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Mechanistic Study for the Photochemical Reactions of d6 M(CO)5(CS) (M = Cr, Mo, and W) Complexes
journal contribution
posted on 2017-06-19, 17:59 authored by Zheng-Feng Zhang, Ming-Der SuThe mechanisms of photoextrusion
reactions are determined theoretically
for the model system of six-coordinated M(CO)5(CS) (M =
Cr, Mo, and W), using both CASSCF and MP2-CAS methods and the Def2-SVPD
basis set. Three types of elimination reaction pathways (i.e., path
I, path II, and path III for axial CO extrusion, equatorial CO extrusion,
and CS ligand extrusion, respectively) are considered in this study.
Theoretical findings show that the photoextrusion mechanism for Cr
and Mo complexes proceeds as follows: M-S0-Rea + hν → M-S1-FC → M-CI →
M-Pro + CO. This study shows that when the reactant, M(CO)5(CS) (M-S0-Rea), is photoirradiated by UV light, it is
excited vertically to many low-lying singlet excited states. It then
relaxes to the first singlet excited state from the Franck–Condon
point (M-S1-FC). After passing through a conical intersection
point (M-CI), this species eliminates a CO group to yield a five-coordinated
product, M(CO)4(CS) (M-S0-Pro). However, for
the W analogue, the photolysis mechanism is represented as W-S0-Rea + hν → W-T1-Min
→ W-T1-TS → W-T1/S0 → W-S0-Pro + CO. That is to say, when the reactant,
W(CO)5(CS) (W-S0-Rea), absorbs UV light, it
is excited to its several low-lying excited states by a vertical excitation.
This species may then return to an intermediate at the first triplet
excited state (W-T1-Min) by means of intersystem crossings
or conical intersections. After passing through a triplet transition
state (W-T1-TS) and a subsequent intersystem crossing (W-T1/S0), this molecule finally loses a CO ligand to
produce a photoproduct at the ground singlet state (W-S0-Pro). In other words, conical intersections and intersystem crossings
play a decisive role in these photoextrusion reactions for M(CO)5(CS). Theoretical evidence from a kinetic viewpoint strongly
supports the theory that the photolysis of M(CO)5(CS) only
produces CO-loss photoproducts rather than the CS-loss photoproduct.
Theoretical analysis using the results of this study allows a good
interpretation of the available experimental observations.
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Keywords
MP 2-CAS methodsM-CIMo complexes proceedsphotoextrusion reactionselimination reaction pathwaysDef 2-SVPD basisProCASSCFconical intersectionsReaCO extrusionphotoproductground singlet stateFCW-T 1conical intersection pointCS ligand extrusionCrtriplet transition stateW-S 0TSmechanismIIITheoretical findings showIIM-S 0UV lightintersystem crossings
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