ABSTRACT

Cu(II) is an open shell transition metal ion with a d9 configuration, with one unpaired electron located in the d^ .y orbital. Instead, Cu(I) possesses a d10 closed shell, being diamagnetic. As a consequence, the reduced ion is devoid of most spectroscopic prop­ erties, and its study has been limited to X-ray absorption [138] and photoelectron spectroscopies [225]. On the other hand, the unusual coordination features of blue copper sites endow them with unique spectroscopic properties relative to normal Cu(II) complexes. Oxidized BCPs display a strong absorption at 600 nm, assigned to a TiCys -> Cu(II) LMCT transition [229-231]. Its intensity is attributed to the orientation adopted by the d*2 _y 2 orbital, which favors overlap with a n orbital of the Sy(Cys) and gives rise to a strongly covalent Cu(II)-Cys bond [231,232]. A second band, at about 450 nm, has been assigned as a pseudo-aCys -> Cu(II) LMCT transi­ tion based on resonance Raman experiments [233,234]. The relative intensity of these bands is a distinctive feature of each BCP (Table 4) [235].