[Anthasci]

I have a question I'm quite unable to find the answer to ... For example, triphenylcarbocation is a relatively stable carbocation and I can understand that, it's able to stabilize itself via resonance structures and the transfer of the positive charge along the phenyl rings. I can't however understand what logic also makes the triphenylcarbanion stable? The resonance stabilisation of the negative charge here isn't possible (or I just don't get it), and moreover, don't all alkyl groups work on a +I principle, meaning the polarisation goes towards C(-)? Wouldn't that destabilize the carbocation even more? Any answers would be greatly appreciated

[Dan]

I have a question I'm quite unable to find the answer to ... For example, triphenylcarbocation is a relatively stable carbocation and I can understand that, it's able to stabilize itself via resonance structures and the transfer of the positive charge along the phenyl rings. I can't however understand what logic also makes the triphenylcarbanion stable?

This is also due to resonance. The negative charge is delocalised, to use your words:

it's able to stabilize itself via resonance structures and the transfer of the negative charge along the phenyl rings.

Try drawing the resonance structures and you will see that the conjugated phenyl rings can delocalise both negative and positive charge.

Hint:

don't all alkyl groups work on a +I principle, meaning the polarisation goes towards C(-)? Wouldn't that destabilize the carbocation even more? Any answers would be greatly appreciated

Remember that resonance (mesomeric) effects can often override inductive effects that oppose them. For example, a substituent that is -I but +M is usually electron donating overall (can you think of one?).

You do not have alkyl substituents in this case, you have aryl substituents, which have very different electronic characteristics.

Simple alkyl groups have a +I effect and no M effect, but you need to be careful when generalising. Consider that CH₃CH₂- is +I, but CF₃CH₂- is -I. Both are alkyl groups. You can't just learn a set of "rules" and apply them to everything in chemistry. You need to understand the theory behind a given "rule" it and appreciate its limitations. They are guidelines only.

[Anthasci]

That was quite a hint, yeah, so the C-atoms of the phenyl ring are able to "absorb" the double bond and thus have a negative formal charge, while the negative charge on the central C(-) is donated into the bond with an "outermost" C of the phenyl ring to form a double bond. Got it, thanks. I'd think that Cl, Br and I would be -I, but generally +M because of their excess amount of valence electrons. It's good to know that mesomerism often prevails over induction, I'll keep that in mind (but with a little reserve, as with everything in organic chemistry).

[Dan]

I'd think that Cl, Br and I would be -I, but generally +M because of their excess amount of valence electrons.

Yes, actually funny you mentioned them because they are usually overall withdrawing (I beats M in these cases, consider that a bromo substituent on benzene deactivates it towards electrophilic substitution). I was thinking more of O, e.g. anisole is much more nucleophilic than benzene due to a +M effect that overrides a -I effect.