Download Chapter 19, Coordination and Organometallic Compounds

Greenwood & Earnshaw
2nd Edition
Chapter 19
Coordination & Organometallic
Compounds
The Transition Elements
All the elements arising from the filling of the 3d, 4d and
5d shells are metals and comprise Groups 3-12.
As a group they are lustrous, deformable, have high
electrical and thermal conductivities and mp/bp are high.
Display multiple oxidation states which vary by 1 rather
than by 2 as found in the main groups.
They have an unparalleled propencity for forming
coordination compounds with Lewis bases.
Werner Theory: Primary Valency = Oxidation State
Secondary Valency = Coordination Number;
the number of Ligands directly bonded to the metal atoms.
Ligands
Mono- or Unidentate: Capable of forming one coordinate
covalent bond with the metal atom, commonly through one
orbital bearing a lone pair of electrons.
Bidentate: Capable of forming two coordinate covalent
bonds resulting in a ring, result is a chelate (Gk. claw).
Tri- or Terdentate: forming three coordinate covalent
bonds resulting in two rings.
Tetra- or Quadradentate: forming four CC bonds.
Penta- or Quinquidentate => 5
Hexa or Sexidentate => 6
1
Chelating Ligands
Chelate Effect: A bidentate ligand, L-L, will commonly
displace two unidentate ligands, L, in a kinetically labile
coordination compound given both ligand sets have similar
individual bond energies. Rationale: When one donor atom
binds to the initial coordination site, the second donor atom
is held in close proximity to the second coordination site.
Chelating ligands forming five and six-membered rings
are especially favored. Smaller rings induce ring strain.
Larger rings suffer from entropy considerations.
Chelating ligands can flexibly conform to the metal or
can rigidly demand a specific coordination geometry.
Stereochemistry & Coordination Number
CN Coordination Number:
L
M
L
2 Linear
3
Trigonal
4
4
Square Planar
Stereochemistry:
L
L
M
L
Tetrahedral
L
L
M
5
L
L
5
Trigonal Bipyramid
L
5
Square Pyramid
Pentagonal Plane
unknown
L
L
L
L
L
L
M
L
L
L
L
L
L
M
L
M
M
L
L
L
L
L
Stereochemistry & Coordination Number
CN
6
Coordination Number:
6
Octahedron
L
Stereochemistry:
L
L
M
L
7
Capped Trigonal Prism
7
L
L
Capped Octahedron
L
L
L
L
L
L
L
L
L
M
L
L
M
L
L
L
Pentagonal Bipyramid
L
L
L
L
L
7
Hexagonal Plane
unknown
L
L
M
L
6
Trigonal Prism
M
L
L
L
L
M
L
L
L
L
L
L
L
2
Factors Affecting Coordination Numbers
With dominant electrostatic forces a higher CN will be
favored by high cation charge and low anion charge.
The limiting CN is ultimately affected by the radius ratio
of cation and anion.
Where covalency is important charge is distributed,
ligand polarizability will lower the CN required to satisfy a
given cation. Pi (π) acceptor character of the ligand will
again raise CN.
The availability of empty orbitals for ligand donation
affects CN. CN is lowest for TM with near-filled d
subshells, heavier elements gps 11-12. However at the
converse extreme for groups 3-4 CN is dominated by
electrostatic interactions.
Isomerism
1) Conformational or "Polytopal"
vs
Tetrahedral
L
Square Planar
L
L
M
Cis
vs
Facial (fac)
B
A
A
B
M
B
B CN 4 or 6
3) Optical Isomers
M
A
B
A A
B
B
A
B
M
A
B
B
B
Meridional (mer)
B
B
B
A
B
B
M
A
A
B
M
B
L
Trans
B
A
A
L
L
L
2) Geometrical
L
M
A
B
A
M
B
B
B
M
A
B
A
B
B
Crystal Field Theory
An Electrostatic Point Charge Model - absurd bonding but
gave fundamental understanding of visible spectra.
∆ t = 4/ 9 ∆ h
∆h and ∆t = hνν,
ν is in the range
of visible light.
∆h
Spherical
Crystal Field
∆t
Tetrahedral
Crystal field
Octahedral
Crystal Field
Free Ion
Ligand Field Theory - Introduces some covalency into
ligand-metal interactions.
Valence Bond Theory - Allowed chemists to understand
the bonding picture but said nothing about excited states.
Molecular Orbital Theory - Gives a comprehensive bonding
picture for Metal-Ligand interactions.
