometimes classified as
being bidentate (2 points of attachment),
tridentate (three points of attachment), or
tetradentate (4 points of attachment).
Kappa convention (?) The kappa convention is
sometimes used to indicate the coordinating atoms
of a polydentate ligand.
292. The 18 electron rule
1) Thermodynamically stable transition metal
organometallic compounds are formed when the sum
of the metal d electrons plus the electrons
supplied by the ligands equals 18.
In this way, the metal formally attains the
electronic configuration of the next noble gas.
The 18 electron rule 18 VE rule inert gas
rule effective atomic number rule (EAN rule).
e.g.
10 4x2 18
8 5x2 18
6 6x2 18
Saturated complexes 18 VE complexes Unsaturated
complexes lt18 VE complexes
Why 18?
30Why 18?
Simple reason
312) Ways to count valence electrons (VE) VE
valence e- of M (or Mn) e- from ligands Two
Models covalent model and ionic model Covalent
model Both M and L are considered as neutral
VE valence e- of M e- from ligands
charge e.g.
For TM, valence e- of M group number
32For TM, valence e- of M group number. e.g. Fe,
8 Pt, 10
33Ionic model Metal complexes are formed from
Mn L X- VE valence e- of Mn e- from
ligands
34Exercise Determine the valence electron count
of the following complexes.
35Exercise Determine the valence electron count
of the following complexes.
7 (Mn) 5x2 (CO) 1 18
7 (Mn) 5x2 (CO) 1 (Mn-Mn) 18
8 (Fe) 3x2 (CO) 1 (Fe-Fe) 3x1(m-CO) 18
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379 (Rh) 2x2 (CH2CH2) 2 (Cl) 1 (Cl) 16
8 (Fe) 3x2 (CO) 2 (CO) 3 (h3-allyl) -1 (1
charge) 18
8 (Fe) 3x2 (CO) 4 (h4-diene) 18
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398 (Fe) 3x2 (CO) 5 (h5-allyl) -1 (1
charge) 18
6 (Cr) 2x6 (h6-benzene) 18
8 (Ru) 6 (h6) 4 (h4) 18
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417 (Mn) 5 (h5) 3x2 (CO)) 18
8 (Fe) 5 (h5) 1 (h1) 2x2 (CO)) 18
6 (W) 5 (h5) 3 (h3) 2x2 (CO)) 17
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439 (Rh) 6 (h6) 2 (Rh) 2 (CO)) 19
44Do the following complexes follow the 18e rule?
ReH7(PPh3)2
45Do the following complexes follow the 18e rule?
7 (Re) 7 (H) 2x2 (PPh3) 18
ReH7(PPh3)2
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48Additional exercise
49Additional exercise
50Additional exercise
51Additional exercise
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563. Limitations of the 18 electron rule
Most OMC follow 18e rule. But there are many
stable compounds that do not have 18 valence
electrons. e.g. WMe6 (12e) Pt(PCy3)2
(14e) Cu(NH3)62 (21e) RhCl(PPh3)3 (16e)
Question When the 18e rule works?
Some general observations 1). Many main group
complexes do not follow 18e rule
e.g. ZnMe2 (14e), Cp2Be (12e), IF7
(24e), SbF6- (22e)
572). F-block metals do not follow 18e rule. e.g.
Why? because electrons can go to (n-2)f orbitals)
583). Transition metal complexes gt three classes
Class number of valence electrons 18e
rule I ....16, 17, 18, 19, 20 not
obey II ....16, 17, 18 not obey III
18 obey
59Class I 3d metals with weak-field ligands Class
I n(VE) 18 22
n(d) n(VE) TiF62- 0 12 VCl62-
1 13 V(C2O4)33- 2 14 Cr(NCS)63- 3 15 Mn(CN
)63- 4 16 Fe(C2O4)33- 5 17 Co(NH3)63 6 18 C
o(H2O)62 7 19 Ni(en)32
8 20 Cu(NH3)62 9 21 Zn(en)32
10 22
60Class II 4d and 5d metals with weak-field
ligands Class II n(VE) ? 18
n(d) n(VE) ZrF62- 0 12 WC
l6 0 12 WCl6- 1 13 WCl6 2-
2 14 TcF62- 3 15 OsCl62- 4 16 PtF6 4
16 PtF6- 5 17 PtF62- 6 18 PtCl42- 8 16
Metals in relatively High oxidation state
61Class III Complexes with p good acceptors
Class III n(VE) 18 n(d) n(VE) V(CO)6- 6
18 CpMn(CO)3 7 18 Fe(CN)64- 6 18 Fe(CO)42- 1
0 18
62MO levels in the absence of p acceptors.
Explanation
If D is small, eg can be occupied. (D lt
pairing energy) gt eg (antibonding)
0 - 4e t2g (nonbonding) 0 6e a1g, t1u,
eg (bonding) 12e Total e- Minimum of
e- Maximum of e-
3d metal complexes with weak field ligands e.g.
H2O, NH3 and Cl- belong to this class (class I).
63MO levels in the absence of p acceptors.
