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Gianfranco Pacchioni coauthored research on phosphine.^[ref]

Phosphine is ${\mathrm{PH}}_3$. Its abundance in giant-planet observable atmospheres can depend on transport from deeper layers faster than local chemical destruction.

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.

Paul S. Bagus coauthored calculations revisiting metal-phosphine bonding. The analysis separates sigma donation, pi acceptance, and the proposed role of phosphorus d orbitals in zerovalent metal complexes.

These distinctions explain why calling a phosphine simply a strong ligand loses information. Donor and acceptor interactions are separate components of the bonding problem, and an orbital calculation is not a universal experimental ranking of ligand strength.^[ref]