Question

Which type of bonding can be found in `"As"("C"_6"H"_5)_3`?

1. `p pi(As)-p pi(C)`
2. `d pi(As)-p pi(C)`
3. `d pi(As)-d pi(C)`
4. `p pi(As)-d pi(C)`

Answer 1

I'm assuming that you mean a monohapto complex (bound via one carbon to arsenic, not all six). That would be `(eta_1-"C"_6"H"_5)_3"As"`.

Arsenic has the `4s` and `4p` valence orbitals, while benzene carbons have their `2p` and `2s`.

Arsenic is utilizing `sp^2` orbitals to bond, having a trigonal planar electron geometry. Carbon certainly doesn't have access to its `d` orbitals, so it's not 3.

If we consider the valence atomic orbital energies, according to Miessler et al. Appendix B.9, we have:

• `E_(2s)^"C" = -"19.43 eV"`
• `E_(2p)^"C" = -"10.66 eV"`
• `E_(4s)^"As" = -"18.94 eV"`
• `E_(4p)^"As" = -"9.17 eV"`

In the reference, apparently the `3d` orbitals of arsenic aren't high enough in energy to be considered valence because they aren't listed. This page also labels the 3d as "marginal core", but not "valence".

We can see that:

• The orbitals of arsenic and carbon are close in energy despite being two quantum levels `n` apart.
• Their `p` orbitals are sufficiently close in energy (they differ by only `"1.54 eV"`, or `"148.59 kJ/mol"`), so those can interact favorably.

As `sp^2sigma-sp^2sigma` interactions aren't an option up above, I would say `ppi-ppi` interactions.

Apparently, that would imply the `2p_z` of carbon and the `4p_z` of arsenic, then, as that's the carbon `p` orbital perpendicular to the benzene ring that carbon is using to delocalize the electrons, and those are the only `p` orbitals that can interact in a `pi` fashion. Carbon's other three `sp^2` orbitals are occupied in `sigma` bonding.

I expect the `ppi-ppi` interaction to be in addition to the `sp^2` bonding, but to not be a true `pi` bond, as that would draw electron density from the benzene rings, decreasing their aromaticity (which requires delocalization inside the ring).