3
Complementary Colors
Color Absorbed
Violet
Indigo
Blue
Blue-Green
Green
Yellow Green
Yellow
•
•
•
•
•
•
•
•
Color Observed
Yellow-Green
Yellow
Orange
Red
Purple
Violet
Indigo
Molecular Orbital Theory
t1u*
a1g*
4p
4s
t1u
eg*
a1g
∆h
eg
eg
t1u
a1g
six degenerate
ligand donor
orbitals
3d
t2g
t2g
transition metal
atomic orbitals
eg sigma bonding
t1u
a1g molecular orbitals
d2sp3
Molecular Orbital Theory
Mid-Spectrochemical series
ligands are sigma
bonding only.
π Donor Ligands:
π Acceptor Ligands:
eg. C O
CN-
eg. NH3
eg. Cl-
eg*
Weak Field Ligands
are π-donors. The
π-atomic orbitals are
filled and low in energy
they cause the ligand
field splitting to
diminish.
∆h
∆h
∆h
t2g
Strong Field Ligands are
π-acceptors. The π∗-molecular
π∗
orbitals are empty and high in
energy they cause the ligand
field splitting to increase.
4
Electron Configurations – Magnetism and
Ligand Field Stabilization Energy
eg*
3/5
∆h < Epairing
High Spin Complexes (Weak Field)
2/5
t2g
d0
Ca+2
LFSE:
d1
Ti+3
d2
Ti+2
d3
V+2
d4
Cr+2
d5
Mn+2
d6
Fe+2
d7
Co+2
d8
Ni+2
d9
Cu+2
d10
Zn+2
2/5
4/5
6/5
3/5
0
2/5
4/5
6/5
3/5
0
Low Spin Complexes (Strong Field)
∆h > Epairing
Organometallic Compounds
Hapticities the Hapto Nomenclature
M
η2
η1
M
M
M
η3
η4
M
M
η5
bis-η
η5
M
M
bis-η
η2
η6
M
M
η7
η3
The nomenclature has been generalized to include other
atoms such as N, O, P, etc. which occur in ligand
systems besides carbon, eg. pyridine may be η6.
η1 Ligands – Alkyl, Alkylidene, Alkylidyne
Methyl – Methene - Methyne
M
H
σ
C
1e-
π
H
H
σ
M
π
π
M
σ
H
2e-
H
3e-
Two sets of p-bonds at right angles.
π
C
H
π
C
σ
M
C
H
π
5
Alkyl, Alkylidene, Alkylidyne
In most carbyne complexes the metal-carbon triple bond is short
and strong. Bond order-lengths are more normal here:
In many carbene
C(CH3)3
complexes the metalE
C pCCW = 175E
dCW = 176 pm
carbon bond is only
dCW = 194 pm H
CH3
W
slightly shorter than the
pCCW = 150EE
P CH3
metal-carbon single
(CH3)3C C
CH2
bond suggesting limited
dCW = 226 pm
double bond character.
H
pCCW = 125EE CH
2
C
P
Carbenes with N or O
CH3
H
in the alpha position are
C(CH3)3 CH3
stabilized due to a more
extended pi system.
[Ta(η
η 5-C5H5)(CH3)(CH2)]
Carbene complexes are
dTa-CH3 = 225 pm dTa-CH2 = 220.6 pm
usually highly reactive
species. Alkyls without β-hydrogen atoms cannot undergo βelimination (of alkene) reactions and thus are more stable.
M-O Diagram – Carbon Monoxide
C
O
C O
σ∗
Antibonding orbitals very Carbon-like
HOMO-LUMO gap
π∗
2p
Lone pair of electrons on Carbon
σ
2p
*
π
2s
sp hybridization
* MO mixing of s&p
σ
Lone pair of electrons on Oxygen
too low in energy to be good donor.
2s
Carbon Monoxide
C O
σ∗
C
O
π∗
The antibonding molecular
orbitals extend more on the
carbon side, a better π-acid,
relatively low in energy.
σ
C
The carbon lone pair is most
available for σ-donation.
π
C
O
The strong pi-bond places
most electron density on
oxygen.
O
The sigma bond is also strong
placing most electron density
on oxygen.
O
The oxygen lone pair is
a very poor donor.
σ
C
6
Metal - η1 Carbon Monoxide Bonds
π∗
d
O
σ
M
O
C
C
π
Terminal C-O Bonding
π
σ
M
M
O
C
π
σ
M
Bridge C-O Bonding
π
Facial Bridge C-O Bonding
M
M
Carbon Monoxide – Higher Hapticities
a)
b)
c)
O
C
M
O
C
M
O
C
M
M
M
M
M
M
Two versions of the unsymmetrical
facial bridge bond to three metals.
