A complex ion will be formed when HCl will react will

Why are complex ions readily formed when water molecules react with the cation but not with the hydronium ions ?

The IUPAC name of the complex ion formed when gold dissolves in aquaregia is

The IUPAC name of the complex ion formed when gold dissolves in aque-regia is

The solution formed when black CuO reacts with HCl is

The IUPAC name of the complex ion formed when gold dissolves in aqua regia is _____________.