Copper(I) hydride complexes represent a promising entry into formic acid dehydrogenation catalysis. Herein we present the spontaneous decarboxylation of a mu(1,3)-formate-bridged dicopper(II) complex (1(H)) to a hexacopper(I) hydride cluster (2(H)) upon reduction. Isotopic labeling studies revealed that both the H- and CO2 originate from the bound mu(1,3)-formate in 1(H), which represents a key step of the metal-mediated formic acid dehydrogenation. The full reaction equation for the conversion of 1(H) to 2(H) is established. The structure of 2(H) features two Cu-3 triangles, each capped by a hydride ligand. Typical hydride reactivity of 2(H) is demonstrated by the addition of phenylacetylene, leading to the replacement of the hydrides by alkynide ligands C-equivalent to CPh (3) while retaining the hexacopper(I) core. Temperature-dependent dynamic behavior in solution on the NMR time scale was observed for both 2(H) and 3, reflecting the rich structural landscape of the bis(pyrazolate)-bridged hexacopper(I) core (four isomers each for 2(H) and 3) predicted by DFT calculations.