This presentation describes about the nature of phosphine ligands, bonding and reactions of metal phosphine containing complexes. Also explains the similarity and differences of phosphine ligand with NH3 and CO ligands.

1. Dr. Geeta Tewari
Department of Chemistry
D. S. B. Campus
Kumaun University, Nainital
Email: geeta_k@rediffmail.com
Metal π Complexes, Part 9,
Teriary Phosphine as ligand
CC BY-NC-SA 3.0

2. Introduction
 Phosphine ligands are phosphines with a chemical formula of
PRR'R" (R, R', R" = H, alkyl, aryl, etc) that are used
as ligands in metal complexes. When R, R’ and R’’ are alkyl or
aryl groups, it is called as tertiary phosphine.
 In tertiary phosphines, the electronic and steric properties can
be changed with varying nature of R group. Small changes in
ligand can entirely change the chemistry.
 It acts as spectator ligand.
• PR3 (R = Carbon groups) - Phosphine (US), Phosphane
(Germany / Europe)
• PR3 (R = OR groups) - Phosphite
 May be -donors and/or π-acceptors.

3.  Like NR3, phosphines also have a lone pair of electron on the
central atom that can be donated to a metal (Lewis base; σ-
donor).
 Unlike NR3, they are π-acids (Lewis acid; π – accepter
ligands). This nature depends on the nature of the R groups
present on PR3 ligand. For alkyl phosphines, the π-acidity is
weak while for aryl, alkoxy and dialkyl amino groups, π-
acidity is high as these groups promote π- acidity due to
deficiency of electrons.
 For PF3, pi-acidity is very high.
Similarity and dissimilarities between
phosphine and amine ligands

4.  In case of CO, the π* orbitals accept electrons from the metal
while σ* orbital of the P – R bond accepts e– in PR3.
 As the R group becomes more electronegative, the orbital that the
R group uses to bond with phosphorus becomes more stable
(lower in energy). Thus, the σ* orbital of the P–R bond become
more stable. In the same line, the phosphorous contribution to σ*
orbital increases and therefore, the size of σ* lobe points towards
the metal increases.
 These two factors (lower energy of σ* orbital of the P–R bond and
large size of σ* orbital), increase π-acid character of PR3 (π-
acceptor).
Similarity and dissimilarities between
phosphine and carbonyl ligands

5. Similar to CO, N2, O2, when PR3 form bond with
metal, the P – R bonds lengthen slightly with
simultaneous shortening of the P – M bond.

6. PMe3  P (NR2)3 < PAr3 < P (OMe)3 < P (OAr)3 < PCl3 < CO  PF3
M (dyz + P(σ*) = M – P π bond
Similarity and dissimilarities between
phosphine and carbonyl ligands
M P
R
R
R
dyz sigma antibonding MO
(P-C)
(P-N)
(P-F)
(P-F)
P
*


7. Bonding

8. Electronic effect of phosphine ligand
 Electronic effect of phosphine ligand can be compared by
changing the R group in PR3 in (PR3)Ni(CO)3 complex.
PR3 υC-O (cm)
PMe3 2064
PPh3 2069
PCl3 2097
PF3 2111
Ni
CO
PR3
OC
OC

9. Steric effect of phosphine ligand
 Variable size of phosphine ligand.
 Bulky PR3 ligand favours low coordination number in
a complex.
PR3 CN
P(i-Pr)3 2
PPh3 3
PMe2Ph 4
PMe3 5

10. Steric effect of phosphine ligand
 The bulkiness of a phosphine ligand can be
determined by their 3-D space-filling models.
 Tolman gave the name cone angle () to understand
the approximate amount to “space” that the phosphine
ligand consume around the metal center.
P
M
R
R
R


11. Electronic and steric effects of phosphine
ligands given by Tolman (Tolman plot)

12.  Tolman plot is helpful in determining the change in the electronic
effects in phosphine ligand without changing steric effects
by moving vertically (from P(OEt)3 to PF3
 or change in the steric effects without changing the electronic
effects
by moving horizontally [from PMe3 to P(o-tolyl)3]
Electronic and steric effects influences the nature of ligands which is
further useful for selection of desired activity or selectivity of
phosphine ligand (as homogeneous catalysts, reversible binding of a
ligand, high stability, or facile decomposition)
Electronic and steric effects of phosphine
ligands given by Tolman (Tolman plot)

