Question

Which of the following species cannot act as a Lewis base?

a) S²

Answer 1

The S2-, SH- has negative charges so its obvious that it can be used to accept an hidrogen been a lewis base.

The H2S and PH3, the central atoms S y P have unpair electrons that we can see in its electornic configuration, so those unpair electrons can be donate to accept an hidrogen, so they behave like a lewis base.

The BH3 is the only compound that can't act like a lewis base. The Boro has an atomic number of 5, been its electronic configuration 1s2, 2s2, 2p1. In its valence orbital it has 3 electrons 2s2,2p1. Because of the rule of multiplicity 1 electron of the orbital 2s2 will jump to another orbital in 2p so it can form a bigger number of bonds. So the electronic configuration will be 1s2, 2s1, 2p2 because of that the B has 3 electrons to form bond thats why we see 3 hydrogen. But all 3 electrons are used and there are no electrons unpair to act like a lewis base because the 1s2 its in an internal orbital and its not available to form bond.