Question

The coordination number of aluminum ion and the number of bridging hydrogen atoms in [Al(BH4)4]⁻ are, respectively,

• A. 8 and 8
• B. 6 and 6
• C. 4 and 6
• D. 8 and 12

Hint:

In coordination compounds, the coordination number refers to the number of bonds between the central metal atom and its ligands.

Answer 1

The Correct Option is A

Structure of $\mathbf{[\text{Al}(\text{BH}_4)_4]^{-}}$

The complex anion $[\text{Al}(\text{BH}_4)_4]^{-}$ has an $\text{Al}^{3+}$ ion at the center coordinated by four $\text{BH}_4^{-}$ groups.

Nature of the Ligand: The tetrahydroborate anion ($\text{BH}_4^{-}$) acts as a bidentate ligand in this complex, forming a unique type of bond known as a three-center, two-electron ($\mathbf{3c-2e}$) bond through its hydrogen atoms.

Coordination: Each of the four $\text{BH}_4^{-}$ groups uses two of its hydrogen atoms to bridge to the central $\text{Al}^{3+}$ ion. * Each $\text{BH}_4^{-}$ group contributes two hydrogen atoms to the coordination sphere.

1. Coordination Number of $\mathbf{\text{Al}^{3+}}$

The coordination number is the total number of $\text{Al}-\text{H}$ bonds formed.

Number of $\text{BH}_4^{-}$ ligands $= 4$

Number of bridging hydrogen atoms per ligand $= 2$

Coordination Number (number of $\text{Al}-\text{H}$ bonds) $= 4 \times 2 = \mathbf{8}$

2. Number of Bridging Hydrogen Atoms

The bridging hydrogen atoms are those that form $\text{Al}-\text{H}-\text{B}$ bridges.

Since each of the four $\text{BH}_4^{-}$ ligands contributes two bridging hydrogen atoms:

Total Bridging Hydrogen Atoms $= 4 \times 2 = \mathbf{8}$

The coordination number and the number of bridging hydrogen atoms are 8 and 8, respectively.