13.  The phosphine ligands can be classified as good σ- donor and
good π- acceptors.
 The presence of alkyl group in phosphines are strong -
donors. The order of their σ- donor ability is:
PMe3 (118º)>PMe2 Ph (122º)>PEt3 (132º)>PMe Ph2 (136º)>P (t–
Bu)3 (182º)
 Phosphites are relatively poor -donors, but are good π-
acceptor ligands (about half as good as CO). The order of π-
acceptor tendency of phosphite ligands is:
P(OMe)3 (107º) > P(OEt)3 (110º) > P(OPh)3 (128º)

14.  Electron withdrawing groups increase π- acceptor
tendency of phosphate ligands. Hence, PF3 (104º) is
poor -donor and strong π-acceptor ligand (almost as
good as CO). The overall order of σ-donor and π-acid
character of all the phophines is:
PMe3≈P(NR2)3<PAr3<P(OMe)3<P(OAr)3<PCl3<CO≈PF3
π-acceptor tendency increased

15. Some structural aspects of phosphine ligands
1) Phosphines generally have a tendency to orient trans to one
another so that they can minimize steric interactions
(especially true for bulky PR3).
2) Average bond distances of some first row M- PR3:
Ti-P 2.6 Å
Cr-P 2.4 Å
Ni-P 2.1 Å
• These data suggest that M–P distances decreases due to
contraction of the metal atom radius. Distance usually also
decreases due to stronger M–P bonding. Later transition metals
are soft as compared to other metals and prefer bonding to
phosphines.

16. 3) M–P bonds are the strongest for alkylated phosphine
ligands when bonded to an electron deficient metal
atom/ ion.
 Hence, PMe3 (a -donor), bonds strongly to Ti (+4)
d0 center in TiCl4 (PMe3) as compared to PCl3.
 Electron-rich metal centers cannot form bond with
strong electron-donating alkylated phosphine ligand
which leads to weaker M–P bonding and phosphine
dissociation is the main feature of such type of
compounds.
Some structural aspects of phosphine ligands

17. Reactions shown by phosphines
• Phosphine with steric hindrance [di–t-butyl
phosphines, P(BuR)3 (R = alkyl or aryl)] promote less
hindrance features such as hydride formation and
stabilization of unusual oxidation states such as IrII,
IrI, RhI and generates coordinate unsaturation at the
metal center.
RhCl(PPh3)3  Wilkinson’s catalyst (RhI )
IrCl(CO)(PPh3)2  Vaska’s compound (IrI )

18.  A 4-coordinate d8 ion is a 16-electron species and is
coordinately unsaturated species
Rh (I)  d8 (8 + 8 (from ligand) = 16 e-
Ir (I)  d8 (8 + 8 (from ligand) = 16 e-
 Saturation of these metal ions (IrI, RhI ) require the addition of
10e– (5-ligands) to become a 18e- species.
 Similarly Rh(III), a d6 ion can expand its coordination sphere
to accommodate 6 ligands.
Reactions shown by phosphines

19. Rh Cl
PR3
PPh3
PR3
Rh
H
Cl
H
PR3
PR3 PPh3
Rh
H
Cl
H
PR3
PR3
Rh
H
CO
H
PR3
PR3
Rh
H
Cl
C C H
PR3
PR3
Rh
PR3
PR3
Cl
Oxidative
addition
+H2
-PR3
Elimination of
one phosphine
and one vacant
site is being provided
for alkene
Addition of alkene
Alkene insertion
Reductive elimination
PR3
addition
16e-
species
Wilkinson's catalyst
(Coordination unsaturation
is present at RhI
)
18e-
species
(RhIII
)
(RhIII
)
(RhIII
)
((RhIII
)
(RhI
)
C2H6
Reactions shown by phosphines

20. Thank You