Download Chem+174–Lecture12a

Phosphine complexes
 In order to understand what a ligand does, one has to
look at its electronic and its steric properties
 The reaction conditions (kinetic and thermodynamic
control) during the reaction determine the configuration
is observed in the product (cis-trans, fac-mer)
 In many cases, there is an equilibrium in solution,
which can be detected by NMR or infrared spectroscopy
 The polarity of the solvent determines which product
precipitates i.e., SnCl4(THT)2: dichloromethane (trans),
pentane (cis)
 L as p-complex only (C2H4, alkenes)
 Ligands like ethylene form strong p-complexes with
low-valent metals
 The HOMO is the C=C p-bond, which is used to form
the M-L s-bond
 Often times, there is also a back-bonding into the
p*-orbital of the C=C bond
 Example: Zeise’s salt (K[PtCl3(h2-C2H4)])
 L as s-complex only (H2)
 Molecular hydrogen does not exhibit a lone pair or a
p-bond, yet it binds to some metal centers as intact
molecule (meaning it does not perform an oxidative
addition!)
 The s-bond of the H2 molecule is the electron donor
in this bond (red bond), while the s*-orbital acts as
an acceptor for the back-bonding (blue bond)
 In order to maximize the overlap, the H2-molecule
binds side-on
 Example: [W(h2-H2)(CO)3(PR3)2],
[OsCl2(h2-H2)(CO)(P(iPr)3)2]
 Sigma complexes are also found for C-H, Si-H, B-H
and M-H groups
 L as s-donor only (NH3, NR3)
 The metal has to exhibit a medium or high oxidation state
in order for these complexes to be stable
 Metal acts as a hard acid and the ligand as a hard base
 Examples: [M(NH3)4]2+ (M=Cu, Zn),
[M(NH3)6]2+ (M=Co, Ni)
 L as s- and p-donor (H2O, OH-, OR-, NR2-, F-)
 The metal has to exhibit a medium or high oxidation state
in order for these complexes to be stable
 The ligand acts as very hard base and the metal as hard acid
 Examples: [Ni(H2O)6]2+, [CoF6]3-, [Sn(OH)6]2-
 L as s-donor and p-acceptor (CO, CN-, NO)
 The metal has to exhibit a low oxidation state in order for these
complexes to be stable
 The s-bond is formed from the sp-orbital of the carbon atom
with a suitable empty d-orbital of the metal while the
p-backbond is formed by the interaction of a filled d-orbital
of the metal with the p*-orbital of the carbonyl group
 The ligand and the metal act as base
 Examples: Mo(CO)6, [Fe(CN)6]4-, [Co(NO)4]
 L as s-donor and s*-acceptor (PR3)
 In the older literature, phosphine ligands
are often referred to as p-acceptors
 In the more recent literature (after 1980),
they are usually referred to as s*-acceptor
 As electron-withdrawing groups (i.e.,
halogen atoms) are placed on the
phosphorous atom, the s-donating capacity
of the phosphine ligand tends to decrease
 At the same time, the energy of the
s*-orbital on phosphorous is lowered
in energy, providing an increase in
backbonding ability (p-acid)
 The degree of p-acidity largely depends on the substituents
on the phosphorus atom
 While alkyl phosphines are weak p-acids, the acidity increases
for aryl, dialkylamino and alkoxy phosphines
 The extreme cases are PCl3 and PF3, which is equivalent to CO
in its p-acidity because more electronegative elements on the
phosphorous atom stabilize the s-bond and lower the energy
of the s*-orbital (see diagram)
 The contribution of the phosphorus atom to the s*-orbital
increases and the size of the orbital pointing towards the metal
as well allowing for a better overlap
 Based on this argument, the order of p-acidity of phosphines is
 PMe3 < PAr3 < P(OMe)3 < P(OAr)3 < PCl3 < PF3 ≈ CO
 Aside of the p-acidity, the steric impact of the phosphine ligand
has to be considered as well
 C.A. Tolman (Chem. Rev. 1977, 77, 313) summarizes the
electronic parameters and cone angles of phosphine ligands:
 The electronic parameter can be adjusted by changing the
R-group (see above). Stronger donor groups increase the electron
density on the metal atom, which is capable of more backbonding
to ligands like CO, CN, etc.
 Tolman observed for Ni(CO)3L that the carbonyl stretching
frequency decreases as the donor ability of the R-group
increases (i.e., PCy3 (2056 cm-1) vs. P(OMe)3 (2070 cm-1) vs.
PF3 (2111 cm-1)).
 The second important parameter is the steric size, which can also be
controlled by changing the R-group.
 Very bulky phosphines often favor low-coordinate compounds, which
can coordinate additional small ligand as observed in catalytic cycles
 Metals like Mo and W can coordinate up to six PMe3 ligands
(i.e., M(PMe3)6)), while a maximum of four PPh3 ligands
(i.e., M(PPh3)4, M=Pd, Cu+, Ag+, Au+) or two PCy3 ligands
(i.e., Cu+, Ag+, Au+, Ni2+, Pd2+, Pt2+) can be coordinated to a metal center
 Thus, the bulkiness of the phosphine ligand can be quantified by its cone
angle (Q)
 The observed cone angles for phosphines range from Q=87o (PH3) to
Q=212o (P(mes)3) (neither one is shown in the diagram below).
 The cone angles for PMe3, PPh3 and PCy3 are Q=118o, Q=145o and
Q=170o, respectively, consistent with the observations above.
 Generally, phosphines with aryl groups or highly branched alkyl chains
exhibit large cone angles while phosphite have much smaller cone angles
 The ability of a metal to perform backbonding can easily
be tuned by manipulating the electronic effect of the
phosphine ligand.
 For instance, a change of the ligand from PBu3 to P(OiPr)3,
which possess virtually identical cone angles, decreases the
ability of the metal for backbonding as can be seen from the
higher carbonyl stretching frequency in Ni(CO)3L.
 If the same electronic effect is desired but a larger cone
angle to lower the number of coordinated ligands, one could
move from PBu3 to P(iPr)3, which exhibits a 30o larger cone
angle, but is electronically speaking identical.
 These complexes can easily be prepared from Mo(CO)6
by the reaction with one equivalent of L
 The resulting compounds exhibit colors ranging from
white to red depending in the ligand L
 95Mo-NMR and infrared spectroscopy can be used to
assess the effect of the ligand L on the metal and the
remaining CO ligands

