Reaction of Re 2 {µ-O 2 CC(CH 3 ) 3 } 4 Cl 2 with [(CO) 5 M-CN]Na ν(Re-Re) force constants are estimated based on the FT-IR and Raman examinations. Photochemical examinations and (M = Cr, Mo, W) leads to tetranuclear complexes of formula Re 2 {µ-O 2 CC(CH 3 ) 3 } 4 [-NC-M(CO) 5 ] 2 (M = Cr, Mo, W). These TG/MS experiments have also been conducted. The latter method shows that the product complexes decompose complexes were characterized by 1 H-, 13 C-, and 95 Mo-NMR, IR and Raman spectroscopy, elemental analysis and around 100 °C, by first loosing their carbonyl substituents; as do the Cr, Mo, W precursor compounds. The dirhenium examined by cyclic voltammetry. The applied methods show the donor capabilities of the [(CO) 5 M-CN] -ligands which tetrapivalate unit decomposes only at higher temperatures in a distinct second step. shift electron density towards the Re centers weakening the Re-Re quadruple bond. The Re-Re bond lengths and the the reactivity of Re 2 {µ-O 2 CC(CH 3 ) 3 } 4 Cl 2 with indicate the formation of the complexes 5Ϫ7. The 13 C-Campus de Santiago, 3810 Aveiro, Portugal NMR data are summarized in Table 1. All signals are influ-[d] Institute of Isotope and Surface Chemistry of the Hungarian enced by the coordination. The strongest shift difference