Download Assignment_6_solution

CYL 100 Inorganic Part
Home assignment and Solved problems for self study- Part 2
Organometallic Chemistry (18e rule, carbonyls, sandwich
compounds, unique reactions)
Solved problems for self study- Part 2
Points to remember;
• Follow only one counting method initially. Easier one is neutral atom counting
method although more realistic is oxidation count method.
• Note that square planar late transition metal compounds and titanium group
metals may show 16 electron stability as well
• All carbon based ligands even if they have a formal + charge are more
electronegative when compared to the transition metal to which it is bonded
• Compare a metal – metal bond in a complex to the Cl-Cl bond of Cl2 if you are
unable to sort out the electron source of the bond.
• A metal – metal bond like Cl-Cl bond does to contribute to the oxidation state of
the metal to which it is attached
1.
Consider 18 electron rule as a guide and determine the value of n in the following
complexes
i)
iii)
(v)
Na2Fe(CO)n
W(η6-C6H6)(CO)n
Cr(η3-C5H5)(CO)n(CH3)
ii)
iv)
(vi)
MnBr(CO)n
Rh(η5-C5H5)(CO)n
IrBr2(CO)n (PPh3)2(CH3)
solution
i)
iii)
(v)
2.
Na2Fe(CO)n n= 4
n =3
W(η6-C6H6)(CO)n
3
Cr(η -C5H5)(CO)n(CH3) n = 4
ii)
iv)
(vi)
MnBr(CO)n n = 5
Rh(η5-C5H5)(CO)n n=2
IrBr2(CO)(PPh3)2(CH3) n =1
A metal complex having the empirical formula FeC9H7O3Cl, has one ligand as a
substituted cyclopentadienyl group. It is an ionic compound and has poor
solubility in hydrocarbon solvents. The compound gives a white precipitate on
treatment with AgNO3 solution. Assuming that it obeys the 18 electron rule,
suggest its structure.
solution
CH3
Fe
OC
CO
CO
Cl
3.
In a carbonyl complex with linear CO-M-CO group, how will νCO change when
a) one CO is replaced by Et3N b) a positive charge is put on the complex c) a
negative charge is placed on the complex.
Solution
νco will change as follows,
a) Since Et3N is a sigma donor only and not an acceptor, the back donation
from M to the CO’s will increase and hence υco will decrease. b) less back
bonding because of positive charge and hence υco increases c) back
bonding increases because of negative charge, and hence υco decreases.
4.
Give a scheme for the synthesis of Mn(CO)4(PPh3)[C(O)CH3] starting from
Manganese acetate, Mn(OAc)2.
Solution
2 Mn(OAc)2 + 4 Na + 10 CO
high temp
high pressure
Mn2(CO)10 + 4 NaOAc
Mn2(CO)10 + 2 Na
2 NaMn(CO)5
NaMn(CO)5 + CH3I
CH3Mn(CO)5
CH3Mn(CO)5 + CO
CH3C(O)Mn(CO)5 + PPh3
CH3C(O)Mn(CO)5 ( migratory insertion)
CH3C(O)Mn(CO)4PPh3
hv
Or at step 3 direct reaction with acyl chloride instead of MeI. Step 1 other
reducing agents e.g. AlEt3 can also be used.
5.
Fluorene, a polycyclic organic compound, forms an ionic organometallic
compound with a CpFe fragment and obeys the 18e rule. It has the molecular
formula [(C5H5)Fe(C13H10)]+[PF6]− (A). Treatment of A with a base results in
another 18e organometallic complex B with the formula [(C5H5)Fe(C13H9)].
Draw reasonable structures of complexes A and B.
Fluorene
H
H
H
H
H
H
H
solution
H
H
H
PF6
Fe
H
H
A
Fe
B
6. Which among the following compounds will not undergo oxidative addition of
methyl iodide ? Give reasons
(a) Ir(PPh3)2(CO)Cl
(b) [RhI2(CO)2](c) η5-Cp2Ti(Me)Cl
Solution
(c) As titanium is already in its highest oxidation state of +4 and therefore no
d electrons present for further oxidation
7. Mark against each statement the appropriate type of reaction or reactions from
the list (oxidative addition, reductive elimination, migratory insertion and β-H
transfer)
(a) cis orientation of the participating ligands is a must.
(b) This reaction does not occur for d0 metal complexes.
(c) This reaction is enthalpy favoured and entropy prohibited.
(d) A vacant coordination site on the metal centre is a prerequisite.
(e) The more electron rich the metal centre, the more facile is the reaction.
(f) There is an increase in the electron count of the metal complex by two units
during this reaction.
