Vibronic coupling in dicyano-complex-bridged mixed-valence complexes. Relaxation of vibronic constraints in systems with degenerate bridging-ligand and electron-transfer excited states

Journal Article · Mon Feb 07 04:00:00 EST 2000 · Inorganic Chemistry

DOI:https://doi.org/10.1021/ic9900713· OSTI ID:20017279

Macatangay, A V; Endicott, J F

Intense near-infrared (NIR) absorption bands have been found in mixed-valence Ru(NH{sub 3}){sub 5}{sup 2+,3+} complexes bridged by trans-Ru(py){sub 4}(CN){sub 2} and cis-Os(bpy){sub 2}(CN){sub 2}, {epsilon}{sub max {approximately}} 1.5 x 10{sup 3} cm{sup {minus}1} and {Delta}{nu}{sub 1/2} {approximately} 5 x 10{sup 3} cm{sup {minus}1} for bands at 1000 and 1300 nm, respectively. The NIR transitions implicate substantial comproportionation constants (64 and 175, respectively) characteristic of moderately strong electronic coupling in the mixed-valence complexes. This stands in contrast to the weakly forbidden electronic coupling of Ru(NH {sub 3}){sub 5}{sup 2+,3+} couples bridged by M(MCL)-(CN){sub 2}{sup +} complexes (MCL = a tetraazamacrocyclic ligand). A straightforward perturbation theory argument is used to account for t his contrasting behavior. The electronic coupling between a cyanide-bridged, donor-acceptor pair, D-(CN{sup {minus}})-A, alters the properties of the bridging ligand. Such systems are described by a vibronic model in which the electronic matrix element, H{sub DA}, is a function of the nuclear coordinates, Q{sub N}, of the bridging ligand: H{sub DA} = H{sub DA}{degree} + b!{sub N}. Electronic coupling in the dicyano-complex-bridged, D-[(NC)M(CN)]-A, systems is treated as the consequence of the perturbational mixing of the local, D(NC)M and M(CN)A, vibronic interactions. If M is an electron-transfer acceptor, then the nuclear coordinates are assumed to be configured so that bQ{sub N} is larger for D(NC)M but very small (bQ{sub N}{approximately}0) for M(CN)A. When the vertical energies of the corresponding charge-transfer transitions, E{sub DM} and E{sub DA}, differ significantly, a perturbation theory treatment results in H{sub DA}= H{sub DA}H{sub AM}/E{sub ave} independent of M and consistent with the earlier report. When E{sub DM} {approx{underscore}equal} E{sub DA}, configurational mixing of the excited states leads to H{sub DA} proportional to H{sub DM}, consistent with the relatively intense intervalence bands reported in this paper. Some implications of the model are discussed.

• Research Organization: Wayne State Univ., Detroit, MI (US)
• Sponsoring Organization: US Department of Energy
• OSTI ID: 20017279
• Journal Information: Inorganic Chemistry, Journal Name: Inorganic Chemistry Journal Issue: 3 Vol. 39; ISSN 0020-1669; ISSN INOCAJ
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
COUPLING
CYANIDES
ELECTRON TRANSFER
ELECTRONIC STRUCTURE
EXCITED STATES
NEAR INFRARED RADIATION
RELAXATION
RUTHENIUM COMPLEXES
VIBRATIONAL STATES