# Are metals hydrogen bond acceptors or donors?

In a carbonyl or $\ce{-OH}$ group, it's clear the oxygen acts as a hydrogen bond acceptor.

Do metals act as hydrogen bond acceptors or donors and if so, how can I tell as what it's acting?

Hydrogen bonds are three centre 4 electron interactions that lower the energy of the molecular orbitals involved. We can hypothetically imagine two scenarios,

• Metal donor (M$^{\delta ^{-}}-H^{\delta ^{+}}...C$)
• Metal acceptor (M$^{\delta ^{-}}...H^{\delta ^{+}}-C$)

(where C is just something else). Metal-Hydrogen bonds where the metal is a donor are scarce since the metal hydride bond is the opposite polarity of the metal hydrogen bond donor, however they do exist.

Metal acceptors are likely to be low oxidation state late row metals with filled $d$ orbitals to donate via $\sigma$ symmetry interaction.

You get these types of interactions all the time in crystal chemistry where we find bridging hydrogens! So in short, it depends where the metal is in the period, if its $dz^2$ is available to donate etc.

If you had a specific example I would be happy to try and work it out, but it looks like you are aware of any literature I would consult!! :)

Only very electronegative atoms— $\ce{O}$, $\ce{N}$ and $\ce{F}$— participate in hydrogen bonding. The donor is the group with the covalently-bonded hydrogen participating in hydrogen-bonding and the acceptor is the atom not covalently bonded.

A $\ce{C}$ atom may also act as a hydrogen bond donor if there are highly electronegative atoms attached to it such as in the case of chloroform $\ce{CHCl3}$.

• I'm afraid that TMOTTM is right. – Mithoron Jan 22 '15 at 0:58