What is the IUPAC name of the coordination complex [Co(NH3)4Cl2]Cl?

Introduction / Context:
Coordination chemistry introduces complexes where metal ions are surrounded by ligands. Correct IUPAC naming of such complexes requires identifying the central metal, its oxidation state, and the number and type of ligands, as well as the counter ions. The complex [Co(NH3)4Cl2]Cl contains a cobalt centre coordinated to ammonia and chloride ligands with an additional chloride ion outside the coordination sphere. This question checks whether the learner can apply standard naming rules to this structure.

Given Data / Assumptions:
- The complex is written as [Co(NH3)4Cl2]Cl.
- Inside the coordination sphere are four NH3 ligands and two chloride ligands bound to cobalt.
- There is one chloride ion outside the bracket as a counter ion.
- Ammonia is a neutral ligand and chloride carries a -1 charge.
- We assume the overall complex is electrically neutral.

Concept / Approach:
To name the complex, first determine the oxidation state of cobalt. Let the oxidation state of cobalt be x. Ammonia is neutral, so it contributes zero charge. Each coordinated chloride contributes -1. The complex ion [Co(NH3)4Cl2]+ must carry a +1 charge to be balanced by the external chloride, Cl-. Therefore x plus the charges of ligands equals +1. Solving gives cobalt in the +3 oxidation state. In IUPAC naming, ligand names are listed in alphabetical order, with ammine for NH3 and chloro for Cl-. With four ammine ligands and two chloro ligands, the coordination part is named tetraammine dichlorocobalt(III), followed by the anion chloride for the outer Cl-.

Step-by-Step Solution:
Step 1: Assign charges. Ammonia, NH3, is neutral, so it contributes zero charge. Each coordinated chloride, Cl-, contributes -1. Step 2: Let the oxidation state of cobalt be x. The complex ion [Co(NH3)4Cl2]+ must have a +1 charge to balance the external Cl-. Step 3: Write the charge balance equation: x + 4(0) + 2(-1) = +1, which simplifies to x - 2 = +1. Step 4: Solve for x to obtain x = +3, so cobalt is in the +3 oxidation state. Step 5: Name the ligands in alphabetical order: ammine for NH3 and chloro for Cl-, with prefixes tetra for four and di for two, giving tetraammine dichlorocobalt(III), followed by the counter ion chloride, forming the full name tetraammine dichlorocobalt(III) chloride.

Verification / Alternative check:
An alternative check is to verify that the overall charge is zero when cobalt is assumed to be in the +3 state. The coordination entity then carries a +1 charge: cobalt contributes +3, coordinated chlorides contribute -2 in total and the four neutral ammonia ligands contribute zero, giving +1 overall. Combined with the external chloride ion, Cl-, the sum of charges becomes zero, consistent with a neutral compound. Additionally, standard naming examples of similar complexes in textbooks confirm the pattern tetraammine dichlorocobalt(III) chloride for such formulations.

Why Other Options Are Wrong:
Option B, tetraammine trichlorocobalt(III), incorrectly suggests three chloro ligands and does not include the outer chloride counter ion, leading to a different formula. Option C, triammine dichlorocobalt(III) chloride, implies only three ammonia ligands inside the coordination sphere, which does not match the given four NH3 ligands. Option D, tetraammine dichlorocobalt(II), assigns a +2 oxidation state to cobalt, which is inconsistent with the required charge balance for [Co(NH3)4Cl2]Cl. These names therefore do not accurately represent the given complex.

Common Pitfalls:
Students may forget to account for the external counter ion when determining the oxidation state of the metal, or they may confuse the number of ligands indicated by prefixes like di, tri and tetra. Another common mistake is to use amine instead of ammine for NH3 in complex names, though the IUPAC convention for ligands is ammine. Careful counting of ligands and systematic application of charge balance and naming rules helps avoid these errors.

Final Answer:
The IUPAC name of the coordination complex [Co(NH3)4Cl2]Cl is Tetraammine dichlorocobalt(III) chloride.