Electronic spectra over the 50 000-20 000-cm-1 region are reported for well-characterized chromophores having Cu(ll)-imidazole (ImH) and Cu(Il)-imidazolate (Im_) units. For tetragonal Cu(Il)-lmH chromophores, three ligand to metal charge-transfer (LMCT) absorptions originate from the -symmetry nitrogen donor lone pair and from two -symmetry ring orbitals, one having primarily carbon character ( ) and the other having primarily nitrogen character (7 2). These ff(lmH) 2( ) -and TTi(lmH) ->• Cu(Il) LMCT absorptions occur at ~220, ~260, and ~330 nm, respectively. Ligand rotation causes the -symmetry absorptions to be broadened for solutions containing geometrically unconstrained Cu(ll)-ImH complexes. The -symmetry absorptions generally are well-resolved spectral features of crystalline complexes, and may be split when the ImH groups have nonequivalent orientations. The cr(ImH) -Cu(II) absorption at 220 nm is insensitive to ligand rotation about the Cu-N axis, and is well resolved from the ligand-localized absorption at ~205 nm. The Cu(l l)-lm" complexes exhibit an additional and characteristic broad absorption at --375 nm for which a tentative assignment has been suggested. Tetragonal type 2 and type 3 copper protein chromophores are expected to exhibit corresponding ( ) -* Cu(ll) LMCT transitions in the near-UV region. Such absorptions are expected to be red shifted for the approximately tetrahedral type 1 copper chromophores. The reported spectra of the above types of proteins briefly are reconsidered from this point of view.

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