MCQ 529 of 933 | IITJAM (IIT Joint Admission Test) Chemistry

Boranes

There are three important structures of borane (Closo-, Nido-, Arachno-). In these structures, the boron atoms are occupying the corners of a polyhedron in which the boron atoms can be bound together or not. These structures are so-called cage structures.

• ${\displaystyle B_n{H}_n^{{\displaystyle 2-}}}$ and ${\displaystyle B_n{H}_{{\displaystyle n+2}}}$ is closo-type.
• ${\displaystyle B_n{H}_{{\displaystyle n+4}}}$ is nido-type
• ${\displaystyle B_n{H}_{{\displaystyle n+6}}}$ arachno-type

According to wades rule, the clusters having n skeletal atoms will adopt closo structure if it is held together by n + 1 skeletal bonding electron pairs. Nido if held together by n + 2 skeletal electron pairs, Arachno if held together by n + 3 skeletal electron pairs, hypo if held together by n + 4 skeletal electron pairs, klado if held together by n + 5 skeletal electron pairs.

Carboranes

A carborane is a cluster composed of boron, carbon and hydrogen atoms. These clusters are also classified as Closo, Nido, Arachno and Hypho.

The number of valence electron in carbon atom is equal to the boron atom plus a hydrogen atom. Hence BH units can be substituted by carbon in carboranes.

The general formula of carboranes is C₂BnHn^{+ 2}, where n is an integer. The smallest carboranes are closo-boranes, which are closed cages. The first carboranes were synthesized in the 1950s, but the field really took off in the 1960s and 1970s.

Carboranes are a versatile class of compounds with a wide range of properties. They are used in a variety of applications, including:

• Catalysis: Carboranes are used as catalysts in a variety of reactions, including hydrogenation, hydroboration, and polymerization.
• Reagents: Carboranes are used as reagents in a variety of reactions, including organic synthesis and organometallic chemistry.
• Materials: Carboranes are used in the development of new materials, such as polymers and ceramics.
• Medical applications: Carboranes are being investigated for use in a variety of medical applications, such as cancer chemotherapy and drug delivery.

Carboranes are a fascinating class of compounds with a wide range of potential applications. They are a valuable tool for chemists and materials scientists, and they are likely to continue to be investigated for new applications in the future.

Metal clusters

Metal carbonyl clusters are metal clusters with carbonyl group as ligand , 18 electron rule has been proven of greatest significance in structral rationalization of low nuclearity carbonyl clusters . But if the number of metal centres per cluster is equal or greater than five, then the conventional approach is not significant.

A British chemist, n Kenneth Wade, solved the unexplained problem of structure and bonding predictions in high nuclearity carbonyl clusters by developing a new scheme of electron counding. The idea behind the extension was that the M (CO)₃ unit is actually isolobal with the BH unit.

• [B₁₂H₁₂]^2-

The above hydroborane has a structure similar to B_nH_n^2-. where the BnH_n^2- is a closo type hydroborane. Hence [B₁₂H₁₂]^2- is a closo structure.

• [NI₅ (CO)₁₂]^2-

Here, n = 5

The total electron count for [NI₅ (CO)₁₂]^2- is

${\displaystyle \begin{array}{cc}{\displaystyle (5\times Ni)}+{\displaystyle (12\times CO)}+2& ={\displaystyle (5\times 10)}+{\displaystyle (12\times 2)}+2\\ & =76\end{array}}$

since n = 5, 12 n + 2(n + 3) = 76.

So structure of [NI₅ (CO)₁₂]^2- is Arachno.

• [C₂B₉H₁₁]^2-

We know that C atom is equal to BH group. So here there is two carbon atom so we can write [C₂B₉H₁₁]^2- as [B₁₁H₁₃]^2-.

Each BH furnishes 2 electrons, each H furnishes 1 electron. Also, ionic charges must be included in the electron count.

So, here

11 BH = 22 electrons

2 H = 2 electrons

ionic charge = 2 electron

Total, there is 26 electrons, means 13 electron pair. since n = 12, the total electron pair in [C₂B₉H₁₁]^2- is n + 2.

Here, the skeletal atoms are held together by Nido structure.

• RH₆ (CO)₁₆

n = 6

The total electron count of RH₆ (CO)₁₆ is

${\displaystyle \begin{array}{cc}{\displaystyle (6\times Rh)}+{\displaystyle (16\times CO)}& ={\displaystyle (6\times 9)}+{\displaystyle (16\times 2)}\\ & =86\end{array}}$

Since, n = 6.12 n + 2(n + 1) = 86

So structure of RH₆ (CO)₁₆ is Closo.

• Os₅ (CO)₂₀

n = 5

The total number of electron count of Os₅ (CO)₂₀ is

${\displaystyle \begin{array}{cc}{\displaystyle (5\times Os)}+{\displaystyle (20\times CO)}& ={\displaystyle (5\times 8)}+{\displaystyle (20\times 2)}\\ & =80\end{array}}$

Since n = 5, 12 n + 2(n + 5) = 80

So the structure of Os₅ (CO)₂₀ is Nido.

• B₅H₁₁

The above hydroborane has a structure similar to B_nH_{n + 6} . where the B_nH_{n + 6} is an arachno type hydroborane. Hence B₅H₁₁ has arachno structure.

• B₆H₁₀

The above hydroborane has a structure similar to BnHn + 4 . where the B_nH_n^{+ 4} is a Nido type hydroborane. Hence B₅H₁₁ has an Nido structure.

Among the given species, the species showing Nido structure are:

[C₂B₉H₁₁]^2- , B₆H¹⁰