SeF4 Lewis Structure — the quick version
Selenium sits in the middle, four fluorines around it, and one lone pair on top. That is the basic picture. But the lone pair changes everything about how the molecule actually behaves, so let me walk through it the way I explain it to students who keep getting the geometry wrong on exams. Here is what you do when you first sit down with this one. Count your valence electrons. Selenium is in group 16, so that is six. Fluorine is group 17, so each contributes seven. You have four fluorines, so that is 6 + (4 × 7) = 34 total valence electrons. Draw Se in the center, put single bonds to each F, and that uses 8 electrons. Subtract those from 34, leaving 26. Fill the fluorine octets first, which takes 24 more electrons. Now you have 2 left, and they go as a lone pair on the selenium. I remember grading a mid-term where three students drew SeF4 as a perfect tetrahedron because they forgot to account for that lone pair. They got the connectivity right but completely missed the geometry. The answer is not tetrahedral. It is seesaw.
Why the geometry is seesaw, not tetrahedral
The electron domain geometry around selenium is trigonal bipyramidal. Five regions of electron density: four bonding pairs and one lone pair. According to VSEPR theory, the lone pair occupies an equatorial position because that minimizes repulsion. Equatorial positions have 120-degree angles to their neighbors, while axial positions are only 90 degrees from three other domains. The lone pair pushes harder, so it wants the roomier equatorial spot. This gives the molecule its characteristic seesaw shape. Two fluorines are axial, two are equatorial, and the lone pair sits where you would expect a third equatorial bond. The axial F-Se-F angle is slightly less than 180 degrees because the lone pair compresses things. The equatorial F-Se-F angle is also compressed, usually to around 102 degrees instead of the ideal 120. When I demonstrate this in lab, I use a ball-and-stick model and physically show how the lone pair takes up more space. Students always say "oh" when they see it. It is one of those things that clicks once you visualize it.
Bonding details and electronegativity effects
The Se-F bonds are highly polar. Fluorine is the most electronegative element at 3.98 on the Pauling scale, while selenium sits at about 2.55. That creates a significant dipole along each bond. The molecule as a whole is polar because the bond dipoles do not cancel in a seesaw geometry. One thing beginners miss is that the axial and equatorial bonds are not the same length. Axial bonds are longer than equatorial bonds in a seesaw structure. For SeF4, the axial Se-F distance is approximately 176 pm while the equatorial Se-F is about 163 pm. This difference comes from the fact that axial bonds experience more repulsion from the equatorial lone pair and the other equatorial bonds. I encountered an edge case once when a student asked why SeF4 reacts differently with water than SF4, even though they are structurally similar. The answer lies in selenium's larger size and lower electronegativity compared to sulfur. Selenium can expand its coordination sphere more easily, making SeF4 more reactive toward nucleophiles like water. SeF4 hydrolyzes readily to give selenium dioxide and HF, while SF4 is comparatively more stable in moist air.
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Practical implications and reactivity
SeF4 is a useful fluorinating agent in organic synthesis. It can convert carbonyl groups to difluoromethylene groups under the right conditions. The reaction mechanism involves nucleophilic attack by oxygen on selenium, followed by fluoride transfer. It is milder than some alternatives like SF4, which requires dangerous conditions and produces toxic byproducts. One limitation is that SeF4 is moisture-sensitive. It fumes in air and reacts violently with water. You need to handle it in a glovebox or with proper Schlenk techniques. Storage should be in sealed containers under inert atmosphere at low temperature. I always tell my students to check the integrity of the container before opening, because even small leaks can degrade the sample over time. Another consideration is that SeF4 is a gas at room temperature with a boiling point around -46°C. Working with gaseous fluorinating agents requires careful pressure management. Some labs prefer to generate SeF4 in situ from selenium and fluorine gas, though that approach has its own safety challenges.
The compound has been studied extensively by microwave spectroscopy, which confirmed the seesaw structure and provided precise bond lengths and angles. These experimental results match well with computational predictions from DFT calculations using modern functionals. When teaching this topic, I usually spend extra time on the distinction between electron geometry and molecular geometry. Students conflate them constantly. The electron geometry describes the arrangement of all electron domains, while the molecular geometry describes only the positions of atoms. For SeF4, electron geometry is trigonal bipyramidal but molecular geometry is seesaw. That distinction matters for understanding properties like polarity and reactivity.