Explanation
If D is small, eg can be occupied. (D lt
pairing energy) gt eg (antibonding)
0 - 4e t2g (nonbonding) 0 6e a1g, t1u,
eg (bonding) 12e Total e- Minimum of
e- 12e Maximum of e-
22e
3d metal complexes with weak field ligands e.g.
H2O, NH3 and Cl- belong to this class (class I).
64MO levels in the absence of p acceptors.
If D is large, eg can be occupied. (D gt
pairing energy) gt eg (antibonding)
0 e t2g (nonbonding) 0 6e a1g, t1u, eg
(bonding) 12e Total e- Minimum of e-
Maximum of e-
Complexes of 4d and 5d metals in high oxidation
state belong to this class (class II).
65MO levels in the absence of p acceptors.
If D is large, eg can be occupied. (D gt
pairing energy) gt eg (antibonding)
0 e t2g (nonbonding) 0 6e a1g, t1u, eg
(bonding) 12e Total e- Minimum of e-
12e Maximum of e- 18e
Complexes of 4d and 5d metals in high oxidation
state belong to this class (class II).
66 In the presence of p-acceptors
Since organometallic compounds usually have
?-acceptors, D is large and t2g are bonding MOs.
They usually have 18e valence electrons (class
III).
D is large. t2g are bonding MOs, and prefer to be
occupied.
gt eg (strongly antibonding) 0 e
t2g (bonding) 6 e a1g, t1u, eg
(bonding) 12 e gtTotal e
67 In the presence of p-acceptors
Since organometallic compounds usually have
?-acceptors, D is large and t2g are bonding MOs.
They usually have 18e valence electrons (class
III).
D is large. t2g are bonding MOs, and prefer to be
occupied.
gt eg (strongly antibonding) 0 e
t2g (bonding) 6 e a1g, t1u, eg
(bonding) 12 e gtTotal e
18e
68Note, just like the octet rule, the 18-electron
rule is not an absolute requirement. There are
many exceptions.Common exceptions to the 18
electron rule
d8 metals The d8 metals (groups 8 - 11) have a
tendency to form square-planar 16 electron
complexes. This tendency is weakest for group 8
(Fe(0), Ru(0), and Os(0)) and is very strong for
groups 10 and 11 (Pd(II), Au(III)).
Because 2b1g orbital is very high-lying and is
usually empty
69- d0 metals The high-valent d0 complexes often
have lower electron counts than 18.
Complexes with bulky ligands Sterically
demanding ligands will often result in lower than
expected electron counts.
70- gt 18 electron complexes Complexes with formally
19 or 20 electrons are known, but they are
usually unstable, or adopt alternate
configurations.
71The 18 Electron Rule Is Empirically Justified
The rule is particularly useful for Groups 6-8
16 e- Compounds
14 e- Compounds
724. Oxidation number of metals and d electron count
The oxidation state of a metal in a complex is
simply the charge that the metal would have on
the ionic model.
d electron count of d electrons in the valence
shell group - oxidation state.
e.g. What is the oxidation number of metals in
the following complexes?
734. Oxidation number of metals and d electron count
The oxidation state of a metal in a complex is
simply the charge that the metal would have on
the ionic model.
d electron count of d electrons in the valence
shell group - oxidation state.
e.g. What is the oxidation number of metals in
the following complexes?
Fe(II), d6
W(VI), d0
Os(II), d6
74Problems with O. S. a) Ambiguous oxidation
states
Convention used in this course.
75b) charge density and formal oxidation states
Consider complexes W(CO)6 and WH6(PMe3)3. Which
one would you expect to have a higher positive
charge on W?
W(CO)6 W(0) W CO
backdonation WH6(PMe3)3 W(VI) actually more
e-rich
Another example
There is no strict correlation between charge
density and formal oxidation states! Oxidation
states in organometallic complexes are merely
formalisms that may bear little resemblance to
the actual positive charge on the metal.
76Any Uses of formal oxidation states? Oxidation
number usually can not be higher than group
member ! gt predict if a compound/intermediate
is possible gt Help to formulate the structure
of a compound.
e.g. (1) Are the following species possible?
WMe6 TiMe6 VH7(PMe3)2 (2)
WH6(PMe3)3 HBF4 ------gt WH7(PMe3)3 BF4
Which of the following is unlikely the structure
for WH7(PMe3)3 ?
77Any Uses of formal oxidation states? Oxidation
number usually can not be higher than group
member ! gt predict if a compound/intermediate
is possible gt Help to formulate the structure
of a compound.
e.g. (1) Are the following species possible?
WMe6 TiMe6 VH7(PMe3)2 (2)
WH6(PMe3)3 HBF4 ------gt WH7(PMe3)3 BF4
Which of the following is unlikely the structure
for WH7(PMe3)3 ?
W(III), OK
W(VIII), x
W(VI), OK
785. Coordination number (C.N.) and geometry
- It is easy to define C.N. for complexes with lone
pair donors. - Monodentate ligand L
- C.N. of L present of atoms bound
to metal - of electron pairs involved in
M-L s bonds. - e.g.
