What reaction takes place when potassium iodide is added to a tetraammine copper(II) complex?
Copper(II) is a hard Lewis acid that would be octahedrally coordinated but for Jahn-Teller distortion.  Instead, it is square-planar coordinated with two distant weakly bound apical ligands.  The lovely deep blue color, hard Lewis base ammonia versus hard Lewis acid water coordination, illustrates the nephelauxetic effect.  What would obtain with triethylamine coordination of copper?  Cu(II) is substitution labile.


Jahn-Teller effect
Nephelauxetic effect
Kinetics (and thermodynamics) of substitution reactions


Adding acid protonates the ammonia, its lone pair now in a dative bond with a proton and unavailable for coordination.  Tetraamminecopper(II) (note the spelling of "ammine" when coordinated) becomes tetraaquocopper(II) with fast kinetics.

Aqueous Cu(II) is a modest oxidizing agent; iodide is a modest reducing agent.  Cu(I) is a soft Lewis acid, iodide is a soft Lewis base, $\ce{CuI}$ is very insoluble (look up its $\ce{K_{sp}}$ -- see here.)

Iodine reacts with excess iodide to give triiodide anion.  However, isolate the "$\ce{CuI}$," wash, and titrate with thiosulfate.  $\ce{CuI}$ strongly adsorbs elemental iodine.  It is also modesty light sensitive.  Cyanide, being a pseudohalide, follows the same path to afford exceptionally insoluble CuCN plus cyanogen.

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