Theoretical investigation on structures and bonding of environmentally important mercury aquo ions and its complexes

Mercury and organomercury compounds are extremely hazardous, accumulating i n the brain and causing irreversible damage to the nervous system. In order to understand the behavior of Hg(II) ion in aqueous solution, we have stud ied the structures and bonding of its aquo complexes. We have carried out t heoretical calculations on mono, bis, tris, tetrakis and hexakis aquo speci es of Hg(II) ion and observed some unusual bonding characteristics. While d ichloromercury(II) is linear, the diaquo ion is nonlinear. The heat of aqua tion of these aquo species was calculated. Coordination behavior of chlorid e, carbonate and sulfohydryl anions with tetraaquo ion, as a model, was als o studied. The mono, di, tri and tetrachloro complexes were found to have t he expected structures. The Hg-Cl distances agreed well with those found fr om the X-ray studies of the crystal structures. Among the organomercury com pounds methylmercury(II) cation is environmentally very important. It is pr oduced as a result of degradation of Hg(II) compounds specially in the mari ne environment. Interactions of methylmercury cation and dimethylmercury wi th water, chloride, carbonate and sulfohydryl ions were investigated theore tically. In all cases considerably strong bond order for the interacting an ions was observed. The solution equilibrium studies and our calculations ar e in good agreement with each other. The bond distances were compared with the crystal data where available. Most interestingly, it was found that dim ethylmercury reacts with carbonate ion, behaving as a bidentate anion, and producing a distorted tetrahedral arrangement around Hg(II). This observati on leads to the speculation of a transport mechanism of dimethylmercury thr ough marine aquifers. The structures and bonding of the above species are d iscussed.