Five moles of

react with two moles of

resulting in the evolution of nine moles of

​and the formation of a monoanionic borane cluster that has a

Correct option is A

The reaction described leads to the formation of a monoanionic borane cluster with a Nido structure. In borane chemistry, the classification of borane clusters is based on the number of boron atoms and the bonding structure, which correlates with their corresponding shapes and classifications.

It is a general feature of closo-

anions that there are no

​and each B-H bond is a normal 2e bond requiring one electron from B and one electron from H. As boron starts with three electrons, it has two electrons left to contribute to the cluster and these electrons are regarded as skeletal or framework electrons. Thus,

anions have 2n + 2 skeletal electrons (the 2n electrons from nB atoms and the 2 electrons from the anionic charge). In borane clusters, each BH unit has an sp orbital that points directly towards the centre of the cluster and the other two

orbitals that point along the surface of the cluster.

The nB atoms have 4n atomic orbitals which are always distributed as follows:
n in the n(B-H_t) bonding orbitals, (n + 1) in cluster bonding molecular orbitals, (2n - 1) in non-bonding and antibonding cluster molecular orbitals.
In 

cluster, there are (n + 1) cluster bonding molecular orbitals and 2n electrons contributed by n B-H units, two additional electrons are required for 

ion. This is the reason why the

ions are more stable than

itself.

Since the shape of the cluster depends only on the number of skeletal electrons, it is possible (conceptually) to remove a vertex group, BH_t from the cluster leaving two skeletal electrons that the vertex BH_t group was contributing. This is possible only if BH²⁺ is removed instead of a BH_t unit leaving one vertex of the cluster empty. If BH²⁺ unit is removed from 

ion is obtained. This will have the same polyhedral structure because the electron count has not changed but one vertex is now unoccupied. The neutral 

borane can be obtained by adding the appropriate number of protons (four protons in this case), this does not alter the number of electrons in the system because protons (H⁺) are the zero electron species. Hence all the bonding molecular orbitals are just filled. The protons bridge the faces of polyhedron which include the missing vertex. In general, the number of skeletal electron pairs in 

will be equal to 

and the number of vertices (V) is 

The boranes are classified as: closo, nido, arachno, hypho and klado boranes.
The boranes in which each corner is occupied by B atoms are called closo boranes. These have the formula

The number of vertices and skeletal electron pairs are respectively n and (n + 1). This indicates that the vertices in a closo borane will be one less than the number of skeletal electron pairs.

If one B atom is removed from a vertex of a closo structure, a nido (Latin: nest like) structure will result. The nido boranes have the formula

The number of vertices will be (n - 1) of an n cornered polyhedron and the number of skeletal electron pairs is (n + 2).