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The carbonyl cis effect is preferential labilization of a ligand cis to another ligand in many six-coordinate metal carbonyls. Site preference in five-coordinate intermediates and transition states matters. Isotopic exchange can distinguish departure sites more directly than substitution-product geometry alone.

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 trans effect is a kinetic influence whereby a ligand accelerates substitution opposite itself, classically in square-planar complexes. It must be distinguished from ground-state bond weakening and from the carbonyl cis effect.