Question

1) A. How does the following modifications affect the rate of a square planar substitution reaction?

a) changing the trans ligand from H- to Cl-

b) changing the leaving group from Cl- to I-;

C) adding a bulky substituent to a cis ligand,

d) increasing the positive charge on the complex.

B. Organometallic compounds are heavily used in catalysis. Write out details of the reactions (including reaction cycle) in both Hydrogenation of alkanes with Wilkinson’s catalyst and Hydroformulation with cobalt carbonyl catalyst (name the catalyst in each case)

Answer 1

A)

a)

when you observe the trans effect series( CO>CN>C2H4>PR2>H>Me>Ph>Py>I>Br>Cl>NH3)

Hydride ion lies higher than the chloride ion.thus if the ligand trans to H or Cl is the leaving group, so the rate of substitution will be decreased if H is changed to Cl.

b)

The rate at which a ligand is substituted in a square-planar complex is related to its position in the trans effect series(And as mentioned in the above series (I>cl) ) which means, If a ligand is high in the series, it is a good entering group that is it is a good nucleophile and a poor leaving group. Since iodide ion is higher in the trans effect series than chloride ion, it is a poorer leaving group than chloride ion. Therefore, the iodide complex will undergo I- substitution more slowly than the chloro complex will undergo cl- substitution.

c)

when a bulky group is added the complex substitution will undergo steric hindrance and hence will slow down the complex formation.

so the rate of square planar substitution decreases with the addition of the bulky group.

d)

the increase in positive charge increases the rate of reaction because it would be difficult to carry out the dissociative reaction which would in turn make the metal-ligand bond breaking difficult by increasing the electron density in the solution

B)

name : chloridotris(triphenylphosphane)rhodium(I)

name of the catalyst: Co2(CO)8