The reaction of Cp*(CO)(2)Re=Re(CO)(2)Cp* (1) with dimethyl acetylenedicarboxylate (DMAD) produced the 3,4-dimetallacyclobutene Cp*(CO)(2)Re(mu-eta>(1),eta(1)-CH3O2CC=CCO2CH3)Re(CO)(2)Cp* (2). Photochemical rearrangement of 2 produced Cp*(CO)(2)Re(mu-eta(2),eta(2)-CH3O2CC=CCO2CH3)Re(CO)(2)Cp* (3), in which both Cp*(CO)(2)Re units are coordinated to the alkyne. Upon heating at 72 degrees C, 3 was converted (t(1/2) approximate to 120 min) to a 78:22 equilibrium mixture of 2:3. Photolysis of 3 in a Rayonet photoreactor (maximum emission 300 nm) resulted in CO loss and formation of Cp*(CO)Re(mu-eta(2),eta(2)-CH3O2CC=CCO2CH3)(mu-CO)Re(CO)Cp* (4). Compounds 2, 3, and 4 were characterized by X-ray crystallography : 2, orthorhombic, Pcba, a = 14.480(3) Angstrom, b = 16.246(3) Angstrom, c = 26.027(3) Angstrom, Z = 8, D-c = 1.946 Mg/m(3); 3, monoclinic, C2/c, a = 12.854(2) Angstrom, b = 14.729(3) Angstrom, c = 16.650(3) Angstrom, beta = 108.188(12)degrees, Z = 4, D-c = 1.989 Mg/m(3); 4, trigonal, P3(2), a = 16.277(2) Angstrom, b = 16.277(2) Angstrom, c = 9.408(3) Angstrom, alpha = 90 degrees, beta = 90 degrees, gamma = 120 degrees, Z = 3, D-c = 2.005 Mg/m(3).