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ammonia and cyclohexylamine are sigma-donor ligands; chloride and acetate are sigma- and pi-donor ligands. The nitrogen lone pair of each amine points toward the metal and forms the donor interaction. These saturated amines have no low-lying conjugated acceptor system that would make them conventional pi-acceptor ligands.

chloride uses a lone pair for sigma donation and has additional occupied $p$ orbitals perpendicular to the bond that can donate with pi symmetry. An oxygen-bound acetate similarly has occupied oxygen-centred orbitals available for sigma and pi donation, although its carboxylate resonance in chemistry changes their energies and distribution. Neither should be classified as a pi acceptor merely because its bonding can be drawn with multiple bonds or resonance in chemistry contributors. None of these four is a standard sigma-donor/pi-acceptor ligand.

The relevant spectrochemical series order is O-bound nitrito $<$ ammonia $<$ N-bound nitro. A pi-donor ligand tends to raise the metal orbitals derived from $t_{2g}$, reducing the octahedral splitting; a pi-acceptor ligand stabilizes those orbitals, increasing the splitting. Since photon energy is $E=hc/\lambda$, the stronger-field complex absorbs at the shorter wavelength. ThusThe corresponding spectroscopic wavenumbers are approximately $21834$, $21097$ and $20576\mathrm{c}{\mathrm{m}}^{-1}$. These are the observed band energies, not three exact independent determinations of ${\Delta }_o$: low-spin $d^6$ transition energies also contain electron repulsion contributions, and replacing one ligand lowers the ideal octahedral symmetry. The relative assignment is nevertheless robust.

The usual qualitative order of increasing ligand field isA chloride ligand is a sigma-donor ligand and pi-donor ligand; its occupied pi orbitals raise the metal $t_{2g}$-derived levels and reduce the splitting. A carbonyl ligand is a particularly effective pi-acceptor ligand, lowering those levels and producing a large splitting. Both phosphine ligands are sigma donors with acceptor orbitals of mainly P–C antibonding molecular orbital character. Replacing phenyl groups by electron-releasing tert-butyl groups increases sigma basicity but generally weakens pi acceptance, so the alkyl-rich phosphine is placed below triphenylphosphine in this qualitative pi-bonding comparison. It would be wrong to equate a stronger sigma donor automatically with a stronger-field ligand, or to invoke empty phosphorus $3d$ orbitals as the essential acceptors.^[ref]

This is a qualitative spectrochemical series prediction, not a universal numerical ranking for all metals and geometries. The opposing sigma and pi trends mean that precise comparisons require matched complexes and measured splittings; a Tolman electronic parameter measures net effects on CO backbonding and is not itself a ligand-field splitting.

A pi-donor ligand has an occupied orbital that can overlap sideways with a metal orbital. In the standard octahedral splitting picture it raises the metal $t_{2g}$-derived levels, tending to reduce their separation from $e_g$.

A sigma-donor ligand supplies an occupied orbital for sigma donation. Additional occupied or empty orbitals can make it a pi-donor ligand or a pi-acceptor ligand as well.

The spectrochemical series orders ligands empirically by the splitting they produce in comparable complexes. Pi-donor ligands tend toward smaller splittings and pi-acceptor ligands toward larger ones. Metal, geometry and oxidation state still matter; a sigma-basicity scale is not an interchangeable substitute.