Beryllium hydride is prepared by the reaction of dimethyl beryllium (Be (CH3)2) with lithium aluminum hydride (LiAlH4). Be (CH3)2 + LiAlH4 —–> BeH2 + LiAl (CH3)2H2 Beryllium hydride decomposes in water to beryllium hydroxide and hydrogen gas. It is insoluble in toluene and diethyl ether. The molar mass of beryllium hydride is 11.03 g/mol. It decomposes on its melting point i.e., 250 °C. Beryllium hydride is a Lewis acid and hence, form dimeric or trimeric adducts with Lewis bases such as dimethylamine and trimethylamine, respectively. Let us discuss the chemical bonding in the beryllium hydride molecule. We will start by drawing its two-dimensional structure i.e., Lewis structure by following the octet rule. Then, we will understand its three-dimensional structure i.e., the actual shape of the molecule by VSEPR theory and VBT. In last, we will learn whether the beryllium hydride molecule is polar or nonpolar.
BeH2 Lewis Structure
Lewis structure is the simplest representation of any molecule and it includes atoms of the molecule and the valence electrons of the atoms. Why valence electrons but not core electrons? Because only the valence electrons, electrons present in the outermost shell of the atom, are available for the formation of the chemical bond between two atoms. Core electrons are tightly bound to the nucleus of the atom and hence, do not contribute towards chemical bonding. Not all valence electrons of an atom participate in the bond formation. Some of them can remain as nonbonding and act as lone pair (s). The valence electrons are represented as dots around the atom in the Lewis structure of the atom. Hence, it is also known as the electron dot structure. Let us draw the Lewis structure of the beryllium hydride molecule. The beryllium atom belongs to group 2 (alkaline earth metal) and the hydrogen atom belongs to group 1 (alkali metal) of the modern periodic table. Hence, the Be and H atoms have 2 and 1 valence electrons, respectively. The beryllium hydride molecule consists of one beryllium atom and two hydrogen atoms. Hence, total number of valence electrons in beryllium hydride are 2 + (1 x 2) = 4 electrons. Lewis structure of the molecule is based on the concept of the octet rule. Octet Rule: Every atom of the periodic table tries to complete its octet i.e., eight valence electrons around it, during the bond formation either with the same atom or with another atom in a molecule. However, there is a lot of exceptions, where the octet rule does not apply. a. Formation of the expanded octet of the central atom: In this case, the central atom in the molecule is surrounded by more than eight electrons and they are stable. For example: In PCL5, the phosphorous atom, a central atom, is surrounded by 10 electrons instead of eight electrons but still exists in nature. b. Formation of Incomplete octet of the central atom: Here, the central atom has less than eight electrons around it and these types of molecules are also stable. Incomplete octet is common when the central atom has less than four valence electrons. Beryllium hydride is one of the examples of the incomplete octet. In the BeH2 molecule, the beryllium atom has only two valence electrons, and hence, it cannot complete its octet by forming a bond with the hydrogen atom. In total, we have 4 valence electrons, which we need to arrange in the Lewis structure of the beryllium hydride molecule. The hydrogen atom always forms a duplet (one of the exceptions of the octet rule) by forming a bond with another atom and hence, there will be two electrons between hydrogen and beryllium atom. These two electrons will form a single bond between hydrogen and beryllium atom. Therefore, the most suitable Lewis structure of the beryllium hydride molecule is:
Here, we can observe that each hydrogen atom has completed its duplet by forming a single bond with the beryllium atom but the octet of the beryllium atom is incomplete. Therefore, beryllium hydride is an electron-deficient molecule and acts as a Lewis acid. Now, the next question that will come into your mind is that what is the shape or geometry of the BeH2 molecule. Obviously, we cannot answer this question on the basis of its two-dimensional structure i.e., the Lewis structure. For that, we need to understand the three-dimensional structure of the BeH2 molecule, which can be anticipated by the Valence shell electron pair repulsion (VSEPR) theory. What is VSEPR theory?
BeH2 Molecular Geometry
VSEPR theory, given by Sidgwick and Powell, gives a simple procedure for predicting the shape of the covalent molecule. This theory is based on the repulsive interactions between the valance electrons of the atoms. Due to repulsion between electrons, the bond pairs obtain a position in space to minimize the repulsive interactions and maximize the distance between them. Non-bonded valence electrons are known as Lone pairs and they have more repulsions than bond pairs. As per the Lewis structure of the beryllium hydride molecule, the beryllium atom is the central atom and two hydrogen atoms surround it. Hence, the beryllium atom has two bond pairs without any lone pair of electrons. The following table describes the shape of the molecules, where the central atom has only bond pairs. So, the shape of the molecule can be predicted by knowing the number of bond pairs on the central atom. The general formula of Beryllium hydride would be AX2 and accordingly, it will have a linear shape/geometry. The linear geometry of the Beryllium Hydride molecule leads to the bond angle (H-Be-H) of 180° for minimizing the repulsions between two B-H bonds in the space.
BeH2 Hybridization
The hybridization of the central atom in the molecule and the shape of the molecule can be estimated from the Valence Band Theory (VBT) as well as from the steric number. Steric Number Method: Steric number is defined as Steric number = Number of Bond pairs + Number of Lone pairs Here, two hydrogen atoms are bonded to the beryllium atom without any lone pair on the central atom, beryllium. Therefore, the steric number of the BeH2 molecule is 2. Two steric number leads to sp hybridization of the beryllium atom with the linear geometry of the BeH2 molecule. VBT Method: The hybridization of the beryllium atom in the BeH2 molecule is calculated as follows: The electronic configuration of the beryllium atom in the ground state: [He] 2s2. The electronic configuration of the beryllium atom in the excited state: [He] 2s12p1. Now, the 2s orbital of the beryllium atom fuses with its 2p orbital and hence, form two sp hybrid orbitals of equivalent energy, which align themselves in a linear geometry. The sp hybrid orbital of the beryllium atom overlap with the 1s atomic orbital of the hydrogen atom, which is shown in the orbital diagram of the beryllium hydride molecule as follows:
Hence, the VBT method also leads to the sp hybridization of the Beryllium atom in Beryllium hydride with linear geometry.
BeH2 Polarity
The polarity of the covalent bond depends upon the electronegativity difference between atoms of the bond. Depending upon the electronegativity difference, the covalent bond can be polar or nonpolar. Let us see the nature of the Be-H bond in the BeH2 molecule. The electronegativity of the beryllium atom and the hydrogen atom is 1.57 and 2.20, respectively, on the Pauling scale. The electronegativity difference of the B-H bond is 0.63 and hence, the B-H bond is a polar covalent bond. It will act as a dipole with a partial positive and a partial negative charge on the beryllium and hydrogen atom, respectively. Hence, the B-H bond has a dipole moment pointing towards the hydrogen atom.
The polarity of the BeH2 molecule depends upon the net dipole moment, which further depends on its shape or geometry. The shape of the BeH2 molecule is linear and hence, two B-H dipoles of equal magnitude but of opposite directions cancel the effect of each other. It leads to zero net dipole moment of the BeH2 molecule. Hence, the BeH2 molecule is nonpolar.
Conclusion
In this article, we have studied the chemical bonding in the BeH2 molecule. In brief, the BeH2 molecule is a Lewis acid owing to the incomplete octet of the beryllium atom, which can be observed from its Lewis structure. The shape of BeH2 is linear with the sp hybridization of the beryllium atom. The Be-H bond is a polar covalent bond still the BeH2 molecule is nonpolar due to its linear shape and zero net dipole moment. Suggestions and queries are welcome. Thank You.
