The reaction of the electronically unsaturated (56 valence electrons, ve) tetrahedral cluster [Re-4(mu(3)-H)(4)(CO)(12)] (1) with pyridazine (pydz) gives as the main initial product the tetranuclear cluster [Re-4(mu-H)(4)(mu-pydz)(pydz)(2)(CO)(12)] (2a), with 64 ve and four hydrogen-bridged metal-metal interactions, with a spiked-triangle geometry. One of the three pydz ligands bridges, in a cis configuration, the cluster edge opposite to the vertex bearing the spike, as indicated by the X-ray single-crystal analysis. This species slowly decomposes, affording the dinuclear unsaturated (32 ve) complex [Re-2(mu-H)(2)(mu-pydz)(CO)(6)] (3a) and two isomers of the tetranuclear cluster [Re-4(mu-H)(4)(mu-pydz)(2)-(CO)(12)] (64 ve), sharing an unusual square cluster geometry and differing in the trans (major, 85%, 4a) or cis (4a') configuration of the bridging pydz ligands. The structures of 3a and 4a have been ascertained by X-ray analysis, while the characterization of 4a' was hampered by its instability (slowly transforming into 3a in THF solution). Both the dimer and the square cluster 4a are also formed directly (and quickly) from 1, being present in solution since the beginning of the reaction. Cluster 4a is the main final reaction product. The reaction with phthalazine follows a similar course, with some differences in the relative amount of the final products 3b and 4b. Most of the novel complexes are able to emit light in solution at room temperature, and photophysical measurements were performed in CH2Cl2 solution on the main stable reaction products (i.e., the dinuclear species 3a and 3b and the trans square clusters 4a and 4b). The emission was in the range of 580-645 nm, from MLCT excited states, with lifetimes on the order of a hundred nanoseconds (50-473 ns). The quantum yields were 1 order of magnitude higher for the squares (1.7 and 1.3% for 4a and 4b, respectively, in CH2Cl2) than for the dinuclear complexes (similar to 0.1%). In the case of 4a, a blue shift and an increase of the emission intensity were observed upon decreasing the solvent polarity.