sigma donation
/ SIG-ma do-NAY-shun /
How does a ligand actually attach to a metal? The most basic way is for the ligand to hand the metal a pair of electrons head-on. A water molecule, an ammonia, a chloride — each has a lone pair pointing outward, and it aims that lone pair straight at the metal, forming a bond along the line joining them. Donating a lone pair into the metal this way, end-on along the bond axis, is sigma donation. It is the bread-and-butter of coordinate bonding.
Sigma comes from the symmetry of the resulting bond: a sigma bond is cylindrically symmetric around the metal-ligand axis, with electron density piled up directly between the two atoms. In molecular-orbital language, the ligand's filled donor orbital overlaps with an empty metal orbital that points toward it. In an octahedral complex, the metal orbitals shaped to accept these head-on donations are exactly the ones pointing at the ligands — the eg-type d orbitals (dz2, dx2-y2) together with the metal 4s and 4p. The ligand lone pairs and these metal acceptor orbitals combine into bonding molecular orbitals (filled, holding the donated electrons) and antibonding ones (the eg* set the d electrons partly occupy). This sigma framework alone is enough to push the eg set up and create a crystal-field-like splitting.
Sigma donation is the foundation on which the whole ligand field is built: it sets the baseline size of delta, and the stronger a sigma donor, the larger the splitting. Strong sigma donors like ammonia and the carbanions of organometallic chemistry drive delta up; weaker donors keep it low. Pi bonding, when present, then tunes delta further up or down from this sigma baseline — but without sigma donation there would be no metal-ligand bond and no complex at all.
Ammonia is a pure sigma donor: its nitrogen lone pair points at the metal and donates head-on, with no pi orbitals to complicate things. That clean, strong sigma donation is why ammonia sits fairly high in the spectrochemical series and why ammine complexes like [Co(NH3)6]3+ have a respectably large delta.
A lone pair aimed straight at the metal makes the simplest, most universal coordinate bond.
Sigma donation alone makes the metal more electron-rich, which would build up negative charge on it; in many real complexes a pi-acceptor ligand relieves this by drawing some of that density back out, so sigma and pi bonding often work as a synergic team rather than in isolation.