pi-donor ligand
/ PY-donor (pi as in 'pie') /
Some ligands do not stop at handing the metal one lone pair. After making the head-on sigma bond, they have extra filled lone pairs sticking out sideways, perpendicular to the bond axis, and they push those into the metal too. Because this sideways overlap has the symmetry of a pi bond, such a ligand is a pi donor: it gives electron density to the metal both end-on (sigma) and sideways (pi). The classic pi donors are the halides, oxide, hydroxide, and water.
Here is the crucial consequence for the d orbitals. The metal orbitals with pi symmetry — the ones pointing between the ligands — are exactly the t2g set in an octahedron (dxy, dxz, dyz). When a pi-donor ligand pushes its filled sideways lone pairs at these t2g orbitals, the two filled sets repel: the interaction raises the metal t2g in energy (it becomes weakly antibonding). Since delta-o is the gap between t2g and the eg set above, raising t2g shrinks delta-o. So pi donors are weak-field ligands: they make the splitting smaller and favor high-spin complexes.
This is the missing half of the spectrochemical series. Pure electrostatics says fluoride, being very negative, should split d orbitals strongly — yet fluoride sits near the weak-field end. The reason is exactly its pi donation: its extra lone pairs raise t2g and squeeze delta down. Recognizing pi donors versus pi acceptors is what lets ligand field theory reproduce the real order of the series, something crystal field theory never could. Pi donation also tends to make a metal center more electron-rich, which matters for its reactivity.
Fluoride and chloride are pi donors and sit at the weak-field end of the spectrochemical series, giving small delta and high-spin complexes like [CoF6]3-. The same pi donation that confuses a pure-charge model is exactly what pulls the halides down toward the weak end where they belong.
Pi donors raise t2g and shrink delta — the opposite of pi acceptors.
A ligand's pi character only matters because the metal t2g orbitals have the right symmetry to interact with it; in geometries or for orbitals where that symmetry match is absent, the pi donation has nowhere to go and does not affect delta the same way.