pi-acceptor ligand
/ PY-acceptor (pi as in 'pie') /
The most interesting ligands give and take. A pi-acceptor ligand makes the usual head-on sigma bond by donating a lone pair to the metal, but it also has empty orbitals of pi symmetry hanging off to the side, and the metal pushes some of its own d electrons back into those empty orbitals. So the metal donates electrons back to the ligand — a two-way street. The exemplary pi acceptors are carbon monoxide, cyanide, and the nitrogen molecule; phosphines and alkenes do it more mildly.
Trace the electron flow, because it has a name: pi back-bonding (or back-donation). First the ligand sigma-donates a lone pair forward into the metal. Then the filled metal t2g orbitals (dxy, dxz, dyz, pointing between the ligands) overlap sideways with the ligand's empty pi-antibonding orbitals and push electron density backward into them. This back-donation drains the metal t2g and stabilizes it (lowers its energy), which is the opposite of what a pi donor does. Since delta-o is the gap between t2g and eg, lowering t2g enlarges delta-o. So pi acceptors are strong-field ligands: they make the splitting large and favor low-spin complexes. The synergy is self-reinforcing — sigma donation makes the metal electron-rich, and back-donation relieves it, so the two strengthen each other.
Pi acceptors anchor the strong-field end of the spectrochemical series and explain its biggest puzzle: why neutral CO outsplits negative halides. They are also the heart of organometallic chemistry — metal carbonyls exist because of CO back-bonding, and you can literally watch the back-donation by infrared spectroscopy, since filling CO's pi-antibonding orbital weakens its triple bond and lowers its stretching frequency. Stronger back-donation, weaker C-O bond, lower frequency: a direct experimental window onto an orbital idea.
Free carbon monoxide stretches at about 2143 cm-1. Bound in Ni(CO)4 the frequency drops to roughly 2060 cm-1, and in an anionic carbonyl with even more back-donation it falls further still. That measurable drop is electron density flowing from the metal into CO's pi-antibonding orbital — pi back-bonding caught in the act.
A drop in the C-O stretching frequency is a direct ruler for how much the metal back-donates.
Back-donation needs the metal to actually have d electrons in t2g to give away, so it is strongest for electron-rich, low-oxidation-state metals; a high-oxidation-state, electron-poor metal has little to back-donate, which is why metal carbonyls favor low oxidation states.