Because of the typical instability of copper nanoclusters, atom-precise structural elucidation of these nanoclusters has remained elusive. Herein, we report an air-and moisture-stable 23-copper nanocluster (SD/Cu23a or SD/Cu23b) isolated from the reaction of Cu(CF3COO)(2), (BuC)-Bu-t equivalent to CH, Cu powder, and Ph2SiH2 using a gradient reduction (Cu-II -> Cu-I -> Cu-0) strategy (GRS), which is competent for controlling the kinetics of the reduction reaction, thus avoiding formation of pure Cu-I complexes or large Cu-0 nanoparticles. The solid-state structure of the Cu-23 nanocluster shows a rare [Cu-4](0) tetrahedral kernel surrounded by an outer Cu-19 shell, which is protected by (BuC)-Bu-t equivalent to C- and CF3COO-ligands. The Cu(23)nanocluster is a rare four-electron superatom with a 1S(2)IP(2) electronic shell closure and can be crystallized in two polymorphs (R3c and R (3) over bar) depending on the solvent used. The crystallization of SD/Cu23a in the R3c space group is mainly governed by van der Waals forces and C-H center dot center dot center dot F interactions, whereas additional intermolecular C-H center dot center dot center dot Clc(hloroform) interactions are responsible for the R3 space group of SD/Cu23b. This work not only shows the ingenuity of a gradient reduction strategy for the synthesis of copper nanoclusters but also provides a better fundamental understanding of how to produce the polymorphic copper nanoclusters in a precisely tunable fashion.