SeF4, selenium tetrafluoride, is a colorless toxic and poisonous liquid that boils at a temperature close to that of water. Se has a +4 oxidation state in this molecule. Its molecular weight is 154.96 g/mol.
It is used as a very efficient selective fluorinating agent and has been used to fluorinate aldehydes, ketones, alcohols and carboxylic acids, activated carbon surfaces. It is a more efficient fluorinating reagent than SF4, as SeF4 is a liquid and could be handled easily under mild conditions.
SeF4 quickly reacts with water molecules to get hydrolyzed and releases hydrogen fluoride, which is a very harmful substance. For this reason, SeF4 is rarely used for this purpose.
SeF4 is commercially produced by reacting selenium with chlorine trifluoride (ClF3). SeF4 has many important applications in organic synthesis reactions.
In this article, we will explore whether selenium tetrafluoride is polar and for that we will take the help of some well-accepted theories in chemistry.
So, is SeF4 polar or nonpolar? SeF4 is a polar molecule because of its trigonal bipyramidal geometry and see-saw shape. The asymmetric shape of the molecule results in the non-zero value of dipole moment. Fluorine is more electronegative than Selenium due to which the Se-F bond is also polar. The lone pair and arrangement of four fluorine atoms around selenium cause the unequal distribution of charge across the molecule.
Let us study the concept of polarity in detail.
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Why is SeF4 Polar?
The question is, why SeF4 is polar in nature. SeF4 is indeed a polar molecule. it has trigonal bipyramidal geometry and a see-saw shape.
The arrangement of four F atoms and a lone pair in SeF4 is not symmetrical (we will see why it is asymmetrical in later discussion).
F atom is more electronegative than Se atom, thus the shared pairs of electrons within the Se-F bond are pulled more towards the F atom- giving rise to a dipole with its head at F atom and tail at Se atom.
This molecule has a see-saw shape, with Selenium as the central atom and Fluorine atoms surrounding it in a see-saw fashion in accordance with VSEPR theory. Due to this, the individual Se-F bond’s polarity is not canceled and thus SeF4 has a net dipole moment.
This summary may seem a little confusing, but going through the following article will make it more understandable.
SeF4 Bond Polarity and Dipole Moment
The polarity of a molecule can be determined by checking the polarity of each of the bonds in the molecule, and the arrangement of these bonds in space.
Also, the electronegativity of each atom also plays a major role in affecting the polarity of the molecule. Different dipoles can be added or subtracted following the mathematics of vectors.
Thus, angle(s) between dipoles also affect the net dipole moment between different dipoles.
The polarity of a bond can be quantitatively expressed in the terms of dipole moment.
Mathematically, the dipole moment of a bond is expressed as:
µ = q * r
Here, q = the absolute charge separation between the atoms involved in the bond
r = distance between the atoms involved in the bond
The S.I. unit of dipole moment is Cm (Coulomb meters) or D (Debye).
1 Debye = when 3.336 × 10−30 Coulomb of charge is separated by 1 meter.
A molecule is non-polar if µ = 0
A molecule is polar if µ ≠ 0
One should remember that the dipole moment of a bond is never negative.
In the above explanation of the dipole moment and polarity of a molecule, it is assumed that the molecule is isolated. Things change when a molecule is surrounded by similar or dissimilar molecules.
In such a case, the interaction between these molecules also affects the polarity of the molecule, and often, a non-polar molecule may also become polar.
To explain the polarity of the SeF4 molecule, we will consider various theories to derive the structure of this molecule and see that how the position of each bond and the lone pair results in a net dipole moment in this molecule.
SeF4 Lewis Structure
To draw the Lewis structure of SeF4, will consider the valance shell electronic configurations of Se and F atoms and calculate the total valence electrons in the molecules.
