The borohydride ion (BH2⁻) has a boron (B) atom with three valence electrons, bonded to two hydrogen (H) atoms, each with one valence electron. The negative charge adds an extra electron, totaling 6 valence electrons. The Lewis structure shows two single B-H bonds and a lone pair of electrons on boron, leading to 6 bonding electrons. This results in a bent molecular geometry with a bond angle less than 120°, typical for sp² hybridization. Boron’s incomplete octet and the lone pair contribute to the reactivity of BH2⁻, making it an important intermediate in various chemical reactions.
BH2- consists of one boron atom and two hydrogen atoms. BH2- is an boron hydride which has some more synonyms like dihydridoborate(-1), Borananion, Boranide and BH2- anion. Molecular weight of BH2- ion is 12.83. Here in this editorial we are learning about the BH2- lewis structure and characteristics.
The two main rules for drawing any lewis structure are as follows:
- Note or count the valence shell electron number of each atom or element including in the molecule or ions or chemical compound. If there are ions present then count charges.
- After bonding between elements put the remaining electron pairs on the atoms or elements of the molecule to complete its octet.
How to draw BH2- lewis structure?
While drawing lewis structure we have to follow some rules like counting valence electrons of each atom, then bonding between atoms, then complete the octet of all elements and calculate the formal charge present on each atom in the molecule.
In BH2- lewis structure, there are only two atoms are present i.e. boron atom and hydrogen atom. There is only one boron atom and two hydrogen atoms are present. First check the position of groups of boron and hydrogen atom in the periodic table. Thus, boron atom comes under the 3rd group of periodic table and hydrogen atom comes under 1st group of periodic table.
So, boron atom contains 3 valence electrons and hydrogen atom contains 1 valence electron in their outer shell orbitals. After counting valence electrons we have to make bonding between all the boron and hydrogen atoms and after bonding the remaining valence electrons should put on the B and H atoms to complete their octets and later we will count the formal charge on B and H atoms.
BH2- valence electrons
To count the valence electrons present on BH2- lewis structure, first check the group position of boron and hydrogen atom in the periodic table. So, boron belongs to 3rd group of periodic table and having three valence electrons in its outer shell orbital. Similarly, hydrogen atom belongs to 1st group of the periodic table and having one valence electron in its outer shell orbital.
In BH2- lewis structure, one boron and two hydrogen atoms are present. Thus, we have to add the valence electrons of boron and hydrogen atoms. Also we have to add one more electron for the negative charge present on BH2- molecule while adding the valence electrons.
Thus, valence electrons in BH2- = 3 (B) + 1 x 2 (H) + 1 (-) = 6
Therefore, total six valence electrons are present on BH2- lewis structure.
As there is bonding between boron and hydrogen atoms, so four electrons out of six electrons get involved in bonding and being bond pairs. Thus we are remained with only two valence electrons for further sharing of electrons in BH2- lewis structure.
BH2- lewis structure octet rule
Octet means presence of eight electrons in the outermost shell of any atom. Here in BH2- molecule we have to now try to complete the octet of each boron and hydrogen atom. There are total six valence electrons in BH2- ion i.e. three valence electrons from boron, two valence electrons from two hydrogen atom and one valence electron is added for minus charge present on it.
As we creates the bonding between the one central boron atom and two outer hydrogen atoms, there will be two B-H bonds will be creates. In this bonding total four valence electrons get used, two electrons present in each single B-H covalent bonds. Now we have only two valence electrons left for further sharing to complete the octet.
Hydrogen atom can contains only maximum two electrons in its valence shell, so we cannot put more electrons on H atom as there are already two bonding electrons are present on H atom. So, the remaining two valence electrons will go on central boron atom. Therefore, B atom now contains total six electrons i.e. two bond pairs and one lone pair electron. As the boron atom contains only six electrons so it has incomplete octet and octet rule is not applied on H atom.
BH2- lewis structure formal charge
Formal charge for any lewis structure can be calculated by the following formula:
Formal charge = (valence electrons – non-bonding electrons – ½ bonding electrons)
Formal charge for boron and hydrogen atoms of BH2- ion is calculated as follows:
Boron atom: Valence electrons on Boron atoms = 03
Lone pair electrons on Boron atoms = 01
Bonding electrons on Boron atoms = 04 (two single bond)
Boron atom Formal charge is = (03 – 01 – 4/2) = -1
So, the boron atom has -1 formal charge in BH2- lewis structure.
Hydrogen atom: Valence electrons on Hydrogen atom = 01
Lone pair electrons on Hydrogen atom = 00
Bonding electrons on Hydrogen atom = 02 (one single bond each)
Formal charge on iodine = (1 – 0 – 2/2) = 0
So, all the two hydrogen atoms in BH2- ion have zero formal charges.
BH2- lewis structure lone pairs
BH2- ion have six valence electrons, out of which four electrons are get engaged in bonding between boron and hydrogen atoms i.e. two B-H bond pair electrons are present in BH2- ion. Now, the remaining two valence get placed on the central boron atom as the hydrogen atom cannot contains more than two electrons in its outer valence shell orbital.
The one single boron hydrogen (B-H) bond involves two electrons, thus 2 (bonds) x 2 (electrons) = 4. OR 4 (valence electrons) / 2 (electrons) = 2 bonds (B-H). Therefore, 6 (valence electrons) – 4 (bonding electrons) = 2 valence electrons remain. Thus, there are only one lone pair electrons are present in B atom in BH2- lewis structure. Hence, the two remaining valence electrons get put on central boron atom, so there is only one lone electron pair present on central boron atom in BH2- lewis structure.
