Theoretical analysis of bonding and stereochemical trends in doubly bridged copper(I)-copper(I) dimers

BFT/B3LYP calculations on known and hypothetical doubly bridged Cu(I)-Cu(I) dimers and other d(10)-d(10) analogues have been carried out. The bridging ligands may he only a donors (hydrides) or have added pi -donor (halides) or pi -acceptor (carbonyls, as get unknown) capabilities. In particular, th e few reported LnCu(mu -C drop CR)(2)CuLm frameworks have been investigated , The latter are symmetric (type b) or asymmetric (types a and b), dependin g an the nature and number of terminal ligands (n = 1, 2; m = 1, 2). Beside the accurate geometric and energetic computations, the nature of the chemi cal bonding is explored in terms of perturbation theory arguments (EHMO app roach). Thanks to the cs donor power of the bridges, electron density is dr iven into the bonding combinations (sigma and pi) of empty metal a and p(pi ) orbitals, In the presence of pi -donor ligands, population of the corresp onding sigma* and pi* levels occurs and the M-M bond vanishes. In contrast, insufficient hack-donation from copper d orbitals prevents the formation o f bridged carbonyl dimers and trigonal-planar monomers are favored, A case study is that of the heterobinuclear d(10)-d(10) complex (CO)(2)-Cu(mu -CO) (2)Co(CO)(2) where the lone pairs of the CO bridges are preferentially dire cted toward cobalt for electronegativity reasons. X similar situation is hi ghlighted for the model (PH3)(2)-Cu(mu -C drop CH)(2)Cu(PH3) (type b), wher e both bridges orient toward the unique fragment (PR3)Cu because of the dif ferent hybridization of L2M and LM sigma orbitals. In the species LnCu(mu - C drop CH)(2)CuLn (n = 2 or n = 1, type a or c), the potential energy surfa ce for the symmetric to asymmetric rearrangement, of the central Cu2C2 ring is quite flat. However, a symmetric Cu-2(mu -C drop CR)(2) framework is ac hieved with eta (2)-bound alkynes (type c), This is attributable to the pi* levels of the latter Ligands, which stabilize the metal p(pi) orbitals inv olved in bridge bonding, The asymmetric Cu2C2 arrangement is preferred agai n in models where the terminal alkynes are substituted for by single phosph ine ligands.