[kindpastor]

What do chemists mean when they refer to "complex reagents" and "excess reagents"?

[DevaDevil]

complexes are usually considered metal-ligand compounds that still have a formal charge.
wiki

excess reagent is nothing more than what's left of the starting reagent(s) after the reaction is complete (in other words the reagents that are in excess stoechiometrically)

[Nick]

DevaDevil and Wiki are definitely correct, however, I would expand the definition of "complex reagent" to include any species designed to bind to another species.  While many complex reagents are ligands that bind to metals, ligands have been designed to bind to many other things, and such ligands are also routinely referred to as complex reagents.  For example, a biochemist would refer to streptavidin (protein) as a complex reagent that binds specifically to the small molecule biotin to produce a complex.  An example of a common complex reagent that binds to metal ions is EDTA (ethylenediamine tetraacetic acid).

Again, this is not a disagreement with a previous post in the thread.  I just don't want you to be confused if you hear the term used outside the context of metal-ligand complexes.