The recently synthesized sterically constrained copper(I) 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 (varphi, 5.6%) of all Cu(R(2)phen)(2) complexes. Complex 1 also exhibits a large positive reduction potential for the [Cu(2+)(dtbp)(2)]|[Cu(+)(dtbp)(2)] couple (E(1/2) = 0.70 V vs Fc(+/0)) and a large negative excited-state reduction potential for the [Cu(2+)(dtbp)(dtbp(-*))]|[Cu(2+)(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 CH(3)CN or CO replaces one of the dtbp ligands, a reaction that is readily followed by loss of the unique emission signature of 1. Monodentate CH(3)CN binds to the copper(I) 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 1 equiv of unbound dtbp. The exceptionally strong donor ligand CH(3)NC 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.