CN is known as cyanide which exists as a pseudohalide anion. It belongs to the cyano group and consists of carbon and a nitrogen atom having a triple bond. It carries a charge of -1 and is a conjugate base of hydrogen cyanide (HCN).
Cyanides are released in nature via cyanogenic compounds and also produced by several microorganisms. We can find CN- ion in both organic molecules like acetonitrile and inorganic molecules like potassium cyanide.
Chemical compounds containing CN- ion have a lot of applications in the present-day world. It can act as a reducing agent.
As HCN, we find use in gas chamber executions and preparation of pesticides and insecticides.
As NCN, it is used for gold and silver mining. Apart from this, cyanides have applications as food additives and in jewelry-making industries.
Cyanide salts are however toxic and poisonous for human bodies. Reaction with water can lead to fumes and fire generation as well. It can cause asthma, bronchitis, acidosis, prenatal injuries, and even brain edema.
Let us now discuss the chemical bonding inside a cyanide ion to understand its chemical and physical properties in a better manner.
CN Lewis Structure
What is Lewis Structure?
If we have a look into the above-mentioned diagram, we can see that it is a sketch of an oxygen atom. Here since an oxygen atom has an atomic number of 8, we have six electrons in the outermost shell.
The outermost shell is known as the valence shell that determines the valency i.e the combining capacity of the atom with other atoms for molecule formation.
Lewis Structure is a simplified and easy diagrammatic representation of the internal structure of a chemical compound. Here, the valence electrons are denoted as dots and we use straight lines to denote the type of bond formed between the valence electrons.
Although Lewis Structure does not give us many details about the bonding, it is the initial step towards understanding the 3D molecular shape or finding out the nature of hybridization.
Finding out the Lewis Structure of CN ion
A cyanide anion consists of carbon and a nitrogen atom.
Carbon belongs to group 4 or 14 of the periodic table and therefore has 4 valence electrons.
Nitrogen belongs to group 5/15 and has a valency of 5.
Also, we need to take into account the electron that provides the negative charge to the CN- ion. total number of valence electrons = 4 + 5 + 1 = 10.
Since CN is a diatomic ion, there is no concept of a central atom here.
Now, we will proceed to draw a simple sketch or skeletal diagram of cyanide ion.
We have placed both the carbon and nitrogen atoms as atomic symbols here. We will henceforth place the electron dot notations.
We have put all the 10 valence electrons surrounding the constituent atoms of the CN ionic molecule. Here comes the concept of octet fulfillment.
Octet rule/Octet fulfillment: Let us look at the periodic table.
The main group elements ie. group 1 to group 17 elements tend to attain the noble gas configuration and this is known as the octet rule.
Here, we have carbon and nitrogen atoms. Both of them will tend to achieve the octet configuration of Neon (noble gas of the same period).
If we look into the Lewis Structure illustrated above, we can see that Nitrogen (N) has already achieved the octet fulfillment. Carbon however still has only four surrounding electrons.
Now, Carbon has 6 valence electrons. It still has not achieved Neon configuration.
Now, both the constituent carbon and nitrogen atoms have achieved octet fulfillment.
Now, we will find out the Formal Charge values of the atoms inside the anion.
For Carbon, Formal Charge = 4 – 0.5*6 – 2 = -1.
For Nitrogen, Formal Charge = 5 – 0.5*6 – 2 = 0.
Since the elements are present in their least possible formal charge values, we have got our most suitable Lewis Structure sketch for CN.
Also, the net formal charge value = -1.
We have achieved the negative charge inside the CN anion.
This is the Lewis Structure for CN anion.
Lewis Structure gives us the 2D pictorial illustration of a molecular or ionic structure.
Also, it helps us get to the next step of understanding chemical bonding i.e predicting the 3D molecular shape of any molecule. We are going to find out the molecular geometry via VSEPR theory.
VSEPR stands for Valence Shell Electron Pair Repulsion model.
This theory points to the fact that electrons tend to form a negatively charged cloud atmosphere surrounding the constituent atomic nuclei inside any molecule.
Since these are like charges they repel each other and to get a stable molecule, these repulsive forces need to be minimized.
Here, electrons refer to both the bonded or paired ones (BP) and the lone or unbonded pairs (LP).
