DFT/B3LYP calculations on known and hypothetical doubly-bridged Cu(I) Cu(I) dimers and other d10/d10 analogues have been carried out. The bridging ligands may be only signa donors (hydrides) or have added pi donor (halides) or pi acceptor (carbonyls, as yet unknown) capabilities. In particular, the few reported LnCu(m C=CR)2CuLm frameworks have been investigated. The latter are symmetric (type c) or asymmetric (types a or b) depending on the nature and number of terminal ligands (n, m = 1 or 2). Beside the accurate geometric and energetic computations, the nature of the chemical bonding is explored in terms of perturbation theory arguments (ehmo approach). Thanks to the sigma donor power of the bridges, electron density is driven into the bonding combinations (sigma and pi) of empty metal sigma* and pi* orbitals. In presence of pi-donor ligands, population of the corresponding sigma* and pi* levels occurs and the M/M bond vanishes. By contrast, insufficient backdonation from copper d orbitals prevents the formation of bridged carbonyl dimers and trigonal planar monomers are favored. A case study is that of the hetero-binuclear d10-d10 complex (CO)2Cu(m CO)2Co(CO)2 where the lone pairs of the CO bridges are preferentially directed toward cobalt for electronegativity reasons. A similar situation is highlighted for the model (PH3)2Cu(m C=CH)2Cu(PH3) (type b) where both bridges orient towards the unique fragment (PR3)Cu because of the different hybridization of L2M and LM fragment orbitals. In the species LnCu(m C=CH)2CuLn (n=2 or n=1, type a or c), the Potential Energy Surface for the symmetric to asymmetric rearrangement of the central Cu2C2 ring is quite flat. However, a symmetric Cu2(m C=CR)2 framework is achieved with eta2 bound alkynes (type c). This is attributable to the pi* levels of the latter ligands which stabilize the metal p/pi orbitals involved in bridge bonding. The asymmetric Cu2C2 arrangement is preferred again in models where the terminal alkynes are substituted for by single phosphine ligands.
A Theoretical Analysis of Bonding and Stereochemical Trends in Doubly Bridged Cu(I)-Cu(I) Dimers
Mealli C;
2001
Abstract
DFT/B3LYP calculations on known and hypothetical doubly-bridged Cu(I) Cu(I) dimers and other d10/d10 analogues have been carried out. The bridging ligands may be only signa donors (hydrides) or have added pi donor (halides) or pi acceptor (carbonyls, as yet unknown) capabilities. In particular, the few reported LnCu(m C=CR)2CuLm frameworks have been investigated. The latter are symmetric (type c) or asymmetric (types a or b) depending on the nature and number of terminal ligands (n, m = 1 or 2). Beside the accurate geometric and energetic computations, the nature of the chemical bonding is explored in terms of perturbation theory arguments (ehmo approach). Thanks to the sigma donor power of the bridges, electron density is driven into the bonding combinations (sigma and pi) of empty metal sigma* and pi* orbitals. In presence of pi-donor ligands, population of the corresponding sigma* and pi* levels occurs and the M/M bond vanishes. By contrast, insufficient backdonation from copper d orbitals prevents the formation of bridged carbonyl dimers and trigonal planar monomers are favored. A case study is that of the hetero-binuclear d10-d10 complex (CO)2Cu(m CO)2Co(CO)2 where the lone pairs of the CO bridges are preferentially directed toward cobalt for electronegativity reasons. A similar situation is highlighted for the model (PH3)2Cu(m C=CH)2Cu(PH3) (type b) where both bridges orient towards the unique fragment (PR3)Cu because of the different hybridization of L2M and LM fragment orbitals. In the species LnCu(m C=CH)2CuLn (n=2 or n=1, type a or c), the Potential Energy Surface for the symmetric to asymmetric rearrangement of the central Cu2C2 ring is quite flat. However, a symmetric Cu2(m C=CR)2 framework is achieved with eta2 bound alkynes (type c). This is attributable to the pi* levels of the latter ligands which stabilize the metal p/pi orbitals involved in bridge bonding. The asymmetric Cu2C2 arrangement is preferred again in models where the terminal alkynes are substituted for by single phosphine ligands.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
