Boranes & Carbaboranes:

Boranes & Carbaboranes:

• The Hydrides of Boron is called as Boranes. The general formula of boranes is BnHn. While The Hydrides of Carbon is called as Alkanes. The general formula of alkanes is called as CnHn.

• In Boranes the basic fragments are BH units. The BH- units which has 7 electrons in it, is iso electronic and iso structural with CH group. The replacement of BH- group by CH group will happens. Thus two replacements lead to a neutral molecule Bn-2C2Hn.

• Thus the mixed hydrides of Boron and Carbon are formed these are said to be Carbaboranes or Carboranes. The general formula of carboranes or carbaboranes is Bn-2C2Hn. Carbaboranes or Carboranes are having electron deficient frame work.

• In Carbaboranes or Carboranes skeleton there are no of H- ions are present so these are said to be polyhedral carbaboranes or carboranes.

• The polyhedral carbaboranes or carboranes may be considered as formally derived from the BnHn-2 ions.

• The majority of polyhedral carbaboranes or carboranes contain two skeletal carbon atoms per cage.

• Carbaboranes or carboranes form a distinct class of “neoclassical” molecules in which the carbon atoms participate in delocalized bonding and often achieve coordination number of 5 or 6.

• Carbaboranes or carboranes and the boranes having the same no of electrons in their bonding frame work have similar structure.

• Carbaboranes or carboranes are broadly classified in to the following three categories based on the geometry.

• Closo Boranes:

• Nido Boranes:

• Arachno Boranes:

Wade’s Rule:

• The rule which states about, the correlation between the number of framework electrons and structure adopted by boranes or carboranes is called as Wade’s Rule.

• It was proposed by Kenneth Wade in polyhedral skeletal electron pair theory which provides electron counting rules useful for predicting the structures of clusters,

• Wade’s Rule for Boranes & Carbaboranes:

The Number of framework electrons (F) can be determined by the following equation.
F = 3b + 4c + h + x – 2n

Where, F = total number of framework electrons,
b = number of Boron atoms
c = number of Carbon atoms
h = number of Hydrogen atoms
x = the amount of negative charge on the ion
n = number of Boron atoms (b) + number of Carbon atoms (c)

1. If F = 3b + 4c + h + x – 2n = 2n + 0 Then they are Capped Closo Boranes.
2. If F = 3b + 4c + h + x – 2n = 2n + 2 Then they are Closo Boranes.
3. If F = 3b + 4c + h + x – 2n = 2n + 4 Then they are Nido Boranes.
4. If F = 3b + 4c + h + x – 2n = 2n + 6 Then they are Arachano Boranes.
5. If F = 3b + 4c + h + x – 2n = 2n + 8 Then they are Clado Boranes.
6. If F = 3b + 4c + h + x – 2n = 2n + 10 Then they are Hypo Boranes.

Examples:

I. Wade’s Rule for B10C2H12:

II. Wade’s Rule for (B9C2H12)2-:

F = 3b + 4c + h + x – 2n = 2n + 0

F = 3(9) + 4(2) + 12 + 2 – 2(11) = 2(11) + 0 (where n= 9+2=11)

F = 27 + 8 + 12 + 0 – 22

F = 47 – 22 = 25

F = 24 = 24 = 24

Note: Question Wrong.