Diphenic acid anhydride (1) is decarbonylated by prolonged heating with mercuric oxide (HgO) in acetic acid to give a diphenic anhydride-mercury compound (3). Since its infrared spectrum showed a shift of carbonyl adsorption to a very low wave number, the O-Hg linkage was considered to be in an iouic form (3'), with the carbon bonded to mercury becoming a carbonium ion. If this is the case, the compound would be expected to react with nucleophilic reagents and indeed 3 is reactive to KI, H₂S and KCN.