Carbonylation of 2,6-diphenylphenoxocopper(I), tetrameric [(CuOC6H3Ph2)(4)] (1, Ph = C6H5), has been shown to result in a [{Cu(OC6H3Ph2)(CO)}(2)] dimer (2). The parent aryl oxide, [(CuOC6H3Ph2)(4)], which has been prepared from mesitylcopper(I) and 2,6-diphenylphenol, has an approximately planar Cu4O4 core, in which copper(I) is two-coordinated and Cu-O bonds range from 1.834(7) to 1.865(7) Angstrom. Its carbonylation product 2 is mu(2)-phenoxobridged dimer, containing three-coordinated copper(I), with longer Cu-O bonds, viz. 1.953(7)-1.995(7) Angstrom. Cu-C bond lengths in [{Cu(OC6H3Ph2)(CO)}(2)] are 1.78(1) and 1.79(1) Angstrom, respectively, with both carbonyl C-O distances equal to 1.12(1) Angstrom, and Cu-C-O angles of 174(1) and 179(1)degrees, respectively. Carbonyl stretching frequencies in the infrared are 2099, 2103, and 2112 cm(-1) for the solid and 2102 cm(-1) in toluene solution, and the C-13 NMR signal (toluene solution) is at 168 ppm. From comparison with other carbonyl complexes of copper(I), the Cu-C bond is judged to be predominantly of sigma character, with minimal metal --> ligand pi* contribution. Both [(CuOC6H3Ph2)(4)] and [{Cu(OC6H3Ph2)(CO)}(2)] retain their aggregation stares on dissolution in nonpolar solvents, as ascertained by cryoscopic molecular weight determinations of the compounds in benzene. Crystal data : 1, triclinic, space group (No. 2), a = 12.738(9), b = 22.426(5), and c = 9.984(3) Angstrom, alpha = 101.62(2), beta = 91.02(4), and gamma = 85.93(3)similar to, Z = 2, R = 0.052 (R-W = 0.058) for 721 parameters and 3843 reflections; 2, triclinic, space group (No. 2), a = 10.67(3), b = 15.72(1), and c = 10.05(1) Angstrom, alpha = 96.99(8), beta = 104.66(16), and gamma = 101.46(12)degrees, Z = 2, R = 0.049 (R-W = 0.054) for 397 parameters and 2152 reflections.