Question

The formula of tetraammineaquachloridocobalt(III) chloride is

• A. $[\text{Co}(\text{NH}_3)_4(\text{H}_2\text{O})\text{Cl}]\text{Cl}_2$
• B. $[\text{Co}(\text{NH}_3)_4\text{Cl}_2] \times \text{H}_2\text{O}$
• C. $[\text{Co}(\text{NH}_3)_4]\text{Cl}_3 \times \text{H}_2\text{O}$
• D. $[\text{Co}(\text{NH}_3)_4(\text{H}_2\text{O})\text{Cl}]\text{Cl}$

Hint:

When writing structural formulas from IUPAC nomenclature: 1. Identify the central metal and its oxidation state. 2. Sum up the charges of all internal ligands to find the net charge of the bracketed complex. 3. Add a sufficient number of outer counter-ions to make the total overall charge of the formula exactly equal to zero.

Answer 1

The Correct Option is A

Concept: According to the International Union of Pure and Applied Chemistry (IUPAC) nomenclature rules for coordination compounds:

• The coordination sphere is enclosed within square brackets $[\dots]$. The central transition metal cation is written first, followed immediately by the coordinated ligands.

• Ligands are listed alphabetically based on their IUPAC chemical names regardless of their net electrical charge.

• The prefixes 'tetra', 'tri', 'di' signify the quantity of that specific ligand inside the coordination sphere.

• The oxidation state of the central transition metal is specified in Roman numerals enclosed inside parentheses directly after the name of the metal.

• Ionizable anions outside the coordination sphere balances the net charge of the complex cation.

Step 1: Identifying the central metal ion and individual ligands inside the coordination sphere.
Let us systematically deconstruct the IUPAC name provided in the question text: tetraammineaquachloridocobalt(III) chloride.

• Central Metal Ion: The name contains "cobalt(III)". This informs us that the central transition metal atom is Cobalt, denoted by the chemical symbol Co, and it possesses a positive formal oxidation state of $+3$ (i.e., $\text{Co}^{3+}$).

• Ligand 1: "tetraammine" points to the presence of ammine ligands, which represent neutral ammonia molecules ($\text{NH}_3$). The prefix "tetra-" mathematically corresponds to exactly four such molecules. Hence, we write this part as: $(\text{NH}_3)_4$.

• Ligand 2: "aqua" explicitly denotes a neutral water molecule acting as a coordinating Lewis base ligand ($\text{H}_2\text{O}$). Since there is no multiplying prefix, its stoichiometry is exactly one: $(\text{H}_2\text{O})$.

• Ligand 3: "chlorido" corresponds to the anionic chloride ligand ($\text{Cl}^-$) bound inside the coordination sphere. There is no multiplying prefix, meaning there is exactly one internal chloride ligand: Cl.
Assembling the coordination entity within square brackets gives: \[ [\text{Co}(\text{NH}_3)_4(\text{H}_2\text{O})\text{Cl}] \]

Step 2: Calculating the total net charge of the coordination complex sphere.
To determine the number of counter-anions present outside the square brackets, we calculate the net algebraic charge of the complex coordination sphere by summing the individual oxidation states and charges of the constituent metal and ligands: \[ \text{Net Charge} = (\text{Charge of Co}) + 4 \times (\text{Charge of }\text{NH}_3) + 1 \times (\text{Charge of }\text{H}_2\text{O}) + 1 \times (\text{Charge of }\text{Cl}^-) \] Substituting the known electrical charges into this equation:

• Cobalt (Co) oxidation state = $+3$

• Ammine ($\text{NH}_3$) ligand charge = $0$ (neutral molecule)

• Aqua ($\text{H}_2\text{O}$) ligand charge = $0$ (neutral molecule)

• Chlorido ($\text{Cl}^-$) ligand charge = $-1$ (anionic)
\[ \text{Net Charge} = (+3) + 4(0) + 1(0) + 1(-1) \] \[ \text{Net Charge} = +3 + 0 + 0 - 1 = +2 \] Therefore, the coordination sphere is a complex cation carrying a net positive electrical charge of $+2$, which can be written as: \[ [\text{Co}(\text{NH}_3)_4(\text{H}_2\text{O})\text{Cl}]^{2+} \]

Step 3: Balancing the complex charge with counter chloride ions.
The compound name concludes with the word "chloride", indicating that ionic chloride anions ($\text{Cl}^-$) act as counter-ions situated outside the coordination sphere to ensure overall electrical neutrality of the crystalline compound. Let $x$ be the number of ionizable chloride counter-ions needed. Since each individual chloride ion carries a static charge of $-1$: \[ \text{Net Charge of Complex Cation} + x \times (\text{Charge of Chloride Anion}) = 0 \] \[ (+2) + x(-1) = 0 \quad \Rightarrow \quad 2 - x = 0 \quad \Rightarrow \quad x = 2 \] This demonstrates that exactly two chloride anions ($\text{Cl}_2$) must reside outside the square coordination brackets to successfully neutralize the $+2$ charge of the coordination sphere.

Step 4: Formulating the final chemical notation.
Combining the cationic coordination complex and the external ionizable counter-anions together yields the complete structural formula: \[ [\text{Co}(\text{NH}_3)_4(\text{H}_2\text{O})\text{Cl}]\text{Cl}_2 \] Comparing this derived structure against the options given, it corresponds exactly to Option (1).