After determining the number of total valence electrons in the molecule, we can find out the number and types of electron pairs around the central atom, Se, and then determine the appropriate geometry of SeF4
Number of Valence electrons of Se (atomic no. 34, p block, group 16) = 6
Number of Valence electrons of four F (atomic no. 9, p block, group 17) = 4*7 = 28
Total number of valence electrons in SeF4 molecule = 6+28 = 34
Fluorine is more electronegative than Selenium, thus Se is placed at the center of the molecule.
Number of electron pairs in the molecule = 34/2 = 17
6 electrons are occupied by the valence shell of each F atom, thus 24 electrons out of 34 are now in the valence shell of the F atom.
From the remaining 10 electrons (34-24=10), 4 electron pairs (i.e., 8 electrons) form Se-F bonds and the remaining 2 electrons remain non-bonded.
SeF4 Shape and Structure: Applying VSEPR Theory
VSPER theory considers different types of electron pairs, namely- bond pairs and lone pairs, and the intensity of repulsions between them to justify the shape of a molecule.
According to VSEPR theory, the electrons pairs arrange themselves around the central atom in such an order that the repulsion between them becomes minimum.
The decreasing order of repulsion between various interactions is:
(Lone pair- Lone pair) > (Bond pair- Lone pair) > (Bond pair- Bond pair)
In SeF4, the ideal shape should be trigonal bipyramidal with five electron pairs, but due to the presence of one lone pair, 3 lone pair-bond pair repulsions arise, and the bond pairs move farther to reduce this repulsion, giving rise to a see-saw like a shape.
The Fluorine atom has a value of electronegativity greater than that of Selenium. As a result, the bond pairs are pulled slightly more towards the Fluorine atom.
Thus, the direction of the dipole in the Se-F bond is in the direction of the Fluorine atom. Have a look at the image.
The dipole moments of individual Se-F bonds do not cancel out due to this see-saw shape of the molecule and as a result (the length of the red arrow is a measure of how strong the dipole moment is), the molecule retains a net dipole moment and thus becomes polar in nature.
This is depicted in the figure.
One such similar compound is SF4. I have written a detailed article on it. Have a look at the Polarity of SF4.
SeF4 Hybridization, Molecular Geometry: Valence Bond Theory
In VBT, we consider the valance shell of the central atom and determine its hybridization whenever required, to find the geometry of the molecule.
The valence shell of Se is 4s2 4p4 and it is in a +4 Oxidation state.
Therefore,
valence shell electronic configuration for Se (+4) is ns2 np0
In SeF4, the vacant valance shells will be filled by electrons from F- as shown giving rise to sp3d hybridization.
The shape corresponding to sp3d is trigonal bipyramidal, but due to the presence of one lone pair, the shape becomes like a see-saw.
Due to this see-saw shape, the dipole moments of individual Se-F bonds do not cancel out and a net dipole moment remains in the molecule. Thereby, making the SeF4 molecule polar.
Conclusion
SeF4 is an inherently polar molecule.
In the SeF4 molecule, fluorine is more electronegative than the Selenium atom.
Considering the Lewis Structure of the SeF4 molecule and applying the rules of VSEPR theory, it is evident that Se is the central atom surrounded by 4 fluorine atoms and a lone pair. The molecule becomes asymmetrical due to the presence of this one lone pair and thus attains a see-saw shape.
As a result, the individual dipole moments of the Se-F bond do not cancel out, retaining a net dipole moment in the molecule.
Valence Bond theory also suggests the polar nature of the SeF4 molecule. Observing the valance electronic configuration of Se (+4) and the electrostatic interaction of F- with the vacant valance shells of Se(+4), the hybridization of SeF4 comes out to be sp3d with one of the hybrid orbitals occupied by a lone pair of electrons. sp3d corresponds to trigonal bipyramidal geometry.
Due to the presence of one lone pair, the molecule becomes asymmetric. Yet again, due to this asymmetry, the individual Se-F dipole moments do not cancel out. Thus, the molecule retains a net dipole moment.
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