Gold (Au) reacts in aqua regia, a mixture of concentrated nitric and hydrochloric acids, to form a complex ion. The reaction is:
\[
\text{Au} + 4 \text{HCl} + \text{HNO}_3 \rightarrow \text{H[AuCl}_4] + 2 \text{H}_2\text{O} + \text{NO}.
\]
The resulting complex ion is \( [\text{AuCl}_4]^- \), characterized by chlorido ions (Cl-) coordinated to the central gold ion (Au). This is a typical coordination compound.
Step 1: Determine Gold's Oxidation State.
To find the oxidation state of gold (Au) in \( [\text{AuCl}_4]^- \), follow these steps:
Assign \( x \) as the oxidation state of gold. Chlorine atoms in chloride ligands have an oxidation state of \( -1 \). For the complex ion:
\[
x + 4(-1) = -1.
\]
Solving the equation:
\[
x - 4 = -1 \quad \Rightarrow \quad x = +3.
\]
Therefore, gold in this complex has an oxidation state of \( +3 \).
Step 2: Establish the IUPAC Name.
The complex ion formed is \( [\text{AuCl}_4]^- \). Adhering to IUPAC nomenclature rules:
- "tetra-" indicates four chloride (Cl) ligands.
- The central metal, gold (Au), is referred to as "aurate".
- The metal's oxidation state, \( +3 \), is denoted by Roman numerals as (III).
Consequently, the IUPAC name for the complex ion \( [\text{AuCl}_4]^- \) is Tetrachloridoaurate (III).
Conclusion:
The correct IUPAC name for the complex ion generated when gold dissolves in aqua regia is \( \mathbf{Tetrachloridoaurate (III)} \).
Thus, the correct option is \( \mathbf{(C)} \).