79- Polydentate ligands
- C.N. of atoms bound to metal
- of electron pairs involved in M-L
s bonds. -
- ? of L present
80b) For Organometallic compounds, it is difficult
to define C.N. e.g.
Convention used in this course. We normally
adopt the e- pairs in M-L bonds as of C. N..
81Exercise. What is the coordination number of
following complexes?
6 6 6
82Further notes on coordination numbers a). For
TM, C.N. 9 , why? TM has 9 valence orbitals
((n-1)d, ns, np). b). dn ltgt C.N. and
geometry dn C.N.
geometrical structure d6
6 prefer octahedral d8
4 prefer square planar d0,
and d10 4 prefer
tetrahedral c). Each C.N. is associated with one
or more geometries.
83Prefered dn
d0,, d5, d10
d8
d8
d0,, d5, d10
84d8,, d6
d6,, d7
(benzonitrile)
d6,, d3
d0
856. Effect of complexation
M-L Complexation of L on M may cause the
change of electron density distribution on L
new reactivity of L unreactive gt reactive
- Change the electron density on L.
- s donation will reduce electron-density of L.
- p-accepting will increase electron-density of L.
Examples
Complexation reduce electron-density of olefin
86Another example,
Complexation reduces the e- density on C6H6.
Reactivity towards Nu- increase Reactivity
towards E decrease
87Change the electron density distribution on L
d
(e- to mainly C atom)
d
(e- to on both atom)
d-
d-
d-
d-
887. Differences between metals
1) Electronegativity differences
Sc Ti V Cr Mn Fe Co Ni Cu 1.3 1.5 1.6 1.6 1.6 1.8
1.9 1.9 1.9 Y Zr Nb Mo Tc Ru Rh Pd Ag 1.2 1.3 1.6
2.1 1.9 2.2 2.3 2.2 1.9 La Hf Ta W Re Os Ir Pt Au
1.1 1.3 1.5 2.3 1.9 2.2 2.2 2.3 2.5
Moving from left to right, the electronegativity
of the elements increases substantially.
892). Trend in the stability of high oxidation
states.
Early transition metals are electropositive, so
they readily lose all their electrons to give d0
centers (e.g. Zr(IV), Ta(V)). Low-valent early
transition metals, such as Ti(II) and Ta(III),
are easily oxidized. Late transition metals
are more electronegative, thus they prefer lower
oxidation states (i.e., Rh(I) compared to
Rh(III)).
Stability of high oxidation states
From left to right decrease
From top to bottom
increase e.g. Ti(II) unstable Fe(II)
stable Ti(IV) stable Fe(VIII) not
exist Hf(IV) very stable Os(VIII) stable
902). Trend in the relative energy of d orbitals
and backdonation.
- Question 1 Which of the following species has d
electrons of highest energy? - (a) Ti(0) (b) Ti(II) (c) Ti(III)
- (2) (a) Ti2 (b) Fe2 (c) Ni2 (d) Zn2
- (3) Fe
- Ru
- Os
912). Trend in the relative energy of d orbitals
and backdonation.
- Question 1 Which of the following species has d
electrons of highest energy? - (1) (a) Ti(0) (b) Ti(II) (c) Ti(III)
- (2) (a) Ti2 (b) Fe2 (c) Ni2 (d) Zn2
- (3) Fe
- Ru
- Os
92- Question 2. How would you explain the following
trend in the IR dada of u(C-O) (in cm-1)? - a). V(CO)6 Fe(CO)5 CO Ag(CO)
- 1976 2023 2057 2204
-
-
-
- b) Cr(CO)6 2000
- W(CO)6 1998
-
93 Question 2. How would you explain the following
trend in the IR dada of u(C-O) (in cm-1)? a).
V(CO)6 Fe(CO)5 CO Ag(CO) 1976 2023 2057
2204 u(C-O) depends on M-CO s donation and p
backdonation. Upon complexation with TM,
u(C-O) is decreased, because of p backdonation.
However, the decrease in u(C-O) will be less for
late transition metals. In the case of Ag
(d10), the energy of d e- is very low.
Backdonation is not significant and the major
interaction with CO is s donation. Therefore
u(C-O) is increased. b) Cr(CO)6 2000
W(CO)6 1998 The energy of d e- of W is higher.
gt more backdonation, lower u(C-O).
94Effects of changing net ionic charge, ligands,
and metal on the ? basicity of a metal carbonyl,
as measured by ?(CO) values (cm-1) of the highest
frequency band in the IR spectrum
95Question 3. Compounds of the formula MH4P3 (M
Fe, Ru and Os, P PR3) are known to have the
following structure.
96Question 3. Compounds of the formula MH4P3 (M
Fe, Ru and Os, P PR3) are known to have the
following structure.
If there is excessive backdonation from dp(M) to
s(H2), the H-H bond will be broken. The energy
of d e- increases in the order of FeltRultOs. In
the case of Os, the energy of d e- is high,
excessive backdonation from dp(Os) to s(H2)
breaks the H-H bond.