In chemistry, the term "formula unit" refers to the simplest ratio of ions in an ionic compound, typically consisting of a metal combined with a nonmetal. To determine which compound does not fit this definition, we need to identify the presence of a metal and a nonmetal in each option.

For example, in option A, the compound consists of sodium (Na), a metal, along with carbon (C) and oxygen (O), both of which are nonmetals. This combination qualifies as a formula unit. In the next option, although the compound is more complex, it still contains sodium, a metal, paired with nonmetals, thus also fitting the definition.

However, option C presents a different scenario. It includes boron (B), which is classified as a metalloid, and fluorine (F), a nonmetal. Since boron is not a metal, this compound cannot be associated with the term formula unit. Finally, option D features manganese (Mn), a transition metal, combined with chlorine (Cl), a nonmetal, which again qualifies as a formula unit.

In summary, the only option that does not align with the definition of a formula unit is the one containing boron, as it lacks a true metal component.