The recently synthesized sterically constrained copper(l) complex [Cu(dtbp)(2)](+) (1), where dtbp is 2,9-di-tert-butyl-1,10-phenanthroline, exhibits unique photophysical and reactivity properties. Complex 1 (lambda(abs), 425 nm; epsilon, 3100 L M-1 cm(-1); lambda(emission), 599 nm) has the longest metal-to-ligand charge-transfer (MLCT) emission lifetime (tau, 3260 ns) and largest quantum yield (phi, 5.6%) of all (Cu(R(2)phen)(2)](+) complexes, Complex 1 also exhibits a large positive reduction potential for the [Cu2+(dtbp)(2)]I[Cu+(dtbp)(2)] couple (E-1/2 = 0.70 V vs Fc(+/0)) and a large negative excited-state reduction potential for the [Cu2+(dtbp)(dtbp(-center dot))]I[Cu2+(dtbp)(2)] couple (E-1/2 = -1.66 V vs Fc(+/0)), indicating that this complex is a potent photoreductant in the excited state. The steric constraint imposed by the t-butyl substituents in 1 enables unusual ligand replacement reactivity. Either CH3CN or CO replaces one of the dtbp ligands, a reaction that is readily followed by loss of the unique emission signature of 1. Monodentate CH3CN binds to the copper(l) center with an affinity 2 orders of magnitude greater than that of the displaced dtbp, despite the fact that the displaced ligand is bidentate. CO-induced displacement of dtbp from 1 is reversible, but only in the presence of I equiv of unbound dtbp. The exceptionally strong donor ligand CH3NC displaces both dtbp ligands from 1. In contrast to the facile ligand displacement reactivity With good donor ligands, 1 does not react readily with O-2, by either a ligand displacement or an oxidative pathway. Rather, O-2 induces partial quenching of emission via an outer-sphere interaction with 1.