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.
This Story also Contains
- Pi-complex
- Properties of Pi-complex
- Some Solved Examples
- Summary
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, η5, 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.
