Chem 652 Spring 2013
Dative Ligands
Prof. Donald Watson
"
Assistant Professor"
"
" Chapters 1-2
Read Hartwig
Dative Ligands (Chapter 2 in JFH)
Carbonyl Ligands: CO
C O
Mononuclear:
V(CO)6, Cr(CO)6, W(CO)6, Fe(CO)5, Ru(CO)5, Ni(CO)4
Polynuclear:
Mn2(CO)10, Fe3(CO)12, Ru(CO)5, Ru3(CO)12, Co2(CO)8, Rh4(CO)12, W(CO)6
Why fewer ligands on Mn2(CO)10 than Co2(CO)8?
Synthesis of Carbonyl Complexes
Typical Synthesis: Reductive Carbonylation
WCl6 + CO + Et3Al
W(CO)6
ReO7 + 17 CO
Re2(CO)10 + 7 CO2
What are the reductants and oxidants?
Will CO coordinate the oxidized metal?
Modes of CO Binding
Type I) Linear; M-CO ~ 180°
M
M
C O
O
C
Type II) Bridging Two Metals
M
M
O
C
Type III) Bridging Three Metals
M
M
M
O
C
M M
Type IV) Unsymmetrical, Bridging
O
Type V) Unsymmetrical, Bridging
(ie type I + L)
C
M
M
O M
Type VI) Unsymmetrical, Bridging
(ie type I + LA)
C
M
C O
Recall Bonding Pictures
M
C O
M
O
C
C O
M
M
O
π back-bonding
C
σ donation
σ donation
π backbonding
Physical Properties: IR Stretching Frequencies of CO
•  free CO : 2143 cm-1
•  complexes with d-electrons: 2125-1850 cm-1
Ni(CO)4
Co(CO)4–
Fe(CO)42–
2057cm–1
1886 cm–1
1785cm–1
more negative charge = more back-bonding
Ph3P
Cl
Ir
CO
PPh3
1860 cm–1
Ph3P
Cl
CO
Pt PPh3
2100 cm–1
more positive charge = less back-bonding
Note: these sets of examples are “isoelectronic” = same number
of electrons/configuration
Physical Properties: IR Stretching Frequencies of CO
•  depends on other ligands σ-donor ability
Mo(CO)3(PF3)3
Mo(CO)3(P(OMe)3)3
Mo(CO)3(PPh3)3
2090cm–1
1977 cm–1
1934 cm–1
most donation
least donation
•  depends on other ligands π-acceptor ability
Mo(CO)3(NCMe)3
Mo(CO)3(NR2)3
Mo(CO)3(pyr)3
1915cm–1
1898 cm–1
1888 cm–1
best acceptor
worst acceptor
NO+ > CO > PF3 > RN C > PCl3 > P(OR)3 > PR3 >
RC N
> NH3
Not All Just d-Electrons
•  Predict which is higher wave number?
Might think Ni(CO)4: more d e– ~ more back bonding.
Cr(CO)6
1987 cm–1
Ni(CO)4
2057 cm–1
•  Why?
Early metal less E.N., thus higher orbital energies.
Bridging Carbonyls
O
C
Type II) Bridging Two Metals
M
1850 cm–1 - 1700 cm–1
M
O
C
Type III) Bridging Three Metals
M
1675cm–1 - 1600 cm–1
M
M
CO Bond Lengths
•  free CO : 1.128 Å
•  Terminal CO: 1.12–1.18 Å longer due to back bonding.
•  Follows same trends as IR.
M-CO BDEs
•  Wide Range of BDEs (6 – >160 kJ/mol)
•  Trends (Calc. BDEs):
Group IV
stronger
Group X
Cr(CO)6
147 kJ/mol
Ni(CO)4
106 kJ/mol
Mo(CO)6
119 kJ/mol
Pd(CO)4
27 kJ/mol
W(CO)6
142 kJ/mol
W(CO)6
38 kJ/mol
stronger
iPr3P Rh
Cl
iPr3P
Cl
Ir
CO
PiPr3
35 kJ/mol
CO
PiPr3
84 kJ/mol
•  Note: first row stronger because of better
overlap.
•  2nd and 3rd row = more electrons, more
back bonding
•  early metals less E.N., more back bonding.
•  2 factors: σ – donation and π backbonding.
•  Effects Reactivity.
CO Analogs
•  Isoelectronic to CO.
O C
carbon monoxide
RN C
isocyanates
S C
carbon monosulfide
•  RNC and SC are better back-bonders than CO… will displace CO.
•  Why?
•  Weaker π bond = lower π*. Think about MO diagram.
Phosphines
•
•
•
•
Trivalent phosphines most important ancillary ligands in organometallic chemistry.
Many, many have been made and used.
Vastly modulate reactivity of TM complexes.
