Binuclear Homoleptic Copper Carbonyls Cu_2(CO)_x (x = 1-6): Remarkable Structures Contrasting Metal-Metal Multiple Bonding with Low-Dimensional Copper Bonding Manifolds

摘要

Binuclear homoleptic copper carbonyls Cu_2(CO0_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 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 jonining 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 coper 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 inenergy, with a Cu-Cu distrance shorter by only (approx) 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 structuers 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).