LP-LP > LP-BP > BP-BP ( in case of strength ).
CN- Molecular Geometry
We need to account for VSEPR notations and steric numbers when there are more than two atoms inside a molecular structure.
For CN-, we have only two constituent atoms.
As per VSEPR theory, the 3D shape or geometry of the ion is linear.
C and N each have the symmetric distribution of valence electrons for the formation of the anion. Also, the number of lone pairs on each of them is equal. A linear geometry is therefore essential to minimize repulsion and attain stability.
The bond angle is 180 degrees.
What Does Polarity Mean?
Polarity is an important property of any chemical molecule. It depends on the separation of charges. This characteristic is defined by another concept called electronegativity.
For that, we can consult the Pauling Electronegativity chart:
Here, in this Pauling Electronegativity scale, we can get an idea of the trends in electronegativity value along groups and periods in the periodic table.
Electronegativity: It is an atomic property that describes its tendency to attain or gain more electrons for bond formation.
When two atoms of the same element come together to form a homogeneous diatomic molecule, due to linear arrangement and same electronegativity values ( e.g. Cl2 ), we have a non-polar molecule. Here, the net dipole is zero.
However, when two or more atoms of different elements form a molecule or there is an asymmetry in the molecular shape, we find a polar molecule where the net dipole is not equal to 0 ( e.g. H2O ).
Is CN anion Polar or Non-polar?
Now, let us focus on the Cyanide anion.
If we consult the Pauling chart provided above, we can find out the electronegativity values of both C & N.
C has an electronegativity value of 2.55 whereas that of N is 3.04.
The difference is therefore equivalent to 0.49.
If the difference is around 0.4 to 1.7, we consider the bond to be a polar one. Also, carbon being slightly more electropositive than nitrogen possesses a partial 𝛅+ whereas N possesses a 𝛅- charge.
The triple bond acts as a slightly polar bond and the ionic nature of CN makes it interact with polar solvents like water.
To understand the nature of chemical bonding inside an ion of cyanide, we have already learned how to draw a graphic illustration of a 2D Lewis Structure.
After this, to delve a little deeper, we have discussed the 3D molecular geometry of CN- along with bond angles. We have also talked about the electronegativity difference and polar nature of the anion.
Now comes one of the most interesting and important concepts of chemical bonding: Orbital Hybridization
Atomic orbitals or AOs are of several shapes and types for example s,p,d,f.
Orbitals, as we already know, are mathematical probability functions denoting where the electrons can be present in any region.
For hybridization to occur, we need AOs of the same atom possessing almost equal energies to come together and fuse. This gives rise to hybridized orbitals like sp, sp2, sp3, sp3d, and so on.
For example, 1 s and 3 p orbitals fuse to form sp3 hybridization.
The hybrid orbitals will possess the same energy.
Hybridization in CN
If we look at the Lewis Structure of CN-, we can see that a triple bond has been formed between carbon and nitrogen.
This denotes that the bond has one sigma and two pi bonds. The pi bonds are formed by a side-to-side overlap of p orbitals and do not have any role to play in hybridization.
Steric number = Number of atoms bonded to central atom inside a molecule + Number of lone pair of electrons attached to the central atom
Steric Number = 1 sigma + 1 lone electron for both C and N.
Therefore, we can decipher the hybridization to be sp.
The sp orbitals of both C and N combine to form the sigma bond in C⦀ N.
Molecular Orbital Diagram
Molecular Orbital Theory is slightly different from VBT and orbital hybridization. Here, AOs from different atoms inside the molecule can come together to form molecular orbitals or MOs.
Therefore, valence electrons are shared inside the molecule.
The electronic configuration of both C and N are as follows:
Carbon (atomic no:6)
C: 1s2 2s2 2p2
Nitrogen (atomic no:7)
N: 1s2 2s2 2p3
In MO theory, we have the concept of non-bonding, anti-bonding, and bonding orbitals.
The four electrons in 1s orbitals are non-bonding orbitals.
The 𝝈 and 𝝅 orbitals are bonding orbitals.
The 𝝈*and 𝝅* are the anti-bonding orbitals.
Below, we can see a MO diagram of the CN- ion.
We have discussed in a detailed manner the several important terminologies and concepts surrounding the chemical bonding of the cyanide ion.