Ligand field interpretation of the quantum yields of photosolvation of d6 complexes

Abstract

state reaction mechanisms than has been possible for ground states (see ref 20), experiments with optically active chelates might further restrict the range of possibilities.We have made no attempt in the preceding discussion to assess the spectroscopic description of our reactive excited states. This type of approach has been made, in some detail, by Zink21 using conventional ligand field theory to predict aand -bonding changes in various excited states. This approach should be very helpful in the case of cobalt(III) ammines where it does appear that the primary act is one of direct heterolytic bond cleavage at a labilized position. We retain a general concern that the thermally equilibrated excited (thexi1) states which presumably are the chemically reacting species may have a different geometry (and a different spin multiplicity) than the Franck-Condon states used by ligand field theory in treating absorption spectra. A further caution, mentioned in the Introduction, is that mechanistic interpretations of quantum yield variations may be specious; relative photoinertness could, for example, be due (21)

Ab Initio Calculations of the Geometry and Vibrational Frequencies of the Triplet State of Tungsten Pentacarbonyl Amine: A Model for the Unification of the Preresonance Raman and the Time-Resolved Infrared Experiments

“…A longer W−N bond in the excited state (by about 0.1 Å) is expected from the photochemical behavior exhibited by W(CO) 5 pyr and W(CO) 5 pip; 1a,, the quantum yield for loss of pyridine or piperidine from these compounds is very high, and this is in agreement with previous results. , The change in the W−N bond length is nearly twice as large as the change in any of the other bond lengths. The W−C ax bond lengthens more than the W−C eq (0.06 Å vs 0.01 Å) bond, and this is in agreement with Zink's results 4a.…”

“…The C−O ax bond is shortened (about 0.015 Å), but the C−O eq bonds are lengthened (about 0.007 Å). This latter result is unusual because it is unexpected that both W−C eq and C−O eq bonds would lengthen. 1b,4a, In the simple molecular orbital picture, when the W−C bond lengthens, because of depopulation of the π bonding orbital (d yz ) and population of the σ antibonding orbital (d z 2 ), it reduces back-bonding to C, thus strengthening the C−O bond and decreasing the C−O bond length. But our results show that in the excited state the newly populated a 1 orbital has substantial CO eq π* character (Figure ).…”

On the photosensitivity of liganded hemoproteins and their metal-substituted analogues.

“…Ligand photodissociation was observed spectrophotometrically after flash photolysis. The samples were exposed to light from a Multiblitz III electronic flash with 100-J energy input, and flash duration, (l/e) - 21 The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact.…”

Ligand field model for the prediction of photochemical reactivities for d6 metal complexes. Comments

“…The fundamental postulate of the LFM approach is that the more the excitation energy is "concentrated" along the unique axis the greater will be the quantum yield for loss of ligands on that axis. 2 The distribution of excitation energy is based upon the fractional d-orbital composition, and the unique-axis o-* orbital (dz2 in crystal field theory) population is pinpointed as the key feature determining photolabilization in the absence of strongly -interacting ligands. With one qualification (vide infra) it is an implicit assumption in the LFM as currently applied that the relationship of dz 2 Kthe z-axis photolabilization quantum yield $ is due primarily to an increased rate of solvolysis (ks, Figure 1) as the dz2 population of the reactive excited state increases.…”

“…These processes are illustrated in Figure 1. The quantum yield for formation of substitution products is defined as 8 = ; fe ks + kx + kn (2) where kr and kn are the rate constants for radiative and nonradiative deactivation of the excited state and represents the interconversion efficiency from initially populated excited states to the reactive state. It has been pointed out previously5•6 that there is a dearth of information available on rates ks, ki, and kn for d6 complexes under photochemically relevant k e produces Figure 1.…”