BH2- lewis structure shape
The BH2- lewis structure contains two single B-H covalent bonds forming between central boron atom and outer two hydrogen atoms. Also there is one lone pair electron is present on the central boron atom. Due to the presence of one lobe electron pair there is a repulsion creates between the boron and hydrogen atoms and the lone pair on boron atom repel the two boron hydrogen B-H bonds away from it making the shape of the molecule bent and it has trigonal planar molecular geometry.
Also according to VSEPR theory, the molecule containing three atoms connected with each other with covalent bonds and the lone electron pair is present on central atom, then it has a bent molecular geometry. The generic formula of VSEPR theory which applies on BH2- ion is AX2E. Where, A is central atom, X is bonding atoms with central atom and E is lone electron pairs present on central atom. Hence, the BH2- has trigonal planar geometry and bent shape as per VSEPR theory.
BH2- hybridization
Hybridization of any lewis structure or molecule is determined by the steric number of its central atom. There is a formula to calculate the steric number as follows:
Steric number = addition of no. of central atoms linked bonded atoms and central atom lone electron pair present on it.
Steric number of BH2- ion = 2 (H atoms) + 1 (lone electron pair) = 3
Hence, the BH2- ion has 3 steric numbers thus it has sp2 hybridization according to VSEPR theory. So, the BH2- lewis structure has sp2 hybridization.
BH2- lewis structure angle
The molecular geometry of BH2- ion is trigonal planar and it has bent shape due to the repulsion cause between two outer hydrogen atoms and central boron atom. According to VSEPR theory, it is considered that any molecule with three elements with lone pair of electrons on central atom having trigoanl planar geometry with bent shape having 120 degree angle between central atom and outer attached atoms. Thus, BH2- lewis structure has 120 degree bond angle within hydrogen boron hydrogen (H-B-H) bond.
BH2- lewis structure resonance
Resonance structures of any molecule have variation in electron distribution from one form of structure to another form. There are some rules to draw the resonance structure of any molecule or lewis structure i.e. there should be a multiple bond (double or triple bond) is present in the molecule and the surrounding element or atom should have at least one lone electron pair.
In case of BH2- lewis structure, there is one lone pair of electron is present on central B atom, but all the boron and hydrogen atoms are joined with each other with a single covalent. So, there are no multiple (double triple) bonds are present in the BH2- ion.
Also there is only formal charge is present on central boron atom and there are zero formal charges on both hydrogen atoms. So, the resonance structure of BH2- ion is not possible as all the conditions are not fulfilled in BH2- ion to draw the resonance structure.
BH2- solubility
The solubility of BH2- (boranide) ion is yet not mention in any literature or article. So, we are not considering the solubility of BH2- ion in any solution.
Is BH2- ionic?
Yes, BH2- ion is particularly ionic in nature as there is a formal charge is present on central boron atom.
Why BH2- is ionic?
In BH2- ion, the central boron atom is attached to two H atoms having zero formal charge, but the central boron atom has minus one (-1) formal charge present on it. So, the negative charge present on central boron atom should show on the outside of bracket of BH2- structure with negative charge, making the BH2- lewis structure ionic in nature.
How BH2- is ionic?
Due to the presence of minus one (-1) formal charge on central boron atom the whole BH2- ion is being ionic in nature. As even there is a covalent bond is present between all B and H atoms, which is a stronger bond and cannot easily breaks, and the atoms are not actually ionic in nature of BH2- ion.
As the only central B atom has charge on it and no charge is present on both H atoms. So, the whole BH2- ion is being ionic in nature due to the presence of -1 charge on central boron atom and the charge is shown outside the bracket while writing the BH2- lewis structure.
Is BH2- acidic or basic?
BH2- ion can act as a lewis base in nature as it has a one lone pair of electron present on it which it can donate easily to other atoms and being basic in nature. It can form a coordinate covalent bond when reacts with water or other solvents.
Why BH2- is basic?
The atom or molecule which shows the electron donating capacity is known to be a basic atom or molecule in nature. In BH2- ion the central boron atom having one lone electron pair on it, which it can easily donate to other atoms which reacting to other atoms, thus it shows a basic nature.
How BH2- is basic?
As the boron atom belongs to 3rd group of periodic table it has three valence electrons in its outer orbit. So, boron atom can show the electron donating capacity due to which it already shows the basic nature. Also in BH2- ion the central B atom have one lone electron pair present on it which can easily get donated to other molecules to form other compound. Thus, BH2- is basic in nature.
Is BH2- polar or nonpolar?
BH2- ion is non- polar in nature. It is non- polar due to the its asymmetrical structure and bent shape.
Why BH2- is non- polar?
All the B and H atoms are arranged not in symmetrical manner and due to the presence of lone electron pairs there is repulsion between atoms and creates 120 degree bond angle and having bent shape, making the molecule non- polar.
How BH2- is non- polar?
In BH2- ion, there is a less electronegativity difference between B and H atoms and also the -1 charge and lone electron pair present on central B atom due to which repulsion cause and the BH2- ion has bent shape with trigonal planar geometry having 120 degree bond angle making BH2- ion a non- polar ion.
Conclusion:
BH2- ion consists of only two elements i.e. B and H atoms, which get connected with each other by single covalent bonds. BH2- have bent shape and its molecular geometry is trigonal planar. It is sp2 hybridized with 120 degree H-B-H bond angle. It is ionic in nature. BH2- ion shows basic nature and a non- polar ion.
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Hello everyone, I am Dr. Shruti M Ramteke, I did my Ph.D. in chemistry. I am passionate about writing and like to share my knowledge with others . Feel free to contact me on linkedin