Co(NO2)2 is the chemical formula for cobalt nitrite. Nitrites are compounds derived from nitrous acid. In the case of cobalt nitrite, the negative NO2 ion is joined ionically with a positive cobalt(II) ion to form an ionic compound.
The chemical formula for nitrous acid is HNO2, but acids easily give up their hydrogen atoms, which float in solution. This is what makes acids very reactive. Cobalt nitrite is not to be confused with the related compound cobaltinitrite, which is formed from cobalt(III). It also has the chemical formula of Co(NO2), but instead of two nitrites attached to a single cobalt atom, there are six.