Photooxidation of the Diaquodimercury(1) Cation

The cation Hgz2+ may be regarded as the simplest binuclear complex that contains a metal-metal single bond. This bond consists of an electron pair in a ubonding MO which is formed by the interaction of the 6s orbitals of both mercury ions [ 11. While in numerous studies it has been shown that metal-metal single bonds of binuclear transition metal complexes can be split homolytically by uu* excitation [2, 31, the photochemistry of Hg22+ is virtually unexplored to our knowledge. We present here the first results on the photoreactivitiy of Hg,‘+. Our study was stimulated partially by Mason [l], who assigned electronic transitions to absorption bands in the spectrum of [Hg2(H20)2]2’ and raised the question of whether the uu* state of the Hgz2+ moiety is an unbound state which may lead to a dissociation. The electronic absorption spectrum of [Hg,(H20),12+ [l] in diluted HClO,, (0.01 M) is dominated by a very broad and intense band in the UV (Fig. 1) at X,, = 237 nm (E = 27600). The complex did not show any luminescence (&.,,, = 250 nm) in deaerated aqueous solutions at room temperature or in ethanol glasses at 77 K. Light absorption by