Animated Solution for Chemistry - Coordination Compounds: Which among the following will be named as dibromidobis-(ethylenediamine) chromium (III) bromide?

Imagine you are an architect, and an IUPAC name is the blueprint handed to you by a master builder. Your job is to translate that long, seemingly complex string of text into a beautiful, structurally sound chemical formula. Coordination chemistry has its own distinct language, and once you understand the grammar, decoding names like dibromidobis-(ethylenediamine) chromium (III) bromide becomes an incredibly satisfying puzzle.

Let's embark on this translation journey. The secret to mastering IUPAC nomenclature for coordination compounds is to never read the name from left to right like a normal sentence. Instead, you must dissect it into its functional anatomical parts: the central metal, the ligands, the coordination sphere, and the counter ions.

The heart of any coordination compound is its central metal atom. In our blueprint, we see chromium (III). This immediately tells us that our central anchor is a Chromium atom ($Cr$), and the Roman numeral $(III)$ is its oxidation state. This $+3$ charge is the foundational number we will use to balance the entire molecule later.

Next, we look at the entourage surrounding the metal—the ligands. The name lists them alphabetically. First, we have dibromido. The prefix 'di' indicates two, and 'bromido' refers to the bromide ion ($B{r}^{-}$). So, we have two $B{r}^{-}$ ligands.

Then comes bis-(ethylenediamine). Why 'bis' and not 'di'? Ethylenediamine is a complex organic ligand whose name already contains the prefix 'di' (in 'diamine'). To avoid confusion—so we don't think we have an 'ethylenedi' and an 'amine'—IUPAC rules dictate the use of alternative prefixes like bis, tris, and tetrakis. Thus, 'bis' tells us we have two entire molecules of ethylenediamine. We commonly abbreviate this bidentate ligand as $en$. Crucially, ethylenediamine is a neutral molecule, meaning it contributes a charge of $0$ to the complex.

Now, we must construct the coordination sphere, which is always enclosed in square brackets. Inside these brackets, a tug-of-war of electrical charges is happening. We must calculate the net charge of this sphere to know how it interacts with the outside world.

The math is straightforward. We sum the oxidation state of the central metal and the total charges of all attached ligands:

Substituting our values:

Our coordination sphere is a cation with a net charge of $+1$. We write it as $[Cr(en{)}_{2}B{r}_2{]}^{+}$.

A chemical compound must be electrically neutral. Our complex ion has a $+1$ charge, so it is desperately seeking a negative charge to balance it out. We look back at the very end of our IUPAC name and find the word bromide.

This is our counter ion, residing outside the square brackets. A bromide ion ($B{r}^{-}$) carries a $-1$ charge. Since our complex ion is $+1$, we need exactly one bromide ion to achieve perfect electrical neutrality.

Finally, we assemble the formula. By convention, inside the square brackets, the metal symbol is written first, followed by the ligands. Placing the counter ion outside, we arrive at our final, elegant structure:

And just like that, you have successfully translated the blueprint into reality. The logic is flawless, the math is simple, and the resulting chemistry is beautiful.