Covalent bond formation to proteins is made difficult by their multiple unprotected functional groups and normally low concentrations. A water-soluble sulfonated bathophenanthroline ligand (2) was used to promote a highly efficient Cu(I)-mediated azide-alkyne cycloaddition (CuAAC) reaction for the chemoselective attachment of biologically relevant molecules to cowpea mosaic virus (CPMV). The ligated substrates included complex sugars, peptides, poly(ethylene oxide) polymers, and the iron carrier protein transferrin, with routine success even for cases that were previously resistant to azide-alkyne coupling using the conventional ligand tris(triazolyl)amine (1). The use of 4-6 equiv of substrate was sufficient to achieve loadings of 60-115 molecules/virion in yields of 60-85%. Although it is sensitive to oxygen, the reliably efficient performance of the Cu.2 system makes it a useful tool for demanding bioconjugation applications.