The unsymmetrical
C-O bridge bond.
a) The C-O is η1 to one metal atom and η2 to the other.
b) The C-O is η1 to one metal atom and η2 to the other
two metal atoms. The M(CO)M angle is ~90E.
c) The C-O is η1 to two metal atoms and η2 to the third.
The bis(η
η1) bridge is symmetrical.
Alkene and Alkyne Metal Complexes
C
M
π∗
d
π∗
d
H
H
H
C
M
σ
σ
C
C
H
H
η2-alkene - 2e donor
H
C
H
η2-alkyne - 2e donor
d
π
M σ
C
H
C
M
M
η2-alkyne - 4e donor
µ2,η2-alkyne - 4e donor
7
Alkene and Alkyne Metal Complexes
R
Ph3P
R=H
R
C
Pt
α
dC-C = 137.5 pm
α = 64E
E
dC-C = 146 pm
R=F
α = 80E
E
dC-C = 153 pm
C=C occupies one coordination site
d10 ML3 C=C in ML3 plane
d8 ML4 C=C perpendicular to plane.
C
R
Ph3P
α = 32.5E
E
R = CN
R
cf: C
Ph3P
β
C
Pt
γ
C
C
C
134 pm
C
120 pm
R = Ph β = 140E
E γ = 39E
E
dPt-C = 206/201 pm dC-C = 132 pm
R = CF3 β = 140E
E γ = 30E
E
dPt-C = 202/203 pm dC-C = 125.5 pm
C
Ph3P
C
154 pm
R
R
The alkyne can be a 2e donor or 4e donor;
4e is common in 5th & 6th period TM.
Alkyne Complexes - Reactions
R
Co2(CO)8
C
+ RC CH
R
(CO)3Co
C
Co(CO)3
(CO)3Co
C
(CO)3Co
CH2
Co(CO)3
H
HCl
MeOH
When one alkyne substituent is H,
acid may cause a rearrangement.
R
Catalysis:
3 R C C R
+ 2 CO
Ph3Cr(Thf)3
R
R
R
R
R
Allyl Metal Complexes
Ψ3
C
C
Available Metal Orbitals
π*
C
Ψ2
C
C
px
dxz
πn
C
py
Ψ1
C
C
dyz
π
C
Ligand Molecular Orbitals
s
pz
d z2
8
Allyl Metal Complexes
1. Allylic carbon atoms sp2 hybridized, pCCC .120E
2. dC-C ≅ 140 pm
3. Metal atom out of C3 plane, below centroid of the
triangle formed by the three carbon atoms.
4. Terminal dM-C are commonly 5-15 pm longer than
central dM-C.
5. 2-Allyl substituents commonly tilted toward metal atom
- a result of pπ orbital tilting.
6. When a coordination plane can be identified the C3
plane forms a dihedral angle of from 95E to 120E with
the coordination plane.
Allyl Metal Complexes
C
O
139 pm
188 pm
W 75EE
210 pm
20E
E
3
198 pm
3
(η
η -C3H5)Co(CO)3
Co
C
O
5
(η
η -C5H5)(η
η -C5H5)W(CO)2
C
O
C
O
C
O
140 pm
206 pm
120E
E
Ni
198 pm
CH3
202 pm
H3C
12E
E
Br
C
O
Fe
139 pm
213 pm
C
C
O
O
3
(η
η -C3H5)Fe(CO)3Br
Bis(2-methylallyl)Ni(0)
Conjugated Polyenes & Cyclopolyenes
9
Aromatic and Non-aromatic Cyclopolyenes
Non-aromatic
Cyclopolyenes
Huckel 4n + 2 Aromatic
Cyclopolyenes
n = 0 2e systems
2+
+
n = 1 6e systems
-
2-
+
2+
n = 2 10e systems
-
2-
Sandwich Compounds an
Isoelectronic 18e- Series
6
Cr
Cr
6
6
5
6
7
6
Mn
5
Fe
7
5
5
Co
8
5
5
Ni
9
10
3
4
Dicyclopentadienyls of the First Period
5
V
5
5
Cr
5
Mn
6
5
5
5
5
Fe
7
5
Co
8
5
Ni
9
10
5
5
5
All dicyclopentadienyl complexes are Low Spin Complexes (Strong Field)
except manganese which exhibits Hi/Low Spin temperature equilibrium.
d3
d5
d4
d7
d6
d8
Dicyclopentadienyliron - Ferrocene
Cyclopentadiene Molecular Orbitals
Ferrocene Molecular Orbitals
e2
e1
dxy
a
s
dz2
a1g
e2g
e
2u
nonbonding
dxz
pz
a2u
No suitable
metal orbitals
e1g
dx2-y2
e1u
e
e2g
2u
nonbonding
dyz
px
e1g
No suitable
metal orbitals
py
e1u
10