= Solution
The usual qualitative order of increasing <ligand field> is
$$
\boxed{\mathrm{Cl^-}<\mathrm{P(^{t}Bu)_2Ph}<\mathrm{PPh_3}<\mathrm{CO}.}
$$
A <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.<Metal-phosphine bonding revisited. σ-Basicity, π-acidity, and the role of phosphorus d orbitals in zerovalent metal-phosphine complexes by Gianfranco Pacchioni and Paul S. Bagus>[1992, pp. 4391–4398, sigma basicity, pi acidity and phosphorus orbital participation]{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.
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