Solution
(a) Red elimination and migratory insertion
(b) Oxidative addition
(c) Migratory insertion
(d) Oxidative addition and β-H transfer
(e) Oxidative addition
(f) Oxidative addition and β-H transfer
8. Using the concept of oxidative addition and reductive elimination, give a
suitable mechanism for the formation of each of the products in the following
reaction
(Et3Si-H may be considered as a non polar/weakly polar system)
Ph2
P
OC
Ir
Br
Solution
P
Ph2
H
+ excess Et3Si-H
Et3Si
Ph2
P
+ HBr + Et3Si-SiEt3
Ir
H
CO
P
Ph2
H
Ph2
P
OC
oxidative
addition
Et3Si
Ir
+ Et3Si-H
Ir
Br
16e
Br
P
Ph2
18e
CO
Ph2
P
Ph2
reductive
Et Si
P
elimination 3
Ir
-HBr
OC
16e
P
Ph2
+ Et3Si-H
H
Et3Si
Ph2
P
oxidative
addition
Ir
H
18 e CO
P
Ph2
+ Et3Si-H
Ph2
P
H
Ir
OC
16e
P
Ph2
P
Ph2
oxidative
addition
H
reductive
elimination
-Et3Si-SiEt3
Et3Si
Ph2
P
Ir
Et3Si
18e
CO
P
Ph2
Home Assignement 11 problems Inorganic Part Part 2 (submit on
March 27 Friday)
1. The complex [M(η6-C6H6)(CO)2]2 obeys 18 electron rule and has one metal- metal
double bond. Predict M and sketch the probable structure of the complex.
2. Determine the unknown quantity
a) [CpW(CO)x]2 [has W-W single bond]
b) [(CO)3Ni-Co(CO)3]z
c) [CpMn(CO)x]2 (has Mn = Mn double bond]
d) [(Cp)3Ni3(μ3-CO)2]z (no metal metal bonds) ( hint: consider unit as a whole)
e) [CpM(CO)3]2 (has a single M-M bond; M = Ist row TM]
3. Trimetallic complexes of phosphido bridges are well known. Assuming that the
18e rule is followed, postulate the structure of [Mn(μ-PH2)(CO)4]3.
4. Arrange the following in the decreasing order of back donation
Cr(CO)6; [Ti(CO)6]-2 ; [Mn(CO)6]+ ; [Ir(CO)6]3+; [V(CO)6]-
5. The compound (η5-C5H5)Co(CO)2 (A) upon UV irradiation, resulted in the
evolution of equimolar amount of a gas and formation of a new compound B.
While the infrared spectrum of A showed absorptions around 1988 cm-1, the
spectrum of B showed absorptions only around 1798 cm-1. Compounds A and B
both obey 18 electron rule and both has the cyclopentadienyl group in the η5
mode. Draw the structure of compound B.
6. Select the best choice in each of the following and briefly justify the reason for
your choice
a)
Complex with the shortest C-O bond
Ni(CO)4,
b)
[Co(CO)4]-,
[Fe(CO)4]2-
Complex with the lowest C-O stretching frequency
V(CO)6-
Cr(CO)6
Fe(CO)62+
7. Ferrocene is thermally stable and not air sensitive but cobaltocene is readily
oxidized to cobaltocenium ion. Why ?
8. The following are some reactions of first row transition-metal containing sandwich
compounds, having η5-Cp ligands leading to products with 18 electrons. Identify
the transition metals A, B and C present in these compounds.
K
H2
A
H
Na/Hg
B
A
excess C2H4
- KC5H5
H
B
H
H
CO
C
- 1/2 C5H5 dimer
OC
C
CO
CO
9. Benzene, on heating with D2 and the precatalyst η5- Cp2TaH3, gets converted to
monodeuterobenzene and finally to hexadeutero benzene. The reaction involves
only oxidative additions (both C6H5-H and D-D) and reductive eliminations.
Suggest a mechanism for the formation of monodeutero benzene (C6H5-D) from
benzene.
( hint: the active form of the catalyst is a 16 electron species )
10. The given metal-alkene intermediate is unstable and undergoes a rearrangement
resulting in the formation of three different arylalkene complexes of the general
formula Pd(PPh3)2(Br)(H)(η2 -alkene). Predict the structures of the three
arylalkene palladium complexes.
Br
Pd
Ph3P
PPh3
CH
C
H2
H2C
CH3
11. Wilkinsons catalyst, RhCl(PPh3)3 catalyzes the reaction of trimethylsilyl hydride
[(CH3)3Si-H] with ethylene to give ethyltrimethylsilane[(CH3)3Si-CH2CH3]. Show
steps involved and mark the electron count and type of reaction at each step.