[yang09]

Question: When H₂ binds to a metal ion, is it a sigma donor, pie-donor or pie-acceptor?

The answer to this problem is pie acceptor but I do not understand why. The molecular orbital diagram for H₂ shows that it has 2 electrons in the s orbital; therefore, shouldn't it be a sigma donor? For a ligand to be a pie acceptor, it has to have an empty p-orbital, but H₂ does not have a p-orbital. Thanks for any help

[DevaDevil]

a pi acceptor accepts a pi electron (in hydrogen's care it would go into the antibonding sigma* orbital)