[orgo814]

The whole concept of a carbonium ion is confusing me. How does a carbon with 5 bonds have a positive charge? I would expect a negative charge? Insight appreciated.

[orgopete]

The whole concept of a carbonium ion is confusing me. How does a carbon with 5 bonds have a positive charge? I would expect a negative charge? Insight appreciated.

Five bonds? No. Four bonds is a completed octet, three bonds is an electron short and a net positive charge.

[beardy]

Carb = carbon
Onium = cation

With a cation carbon in mind, how many bonds do you think carbon would need to render a positive charge?

[critzz]

I think that what he means is for example the methanium-ion (CH5⁺), which actually has 5 bonds and is considered a superacid.

So let's consider a hydroxonium-cation (H₃O⁺).

To determine the formal charge on the oxygen you can use this formula: [formal charge = valence electrons - lone pair electrons - 1/2 × bonding electrons].
So if you fill in: formal charge = 6 valence electrons - 2 lone pair electrons - 1/2 × 6 bonding electrons = a formal charge of +1

For an ammonium-cation (NH₄⁺) it works the same way: 5 - 0 - 4 = +1

However, in the case of methanium it doesn't really seem to work like that (as where your confusion probably comes from): 4 valence electrons - 0 lone pair electrons - 1/2 × 10 bonding electrons = -1

What's going on here?

The methanium-cation can be seen as a CH₃⁺-cation, where the empty p-orbital is coordinated to the bond of a H₂-molecule, via a 3-center-2-electron bond.
Viewed in this way, the molecule still has a +1 charge.

[orgo814]

Thank you critzz!

[critzz]

You're welcome