I 5.1
I
A variety of trivalent phosphorus compounds give complexes with transition metals, and these complexes are widely used as starting materials for the preparation of transition metal organometallic complexes. The two common routes for the preparation of trivalent phosphorus compounds:
3 PhMgBr + PCl
3

PPh
3
+ 3 MgBrCl
3 PhLi + PCl
3

PPh
3
+ 3 LiCl are typical of the routes used to prepare many metal-aryl complexes. The phosphines (or phosphanes to use the more modern name), PR
3
, and phosphites,
P(OR)
3
, are donor molecules (via the lone pair on the phosphorus atom) and can act as Lewis bases:
PR
3
+ BR

3

R
3
P-BR

3
The phosphines also have empty d orbitals and back-acceptance into these orbitals is also possible.
The extent to which back-donation occurs is dependant on the electronegativity of the groups attached to the phosphorus atom. For the Group 15 ligands this effect is reflected in the CO stretching frequencies.
Compound

CO (cm

1 )
[(C
2
H
5
)
3
P]
3
Mo(CO)
3
[(C
2
H
5
O)
3
P]
3
Mo(CO)
3
[Cl
2
(C
2
H
5
O)P]
3
Mo(CO)
3
(Cl
3
P)
3
Mo(CO)
3
(F
3
P)
3
Mo(CO)
3
1937
1994
2027
2040
2090
1941
1922
1969
1991
2055

I 5.2
Based on these and other data an extensive series of ligands including phosphorus donor ligands as well as other Group 15 and Group 16 ligands can be arranged in terms of

-acidity ( the

acidity series ).
A consideration of the actual stability of the metal-phosphine complexes shows that steric as well as electronic factors are important. This is revealed in the equilibrium data below:
Equilibrium Constants in Benzene at 25

C
These steric angles can be correlated with a parameter introduced by C.A. Tolman called the cone angle. This cone angle is defined as the apex angle of a cylindrical cone centred 2.28

from the centre of the P atom which just touches the Van Der
Waals radii of the outermost atoms of the model. The smaller the cone angle the greater the competitive binding ability of the ligand in situations where steric interference is important. Cone angle data for a number of phosphorus ligands are given below.
Values of the ligand cone angle,

PMe
3
PMe
2
Ph
PPh
2
Me
PPh
3
PCy
3
118
122
136
145
170

I 5.3
EXPERIMENTAL
Triphenylphosphine is prepared from a Grignard reagent (PhMgBr) and PCl
3
. The product is then used to prepare complexes of Co( II ), Ni( II ) and Cu( I ). U.V. spectra are to be recorded.
PREPARATION OF PHENYL MAGNESIUM BROMIDE
All apparatus must be dried in an oven before use, and assembled as in Figure 1.
Magnesium turnings (0.3 g) are placed in a 3 necked 50 mL round bottom flask fitted with a reflux condenser and a dropping funnel. Bromobenzene (0.4 mL), dry diethyl ether (1 mL) and a stirring bar are added to the flask. (Note: Dry ether must be used.) On warming gently with the hand the reaction should begin as evidenced by bubble formation. If the reaction does not start (a) see a demonstrator, or (b) add a crystal of iodine.
Once the reaction has commenced add, over a 5 minute period, a solution of bromobenzene (1 mL) and dry diethyl ether (5 mL) from the dropping funnel at such a rate that the other refluxes gently. Reflux for a further 15 minutes over a warm water bath to ensure complete reaction. The solution should now be cloudy and brown.
PREPARATION OF TRIPHENYULPHOSPHINE
The reaction should be carried out in a fume cupboard.
The Grignard reagent should be stirred whilst cooling in an ice bath. A solution of phosphorus trichloride (0.21 mL) in dry diethyl ether (5 mL) is added dropwise to the
Grignard reagent from a dropping funnel over a period of 5 minutes. A vigorous reaction occurs, as each drop enters a yellow solid may begin to separate; if it does then the addition is being made too rapidly.
Prepare a solution of hydrochloric acid (3 mL of concentrated acid in 20 mL of water) and place this in a 400 mL beaker surrounded by an ice-salt freezing mixture.
Slowly pour the contents of this reaction flask into the acid, a little at a time, with vigorous stirring. When the hydrolysis is complete the bulk of the triphenylphosphine will be in the ether layer. Decant off the ether layer, extract the aqueous layer with a further 2.5 mL of ether and dry the combined ether fractions over anhydrous sodium sulphate until the ether is clear. Filter off and evaporate the filtrate to obtain a yellow semi-liquid liquid residue which solidifies overnight to yield 0.7 g of crude

