Binuclear homoleptic copper carbonyls Cu(2)(CO)(x) (x = 1-6): remarkable structures contrasting metal-metal multiple bonding with low-dimensional copper bonding manifolds.

Binuclear homoleptic copper carbonyls Cu(2)(CO)(x) (x = 1-6) have been studied using four different density functional theory methods (DFT) in conjunction with a basis set of extended double-zeta plus polarization quality, labeled as DZP. For each homoleptic binuclear copper carbonyl compound, several stationary point structures are presented, and these structures are characterized in terms of their geometries, thermochemistry, and vibrational frequencies. The optimal unsaturated Cu(2)(CO)(x) (x = 1-6) structures are generated by joining 18-electron tetrahedral, 16-electron trigonal, 14-electron linear copper carbonyl building blocks, and/or bare copper atoms with copper-copper single bonds rather than by joining 18-electron copper carbonyl units with multiple copper-copper bonds. For Cu(2)(CO)(6) the eclipsed and staggered ethane-like structure are virtually degenerate and lie significantly lower in energy than other possible structures. The eclipsed Cu-Cu single bond distance is predicted to be 2.61 A, while that for the staggered structure is 2.65 A. The lowest energy structure for Cu(2)(CO)(5) is the eclipsed ethyl radical-like structure, with r(e)(Cu-Cu) = 2.51 A. The staggered ethyl radical-like structure lies only 0.1 kcal/mol higher in energy, with a Cu-Cu distance shorter by only approximately 0.001 A. For Cu(2)(CO)(4) a methylcarbene-like structure is predicted to lie lowest, with Cu-Cu distance 2.40 A. However, twisted and planar ethylene-like structure lie only 3-5 kcal/mol higher. For Cu(2)(CO)(3) a surprising methylcarbyne-like structure with r(e)(Cu-Cu) = 2.38 A is predicted to lie lowest with all four DFT methods. However, a classical vinyl radical-like lies only 2-4 kcal/mol higher. For Cu(2)(CO)(2) theory predicts a vinylidene-like structure with r(e)(Cu-Cu) = 2.34 A to be essentially degenerate with cis and trans bent acetylene structures with copper-copper distances 2.33 A. Finally, and consistent with earlier theoretical studies, the linear end on Cu-Cu-CO structure with r(e)(Cu-Cu) = 2.27 A is the predicted global minimum for Cu(2)(CO).