= Solution
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 \b[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.<Cis labilization of ligand dissociation. 3. Survey of group 6 and 7 six-coordinate carbonyl compounds. The site preference model for ligand labilization effects by Jim D. Atwood and Theodore L. Brown>[1976, pp. 3160–3161, isotope exchange, intermediate rearrangement and limitations of inferring dissociation sites from substitution products]{ref}
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