I 5.4 triphenylphosphine. Recrystallize the product from ethanol, retaining a few particles of the solid to seed the cooled ethanolic solution. Filter off the white crystalline triphenylphosphine in a Buchner funnel, washing the product with 1 mL portions of ethanol until all traces of yellow colour have disappeared. Melting-point 79-80

C.
PREPARATION OF TRIPHENYLPHOSPHINE COMPLEXES
(a) [Co(PPh
3
)
2
Cl
2
]. Dissolve finely powdered pink cobalt( II ) chloride
(CoCl
2
.6H
2
O) (0.11 g; 0.4 mmole) in absolute ethanol (1.5 mL) to obtain a rich blue solution. Add a solution of triphenylphosphine 0.25 g; 0.8 mmole) in absolute alcohol
(2 mL) (heat if necessary) to obtain an immediate precipitate of bright blue crystals which are collected in a Buchner funnel and washed with a little absolute ethanol before drying. Report the yield and the melting point. Record the UV-vis spectrum of the compound in a suitable solvent, give the concentration on the printout (hint: what solvent was the compound precipitated from? Is the compound charged?)
(b) [Ni(PPh
3
)
2
Cl
2
]. Stir nickel( II )chloride (NiCl
6
.6H
2
O) (0.20 g; 0.8 mmole) in acetic acid (8 mL) to obtain a very dark-green crystalline precipitate which becomes almost black on standing. Filter off in a Buchner funnel and wash with ether to remove unreacted triphenylphosphine, and then a little acetic acid before drying.
Report the yield and the melting point.
(c) [Cu(PPh
3
)Cl]
4
. Dissolve copper( II ) chloride (CuCl
2
.2H
2
O) (0.10 g: 0.6 mmole) in ethanol (3 mL) to obtain a green solution. Add a solution of triphenylphosphine (0.25 g); 0.8 mmole) in ethanol (5 mL) and heat the mixture under reflux for a short time until the green colour disappears indicating that the copper( II ) has been reduced to copper( I ) by the triphenylphosphine. At the same time an off-white solid is deposited, which is filtered off in a Buchner funnel and washed with ethanol (2 cm 3 ) to obtain fine pale-beige crystals. Report the yield and the melting point.
Thus, triphenylphosphine is prepared from a Grignard reagent rather than phenyllithium as the latter is air-sensitive and requires an inert atmosphere for good yields. Reaction of triphenylphosphine with the metal salts yields stable complexes.
These are readily characterized by U.V. spectroscopy. They could be further reacted with more Grignard reagent to yield metal-carbon bonded complexes e.g.:
[M(PPh
3
)
2
Cl
2
] + 2PhMgBr [Ph
2
M(PPh
3
)
2
] + 2MgBrCl

I 5.5
Phosphines, (especially triphenylphosphine, because of its stability in air) have been used as ligands in thousands of organometallic complexes. Some of these complexes e.g. Wilkinson's catalyst, Rh(PPh
3
)
3
Cl are of great commercial importance.
REPORT
1. Report any variations from the experimental procedure listed.
2. Report percentage yields and melting points as indicated.
3. Visible spectra and their assignments must also be reported for the Co( II ) complex only.
4. Look up the cone angle of PPh( 1 Bu)
2
. Comment on its size (

value).
5. Would

for PMe
3
increase or decrease if the M

P bond distance increased from 2.28

. Show your reasoning.
6. Explain clearly and concisely the origin of the trends in the carbonyl stretching frequencies of the complexes given on the first page of this practical.
7. Do the complexes Rh
4
(Co)
11
(PPh
3
), (
 5 -C
5
H
5
)Re(CO)(PPh
3
) I , and
Co
2
(CO)
6
(PPh
3
)
2 obey the 18 electron rule? Draw possible structures for the complexes. You may wish to look up related structures in a text book if necessary.
REFERENCES
1. C.A. Tolman, Chem. Reviews , 77 , 313 (1977)
2. F.R. Hartley, " Elements of Organometallic Chemistry ", Chemical Society,
London, 1974.