  Active sites of molybdenum and tungsten enzymes, particularly mononuclear
tungsten formate dehydrogenase (FDH) have been theoretically investigated
towards their interaction with CO2. Obvious {\pi} interaction has been found
between the 2e reduced metallodithiole moiety and the molecular CO2. This weak
{\pi} bonding is predicated both at gas phase, noted as -6.0 kcal/mol and
aqueous solvation level, -3.6 kcal/mol. Such interaction is not only limited to
CO2, but also to the CO2 reduced product, i.e. formate, in the form of anion-
{\pi} interaction, noted as -6.8 kcal/mol and -4.1 kcal/mol respectively in gas
and aqueous solvation model. The Bailar twisted angles from 60o to 0o,
governing structure preference of tungsten dithiolene from octahedron to
triangle prism in their restricted structures, has been explored to evaluate
such {\pi} in-terrelations with CO2 and formate. An octahedral structure with 3
kcal/mol energy lower is preferred over the triangle prismatic when such
interactions are concerned.
