The acid-catalysed complexation of various substituted phthalides with Cr(CO)6 has been studied. Complexation of substituted 3-benzylidene phthalides takes place at the phenyl ring in most cases, but with di-halogeno-substituted 3-benzylidene phthalides the complexation took place at the phthalide moiety. An explanation of this difference is based on molecular modelling quantum chemical calculations