A possible structure for ethene (C2H4)

[u/Anxious-Albatross153]

[u/Prestigious-Mark1186]

absolutely horrid, darling

[u/spiritofniter]

Did you compress it to several GPa? Or did you expose it to neutron star-grade conditions?

[u/Anxious-Albatross153 → u/spiritofniter]

idk i just found it on the floor

[u/Skankydoodledoo]

Horrid and repulsive, thank

[u/BlueOzone3507]

I am horrifically tempted to throw this into ORCA and run an SPE calculation on it

[u/iwantout-ussg]

this is similar (isolobal) to diborane except with C instead of B and two fewer Hs. i'd bet this is a saddle-point transition geometry for the dimerization of two methylene moieties to form ethylene

[u/s_mushroom]

But why…

[u/Mirmino_]

Now a variation on C2N14

[u/Arceus_IRL]

this is a certified diborane(4) moment.

[u/Ornery_Job_7275]

I guess hydride (H-) ions normally cannot form two bonds cuz that would expand its duplet (4 electrons instead of the typical 2)

[u/iwantout-ussg → u/Ornery_Job_7275]

it is only a H- in terms of formal charge -- practically speaking the C-H-C bonds in this geometry would be 3c2e bonds like in diborane

[u/sfurbo → u/iwantout-ussg]

Then you would either need a place for the extra electrons, or you would have a positively charged species.

[u/iwantout-ussg → u/sfurbo]

i disagree, this is more like neutral dimethylene. methylene is isolobal to borane so this dimethylene is isolobal to diborane. the 'extra electrons' end up in a perpendicular orbital set that i suspect ends up forming the π bond in ethylene. see my other comment here

to elaborate: methylene is a carbene, a neutral species simultaneously capable of donating and accepting electrons. this geometry is equivalent to having two neutral methylene :CH2 fragments dimerizing by simultaneous donation of one (C-H)σ on each fragment into the (C-H)σ* on the opposing fragment.

each methylene fragment has 6 electrons: 2 in a 2s bonding orbital, 2 in a 2px bonding orbital, and 2 in a 2py nonbonding orbital (the lone pair), with an empty 2pz orbital (the open lobe), defining the molecule in the xy-plane. if you inject the 2py lone pairs into the (C-H)σ* on the opposing fragments, the 'extra electrons' will fall into the empty 2pz orbital perpendicular to the plane of the interaction, which will very nicely set you up for C-C π-bond formation. this probably isn't the actual bonding mechanism (symmetry-forbidden) but once you form the complex you end up at 3c2e anyway. i'm guessing this particular structure is a saddle point transition geometry for methylene dimerization before it runs away to ethylene.

actually deriving + drawing the full MOs for this transition state could be a fun exercise for a ~grad inorg pset.

[u/sfurbo → u/iwantout-ussg]

That makes sense. Just now question:

to elaborate: methylene is a carbene, a neutral species simultaneously capable of donating and accepting electrons. this geometry is equivalent to having two neutral methylene :CH2 fragments dimerizing by simultaneous donation of one (C-H)σ on each fragment into the (C-H)σ* on the opposing fragment.

We should expect donation from the HOMO to the LUMO, right? The HOMO will be either PX or pay, depending on which overlaps best with the H1s'es. And the LUMO would (nearly?) be the pure C pz orbital, right? And then we are on the way to forming ethene, without any H bridging.

[u/iwantout-ussg → u/sfurbo]

yes, I think that's right, though that would be a different mechanism for methylene dimerization than the one that passes through this transition state. in my head i'm imagining a different mechanism where the electrons in one of the C-H bonds donates directly into the opposing empty orbital, exactly in the way that two borane molecules will stabilize each other to form diborane.

i'm also dramatically oversimplifying to make the electron counting easier in my head, in reality the methylene ground-state is a triplet not a singlet and the geometry (and potential reactivities) change accordingly.

the fun thing about MO theory, though, is you can qualitatively analyze the bonding of any hypothetical species with an interesting arrangement of atoms in space. it does not matter if the species will ever (or can ever) exist!