95Mo-NMR
studies have shown that the chemical shift
varies significantly with the ligand
Ligand
d(ppm)
 Ligands that are good s-donors, but poor or
no p-acceptor causing a significant decrease
in the HOMO-LUMO gap, which results in
a deshielding of the Mo-nucleus
 Ligands that are s-donors and good
p-acceptor i.e., PF3 and P(OR)3 are
comparable to the CO ligand itself
Piperidine -1433
CH3CN
-1440
PCl3
-1523
PCl2Ph
-1615
PClPh2
-1702
PPh3
-1743
PBr3
-1396
PF3
-1860
P(OPh)3
-1819
Mo(CO)6
-1857
PPh3
Mo-C (trans)
C-O (trans)
Mo-C(trans) vs. Mo-P
256
199.6
114.1
P(2-MeOPh)3
258.8
198.1
114.3
P(2,4,6-MeOPh)3
263.6
197.4
114.5
P(NC5H10)3
260.5
198.5
114
PCy3
259.4
197.2
115.4
PMe3
250.8
198.4
115.1
PCl3
237.9
203.5
113
204
Mo-C(trans)
Mo-P
R² = 0.8273
202
200
198
196
230
240
250
260
270
Mo-P (pm)
 If the phosphine ligand is a good p-acid, the Mo-P bond is
very short (i.e. PCl3) and the Mo-C bond is fairly long
 If donor groups are attached to the phenyl group, the Mo-P
bond length increases while the Mo-C bond length
increases because the phosphorus atom becomes a weaker
p-acid
 For the sequence the chemical shift
Mo(CO)5(PPhxCl(3-x))
in the 95Mo-NMR spectrum follows
a straight trend
-1200
 The chemical shift depends linearly
-1400
with the cone angle of the phosphine
 The weaker of a p-acid, the
phosphine is, the more negative the
chemical shift is because the
Mo-atom is more shielded
 The comparison of the first and
the second diagram shows that a
larger number of phosphine
groups increases the effect
(~200-300 ppm/group)
120
125
130
135
140
145
150
-1300
-1500
PCl3
-1600
PPhCl2
-1700
-1800
PPh2Cl
PPh3
R² = 0.972
Mo(CO)4(PPhxCl(3-x))2
-1200
120
PCl3
125
130
135
140
145
150
-1300
PPhCl2
-1400
-1500
PPh2Cl
-1600
-1700
R² = 0.9373
PPh3
 The di- and trisubstituted
Ligand
Piperidine
compounds exhibits the same
CH3CN
trends like the monosubstituted PCl
3
compounds, just to a much larger PCl2Ph
degree i.e., L=PPh3 (d=-1743ppm, PClPh2
PPh3
-1556 ppm, -1265 ppm)
PBr3
 Note that all disubstituted
PF3
compounds are in cisP(OPh)3
configuration while the
Mo(CO)6
trisubstituted compounds are in
fac-configuration.
Mo(CO)4L2
Mo(CO)3L3
-1093
-1307
-1112
-1206
-910
-1369
-1124
-1522
-1320
-1556
-1265
-977
-1860
-1819
-1857
-1860
 Wilkinson’s catalyst (RhCl(PPh3)3)
 It is obtained by the reaction of RhCl3 with
four equivalents of triphenylphosphine as a
red-violet solid (note that the phosphine
acts as ligand and as reducing reagent
here)
 It exhibits a square-planar coordination of
around the Rh(I)-ion (d8)
 It catalyzes the hydrogenation of alkenes
 The complex itself is the 16 VE system
 Step 1:The dissociation of one




triphenylphosphine ligands to
give 14 VE complexes
Step 2: Oxidation addition of H2
to the metal (cis)
Step 3: The π-complexation of
alkene to the metal
Step 4: Intramolecular hydride
transfer (olefin insertion)
Step 5: Reductive elimination
results in extrusion of the alkane
product
 When the triphenylphosphine ligands are replaced by
chiral phosphines (i.e., DIPAMP), the catalyst becomes
chiral and converts prochiral alkenes into enantiomerically
enriched alkanes via the process called asymmetric
hydrogenation (i.e., L-DOPA process, Monsanto)