= Solution
For an octahedral complex, strong axial <sigma donation> raises the <metal>–ligand antibonding $e_g$-derived orbitals. A <hydride> is a very strong sigma donor: its occupied $1s$ orbital overlaps effectively along a <metal>–ligand axis and has no filled perpendicular p orbitals to act as a conventional pi donor. It can therefore produce a large <octahedral splitting> by raising $e_g$ relative to $t_{2g}$.
<Triphenylphosphine> also gives <sigma donation>, but its useful acceptor orbitals additionally receive <metal> <pi backbonding>. This lowers the <metal> $t_{2g}$-derived levels and enlarges their separation from $e_g$. The acceptors have substantial P–C antibonding character; invoking a dominant <phosphorus> $3d$ acceptor is unnecessary. Thus \b[<hydride> is strong-field principally through sigma bonding, while PPh$_3$ combines <sigma donation> with pi acceptance]. The <spectrochemical series> describes the resulting splitting, not just a ranking of ligand basicity.
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