I hate to bring up an old topic, but I believe I got the same result as Something with CuO and CuSO4 except that in both cases I had a green solution that over time yielded a green precipitate. The product yield was too high for trace stabilizers to have been the ligands, I believe. I used 3% H2O2. The reactions with CuO is much slower, and was only partially reacted two days after mixing. Both are very endothermic and are photosensitive. I am quite stumped myself, but my guess is that it's got to be a bivalent cuprate; I'd hypothesize Cu[Cu(OH)4] or cupric tetrahydroxycuprate(II). My guess is based on the fact that every precursor Something and I have used has a particularly basic ligand.