Soft donors, form strong bonds with soft (late) metals.
simple
monophosphines
PPh3, PMe3, P(o-tol)3, P(OMe)3, P(OPh)3, PF3
OMe
complex
monophosphines
PtBu2
iPr
MeO
iPr
PtBu2
Ph
Fe
Ph
Ph
Ph
iPr
BrettPhos
Ph
Q-Phos
Bidentate Phosphines
cis or trans:
cis:
PPh2
PPh2
Me
Fe
PPh2
dppe
Me
O
PPh2
dppf
trans:
Ph2P
PPh2
xanthphos
Ph2P
PPh2
transphos
Cis vs Trans Phosphines?
31P
NMR very useful for diamagnetic complexes.
X
R3P
X
Ni PR'3
R3P Ni
X
X
PR'3
cis JP-P 65 Hz
trans JP-P 300 Hz
Cl
PMe3
Cl
Me3P Ru Cl
Me3P
PMe3
PMe3
Me3P Ru PMe3
Me3P
Cl
–6.63 (s)
9.0 (t), –12.7 (t)
Chiral Phosphines
Me
P
PPh2
PPh2
PPh2
OMe
Me
Me
P
O
P NMe2
O
Me
BINAP
duPhos
P
phosphoramidites
MOP
Me
iPr
P
iPr
Me
chiral at phosphorus
Note: inversion of PR3 29-35 kcal/mol = stable
even at elevated temps
Electronic Properties of Phosphines
phosphine
pKa’
(p-ClC6H4)3P
1.03
(p-FC6H4)3P
1.97
Ph3P
2.73
(o-MeC6H4)3P
3.08
(p-MeOC6H4)3P
4.84
MePh2P
4.59
Me2PhP
6.50
Me3P
8.65
Cy3P
9.70
tBu3P
11.4
increasing
σ-donor ability
Note: Aryl
phosphines less
electron rich than
alkyl phosphines.
Tolman Cone Angle
R
R
P
θ
M
•
•
•
•
R
phosphine
θ
H 3P
87
Me3P
118
MePh2P
122
Ph3P
145
Cy3P
170
tBu3P
182
(o-tol)3P
194
(mesityl)3P
212
“Cone Angle” approximate volume of ligand
Not perfect measurement, as ligands are not really conical.
Note: cone angles can exceed 180 °!
Effects reactivity in many ways!
Tolman Chem Rev 1977, 77, 313
Phosphines Are π-Acceptors!
R
R
P
R
R
π back-bonding
R
R
σ donation
d
Ni(CO)3L
ligand
CO IR (cm–1)
F3P
2110
(MeO)3P
2079
Ph3P
2069
Cy3P
2056
tBu3P
2056
σ*
lower π*
better
back-bonder
Amine Ligands
•  sp3 amine ligands typically polydentate (no significant back bonding).
H
N
Me Me
N
Cl
Pd
Cl
N
Me
Me
HN
NH
Ru
Me3P
R
H
•  sp2 allows for better overlap (back bonding), particularly with late 1st row metals.
Me
Me
O
O
N
N
N
N
R
bypyrdine
bisoxazoline
R
O
N
R
PR2
R
phophinopxazoline
Ar N
R
N Ar
bis imine
Me
Ar
Me
N
N
N
Ar
pyridyl bisimine
Oxygen Ligands
O
THF
MeO
OMe
O PPh3
DME
•  Typically seen with early metals… driven by coulombic interaction, but
there are exceptions.
Cl
Cl
Cl
W
Cl
Me
O
O
Me
OC Re O
OC
Cl
Cl
Ni
Me
O
O
Me
Dinitrogen Ligands
N N
2 π*
•  Kind of like CO, mostly stabilized by back-bonding.
N
N
N2
2+
H3N Ru NH3
NH3
H3N
NH3
Cp*2Zr N N ZrCp*2
N2
PiPr2
Rh
PiPr2
iPr2P
N N Rh
iPr2P
•  Yesterday: Cummins
1/2 N2
(R2N)3Mo
(R2N)3Mo N N Mo(NR2)3
(R2N)3Mo N
𝜼2 dinitrogen
•  Can also be edge bound 𝜼2.
N
Zr
ZrCp'2
N
H
N
H2
Zr
H2
ZrCp'2
N
H
Paul Chirik, Princeton
H
Zr
+ NH3
H
Alkene Complexes
Common Alkene Ligands:
O
Ph
Ph
COD
NBD
Ph
COE
dba
Ph
(chiral!)
σ*
E
π*
dx2-y2
E
dxz
π∗ (C-C)
(or dz2)
π (C-C)
(or dxy)
σ
M
π
M
alkynes, carbonyl, imines, very similar.
Dative “Neutral” Carbene Ligands
Me
O Me
(OC)5Cr
Me
Ar N
Cl
Cl
Fisher Carbene
N Ar
Ru
iPr
Ar N
Me
Ar =
iPr
Cl
PCy3
Grubb's (II)
(IMes)
R
N
Formation of
NHC
Complexes:
Ar =
Pd
Me
Ph
N Ar
base
H
M
N
R
(IPr)
R
N
R
N
M
N
R
N
R
imidazolium cation
Sigma Bond
Pi-Backbond
R
M
N
C
N
R
M
C
N
N
Properties of Arene Ligands
•  Arenes predominantly π-donors (stronger than 3 CO’s).
