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The CO trans to a phosphine ligand competes for metal pi donation with a weaker pi acceptor than CO itself, whereas a cis CO has another CO trans to it. The trans-to-phosphine carbonyl can therefore benefit from stronger M–CO pi backbonding; loss of one of the four cis carbonyls is commonly favored. Capture of the incoming phosphine at that vacancy gives the cis bis(phosphine) product. The fourfold statistical availability of cis sites reinforces an electronic preference but does not establish it by itself.

This is the carbonyl cis effect, rather than the square-planar trans effect applied without qualification. A fuller explanation compares the five-coordinate dissociation transition states and their ligand site preferences. Product geometry alone cannot prove which CO first departed, because the intermediate can rearrange before capture. Isotopically labeled CO exchange is a more discriminating experiment; the classic studies explicitly separate this issue from product stereochemistry.^[ref]

The readily displaced acetonitrile sites have both been exhausted. A second chloride would have to displace a strongly bound carbonyl ligand, open the chelate ring, or associate to an already 18-electron center to make an unfavorable crowded 20-electron species. The product is neutral, so further chloride uptake also lacks the charge-neutralization advantage of $D\to E$ and would generate an anionic complex if substitution occurred. The chelate effect disfavors loss of the tethered pyridine, while strong pi backbonding helps retain CO under the stated conditions. These combined kinetic and bonding arguments explain the absence of further reaction. The 18-electron rule alone would not forbid replacing CO by chloride, since that replacement is isoelectronic; it therefore cannot be the whole explanation.

Strong ligand-to-metal pi donation is favoured by an occupied ligand p orbital and a suitable empty metal d orbital of matching symmetry, with reasonably similar energies and good spatial overlap. Small donor atoms such as O or N can supply short bonds and efficient overlap; oxo, imido and nitrido ligands are familiar examples. Early transition metals in high oxidation states, often $d^0$, are particularly favourable because the metal acceptor orbitals are vacant and relatively low in energy. More spatially extended $4d$ and $5d$ orbitals can strengthen the overlap.

Metal-to-ligand pi backbonding is the complementary case: electron-rich, low-oxidation-state metals donate occupied d orbitals into low-lying ligand antibonding orbitals, as for CO. In both directions, filling a metal–ligand pi-antibonding orbital offsets the multiple-bond stabilization. The relevant prediction is orbital occupancy and overlap, rather than the presence of a formal double-bond line in a drawing.

A phosphine ligand such as $\mathrm{P}{\mathrm{R}}_3$ donates a phosphorus lone pair. It can also accept pi backbonding into orbitals with substantial P–R antibonding character; phosphorus $3d$ orbitals need not be essential acceptors. Alkyl substitution generally increases sigma donation relative to electron-withdrawing substituents.

A phosphite ligand such as $\mathrm{P}(\mathrm{OR}{)}_3$ binds through a phosphorus lone pair and can accept pi backbonding. Its donation and acceptance differ from those of both phosphine ligands and carbonyl ligands.

A pi-acceptor ligand has a suitable unoccupied orbital for pi backbonding. Stabilization of metal $t_{2g}$-derived levels increases the usual octahedral splitting.

Pi backbonding is donation from occupied metal orbitals into ligand acceptor orbitals with pi symmetry about the metal–ligand axis. In a carbonyl ligand, occupation of CO ${\pi }^{\ast }$ weakens C–O while strengthening metal–carbon bonding. Lower CO stretching spectroscopic wavenumber is therefore a useful comparative probe, with electrostatic and mode-coupling qualifications.

Donation from an occupied ligand orbital into a metal orbital of pi symmetry relative to the metal–ligand axis. It differs from metal-to-ligand pi backbonding.