Photophysical aspects of CH bond activation in rhodium dicarbonyl complexes

Electronic Excitation of RhI(CO)2 Species on Al2O3 Surfaces: Photodissociation of the RhI−CO Bond

“…9 For example, dissociation of CpRh(CO) 2 takes place during ultraviolet excitation between 2.7 and 4.3 eV. 21 The rate of CO loss from Rh I (CO) 2 /Al 2 O 3 species increases during irradiation at higher photon energies (Figure 2). The gain in the rate of Rh I -CO dissociation as the photon energy is increased, may be explained by two viable processes: (1) excitation into higher lying vibrational levels of a particular energy level for an excited state that can overcome the barrier for dissociation and (2) excitation into higher lying electronic levels that are dissociative in Rh I -CO.…”

“…11 The loss of a CO ligand from the ultraviolet photolysis of the CpRh(CO) 2 complex has been attributed to the excitation of electrons between the rhodium center d-based orbitals, or ligand-field (LF) excitations. 21 Often, a high degree of covalency is associated with the metal-ligand bond of an organometallic complex (as in the case with the back-bonding interaction between the transition metal and a CO ligand). This indicates that a fair amount of ligand character is incorporated in metalbased d f d transitions.…”

“…Since that initial study, many have investigated bond activation chemistry during the ultraviolet excitation of various Rh-based transition metal carbonyls, particularly the CpRh(CO) 2 and Cp*Rh(CO) 2 complexes. [15][16][17][18][19][20][21][22] The loss of a CO ligand from the ultraviolet photolysis of the CpRh(CO) 2 complex has been attributed to the excitation of electrons between the rhodium center d-based orbitals, known as ligand-field (LF) excitations. [9][10][11][12][13] An electron undergoes a vertical transition from a populated metal-centered d-character molecular orbital into the lowest unoccupied molecular orbital (LUMO) that also has significant metal d-character and is antibonding with respect to the ligand.…”

“…[9][10][11][12][13] An electron undergoes a vertical transition from a populated metal-centered d-character molecular orbital into the lowest unoccupied molecular orbital (LUMO) that also has significant metal d-character and is antibonding with respect to the ligand. Lees and co-workers 21 have studied ligand substitution (CO/C-H or CO/Si-H) during the laser photolysis of CpRh(CO) 2 at 2.7 and 4.0 eV photon energies, where C-H or Si-H bonds were cleaved on the photoproduced Rh site from which a CO ligand had been eliminated. They found that two LF excited states (at 2.9 and 4.3 eV) were involved in the reaction mechanism.…”

Photochemical displacement of co-ordinated ethene from [Rh(η5-C5H5)(PPh3)(C2H4)]

“…[2][3][4][5][6] The need to generate the 16-electron intermediate in order to effect oxidative addition, i.e. the need for a dissociative mechanism, is emphasised by studies involving matrix isolation 7-10 laser flash photolysis, [11][12][13][14][15] and quantum yield measurements 16, 17 on these complexes and related carbonyl systems such as [Rh(η 5 -C 5 H 5 )-(CO) 2 ] and [Rh(η 5 -C 5 H 5 )(CO)(C 2 H 4 )]. Electron-releasing ligands such as PMe 3 and C 5 Me 5 are not essential for C᎐H and Si᎐H bond activation, as is evident from the reactivity of the carbonyl complexes, but they certainly influence the stability of the products {compare [Rh(η 5 -C 5 H 5 )(CO)(alkyl)H] and [Rh(η 5 -C 5 Me 5 )(PMe 3 )(alkyl)H]}.…”