I read that bond vibrations are only IR active if they produce a change in dipole moment. I'm trying to visualise how this works with chromium hexacarbonyl: If I'm not mistaken, this compound has one significant IR absorption at around 1900 cm-1, which is a C-O stretching absorption. Is this produced by an asymmetrical stretch? For example, if the ligand at the top is stretched, but the ligand at the bottom is compressed (as in the bond length is temporarily shorter)? Thats the only way I can see the dipole changing. Then again, what about bending? If one of the C-O bonds bends so that it is no longer colinear with the M-C bond axis, that would change the dipole moment, wouldn't it? Also, what about Jahn-Taller distortions? Would they cause a change in dipole moment? Because all the ligands are identical and the molecule is completely, I'm guessing Jahn-Taller distortions don't change the dipole moment of the molecule. Am I right?