Free CO: 2143 cm-1
Mo(CO)6
CO: 2000 cm-1
Me
Me
Me
Me
Me
OC
Mo
Me
CO
CO
CO: 1970 cm-1, 1932 cm-1,1865 cm-1
π-Basicity Also Seen in Reactivity
O
NH2
Cr
OC
Cr
CO
CO
OC
less basic
than
aniline
OH
CO
CO
more acidic
than benzoic
acid
Why?
Typical Structures of Arene Complexes
Me
Me
Me
Mn
Mo
Ru
PR3
Cl
PR3
Ru
PR3
Cl
“piano-stool”
2+
Ru
Ru
η4
η4
NH3
NH3
H3N Os
H3N
NH3
η2
Preparation of Arene Complexes
Me
iPr
iPr
W
OC
CO
CO
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
OC
W
Me
iPr
+
CO
CO
Usually via displacement of weaker ligands, explosion of CO, etc.
iPr
More Complex π-Ligands
•  Consider metal arene complex.
•  Need MO’s of ligand.
M
ψ4
b2g
ψ3
e2u
ψ2
e1g
ψ1
a2u
Frost circle: easy mnemonic.
3-nodes
2-nodes
1- node
no nodes
Molecular Complex
Consider:
Cr
Cr(0) (d6):
Benzenes:
3 X 4p
1 X 4s
5 X 3d
12 MO’s
12 e–
•  However, more complex because PhH orbitals mix!
•  Need SALC’s!
SALCs Cr(C6H6)2
12 total SALC’s:
MO’s Cr(C6H6)2
Sigma Complexes
Dihydrogen Complexes
Cy2P
CO
OC W
OC
H
PCy3
H2
PCy3
COH
OC W
OC
H
PCy3
Free H2 = 0.74Å
H-H = 0.82Å
IR = 2690 cm-1
Kubas, JACS, 1984, 106, 451
“Arrested Oxidative Addition”
Typical values:
IR: 2300-2900 cm-1
NMR: ∂ 0 to –10 ppm
JH-D: 20-35 Hz
Bond length: 0.82-1.00Å
pKa: 0-15 (typically, compared to 35 for H2)
Sigma Bonds
σ*
π*
E
Sigma bonds provide
both for σ-donation
and π back-bonding.
σ∗ (H-H)
π
σ (H-H)
σ
Chatt-Dewar-Duncanson Model
H
M
H
Similar for C–H and Si–H
Reactions of Dihydrogen Complexes
Et2 H Et2
P
P
Fe
P
P
Et2H H Et2
Ph C N
Et2 H
P
Fe
P N
Et2
C
Ph
Et2
P
P
Et2
+ H2
•  Ligand exchange with strong σ-donor ligands.
PCy3
COH
OC W
OC
H
PCy3
PCy3
CO
H
OC W
OC
H
PCy3
•  Oxidative addition, can be reversible.
Metals and Ligands Matters
Compare:
Et2 CO Et2
P
P
Mo
P
P
Et2H H Et2
PPh3
H H
H Ru
Ph3P
H
PPh3
Et2P
Et2
H
P
W PEt2
P
H
Et2
CO
H
Cy3P
H H
Ru
H
H
Why do these differ?
H
PCy3
H
Si-H and C-H Sigma Complexes
•  Si-H and C-H bonds also form sigma complexes.
•  Important in hydrosilylation and C-H activation.
OC
OC
Me
Me
Me
Mn
H
Si
Ph3
Me
Ir
Me
Me3P
D
H
Me
Ir
Me
Me3P
Me
Me
Me
Me
Me
Me
H
D
Bergman JACS, 1986, 108, 1537
Me
Me
Me3P
Ir
H
D
Bond Strengths
M
H
H
~
M
H
SiR3
>
M
H
CR3
•  Might expect Si-H to be weaker donor because of steric interactions.
•  But:
•  Si-H bond is weak.
•  Low lying σ* Si-H.
•  Better back-bonding.
•  C-H is a weaker donor because of steric interactions.
Intramolecular C-H Dative Bond: Agostic Interaction
Me
Me
H
2.15 Å
H
Me
B N N
N N
MoII
CO
H
Me
C
CO
B
16 e–
First Example:
Cotton, JACS, 1974, 76, 754
1H:
Mo
3.06 Å
–3.8 ppm (b)
Termed “agostic interaction” by Brookhart and Green
Prog. Inorganic Chem. 1988, 36, 1
J. Organomet. Chem. 1983, 250, 395.
Details of Agostic Interactions
M
H
H
H
β-agostic
H
H
H
H
M
H
H
α-agostic
M
H
α-agostic
M
H
L
agostic interaction
with dative ligand