= Solution
Photoinduced CO loss gives a 16-electron fragment. In the ester-substituted complex the pendant <ester> carbonyl <oxygen> can reach <chromium> and capture that vacant site. A suitable structure is \b[$\mathrm{Cr}(\eta^6\!\text{-}\mathrm{C_6H_5CO_2Me})(\mathrm{CO})_2(\kappa O\!\text{-ester})$], retaining the coordinated <arene> and adding intramolecular coordination of its <ester> <oxygen>. The added <oxygen> lone-pair donation restores an 18-electron count. The gaseous by-product is \b[<carbon monoxide>].
<heptane> is a poor donor, but <intramolecular ligand trapping> avoids needing a donor solvent. Benzene has no pendant donor. Although the <methoxy group> has <oxygen> <lone pairs>, its directly attached <oxygen> does not have the same reach and favourable geometry as the projecting <ester> carbonyl <oxygen>; possession of a <lone pair> alone is insufficient. For these two substrates, <acetonitrile> can trap the unsaturated fragment through <nitrogen> to give $\mathrm{Cr}(\eta^6\!\text{-arene})(\mathrm{CO})_2(\mathrm{NCMe})$. Without a suitable trap, CO recombination can reverse the initial photochemical dissociation.
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