Pi - Complex

Pi-complex coordination compounds are a class of chemical species formed through the interaction of transition metal ions with pi-electron systems, such as alkenes, alkynes, and aromatic compounds. The metal complex of these compounds has the ability to coordinate with the pi-electron of the ligands which results in unique electronic and spectroscopic properties. This complex has significance in both theoretical and in practical aspects.

Pi-complex

These are the compounds of metals with alkenes, alkynes, benzene, and other ring compounds. In these complexes, the metal and ligand form a bond that involves the π electrons of the ligand. Three common examples are Zeise's salt, ferrocene, and dibenzene chromium. These are shown in the figure below:

Zeise' salt Ferrocene Dibenzene chromium

The number of carbon atoms bound to the metal in these compounds is indicated by the Greek letter 'η' with a number. The prefixes η², η^5, and η⁶ indicate 2, 5, and 6 carbon atoms are involved in the formation of the bond with the metal atom in the compound.

Properties of Pi-complex

• Bonding Nature:

In bonding involves both the sigma-donation from the ligands and pi-bonding from the metal which enhances the stability of the complex

• Stability

pi-complex are generally very stable and the stability is influenced by the factors such as the nature of the metal, the pi-ligand, and the environment of the complex.

• Magnetic properties

In general the presence of the presence of unpaired electrons in the pi-complex can lead to paramagnetism. The magnetic can also be influenced by the electronic configuration of the central metal atoms and nature of ligands.

Recommended topic video on ( Pi- Complex)

Some Solved Examples

Example 1
Question: Among the given complexes, identify the one(s) which is/are also (pi)-complex(es):
(a) Zeise's salt
(b) Ferrocene
(c) Dibenzene Chromium

d) none

Solution: Zeise's salt, Ferrocene, and Dibenzene Chromium are all (pi)-complexes. These complexes involve the metal-ligand bond formation by the donation of (pi) electrons from the ligands to the metal.

Hence, the correct answer is Option (4): (a), (b), and (c).

Example 2
Question:The sum of bridging carbonyls in W(CO)₆ and Mn₂(CO)₁₀ is ______________

a) 0

b) 3

c) 1

d) 2

Solution:

W(CO)₆ has 0 bridging CO groups.
Mn₂(CO)₁₀) also has 0 bridging CO groups.

Thus, the sum of bridging carbonyls in both complexes is 0.

Hence, the correct answer is Option (1): 0.

Example 3
Question: Given below are two statements: one is labeled as “Assertion A” and the other is labelled as “Reason R”

- Assertion A: In the complex Ni(CO)₄ and Fe(CO)₅, the metals have zero oxidation state.
- Reason R: Low oxidation states are found when a complex has ligands capable of (pi)-donor character in addition to the (sigma)-bonding.

In the light of the above statements, choose the most appropriate answer from the options given below:

1) A is not correct but R is correct.
2) A is correct but R is not correct.
3) Both A and R are correct and R is the correct explanation of A.
4) Both A and R are correct but R is NOT the correct explanation of A.

Solution:
Statement A is correct as the metals have zero oxidation state in both Ni(CO)₄ and Fe(CO)₅.
Statement R is incorrect because ligands must have (sigma)-donor and (pi)-acceptor characters for the metal to exhibit a low oxidation state.

Hence, the most appropriate answer is Option (2): A is correct but R is not correct.

Example 4
Question: Which among the following complexes has the metal in the highest oxidation state?

(a) [Cr(CO)₆

(b) [Mn(CO)₅Br]

(c) Fe(CO)₅

(d) [Co(CO)₄]

Solution:

Cr(CO)₆): Chromium is in the 0 oxidation state.
[Mn(CO)₅Br]: Manganese is in the +1 oxidation state due to the presence of a bromide ion.
Fe(CO)₅): Iron is in the 0 oxidation state.
[Co(CO)₄]: Cobalt is in the 0 oxidation state.

The complex [Mn(CO)₅Br] has the metal in the highest oxidation state of +1.

Hence, the correct answer is Option (b):[Mn(CO)₅Br]

Example 5
Question: Identify the complex where the metal exhibits the highest coordination number:

(a) [Fe(CO)₅]

(b) [Co(NH₃)₆]³⁺

(c) [PtCl₄]^2-

(d) {[Cr(en)₃]³⁺

Solution:

[Fe(CO)₅]): Iron has a coordination number of 5.
[Co(NH₃)₆]³⁺): Cobalt has a coordination number of 6.
[PtCl₄]^2-: Platinum has a coordination number of 4.
[Cr(en)₃]³⁺): Chromium has a coordination number of 6 (each ethylenediamine (en) ligand counts as 2 due to its bidentate nature).

The complexes [Co(NH₃)₆]³⁺)) and [Cr(en)_3]³⁺ both have the highest coordination number of 6.

Hence, the correct answer is Option (b) and (d): [Cr(en)₃]³⁺) and [Cr(en)₃]³⁺).

Summary

The study of pi-complex coordination compounds is vital in various fields, which include material science catalysis, and bioinorganic chemistry, they have a very important role in the olefination, hydrogenation, and polymerization. The pi-complex has a great contribution to the field of coordination chemistry such as in the ligand field theory and also in the bonding.