Metal carbonyls, photodissociation

Abstract In this review we discuss the theory and application of methods of excited state quantum chemistry to excited states of transition metal complexes. We review important works in the field and, in more detail, discuss our own studies of electronic spectroscopy and reactive photochemistry. These include binary metal carbonyl photodissociation and subsequent non-adiabatic relaxation, Jahn-Teller and pseudo-Jahn-Teller effects, photoisomerization of transition metal complexes, and coupled cluster response theory for electronic spectroscopy. We aim to give the general reader an idea of what is possible from modem state-of-the-art computational techniques applied to transition metal systems. 

Nevertheless, we were able to develop a transient absorption apparatus involving IR probe radiation that is suitable for gas phase studies, as have a number of other groups either coincident with or subsequent to our work [1]. In the remainder of this article we will discuss the apparatus and the results of our studies on three prototypical metal carbonyl species Fe(C0>5, Cr(C0>5 and Mn2(CO)] o The discussion in this article will center on the nature of the photolytically generated coordinatively unsaturated species, their kinetic behavior and photophysical information regarding these species. This latter information has enabled us to comment on the mechanism for photodissociation in these systems. Since most of the results that will be discussed have been presented elsewhere [3-10], we will concentrate on a presentation of data that illustrates the most important features that have come out of our research and directly related research regarding the kinetics, photophysics and photochemistry of coordinatively unsaturated metal carbonyls. 

We shall now focus on our results of modelling the potential energy surfaces for photodissociation for three metal carbonyls, Cr(C0)6, 2( 0) , and Fe(CO)5. One common feature of these carbonyls from experiment is that they are known to eject one CO ligand on an ultrafast timescale (2,4-7,34). Although the nature of the initial states that lead to this are still under discussion (vide supra), the mechanism of relaxation of the unsaturated metal carbonyl that remains has also been the subject of much investigation (2,5-7,34,48,78,79). 

The first quantitative measurement of the distribution of excited atomic states produced in the multiphoton dissociation of a metal carbonyl has been made for Cr(CO)6. Photodissociation does not yield spin- or parity-differentiated states, rather the state distribution appears to be statistical. Photofragmentation dynamics of Cr(CO)6 in the gas phase have been measured and two channels of dissociation revealed. One of these is a rapid predissociation (efficiency 36%) and the other a slow process (efficiency... 

Fig. 4 Coherent oscillations observed by Fup and co-workers after ultrafast photodissociation of group 6 metal carbonyls. The Fourier transform of the oscillatory part shows a peak at 96cm which compares to 98cm found for equatorial L-M-L bending in Jahn-Teller moat using semi-classical direct dynamics [43] (reused from [70] with permission)...

Photochemical studies of simple mononuclear metal carbonyls have been relatively few in the period under review. The ultrafast photodissociation dynamics of Cr(CO)6 following excitation at 200 and 267 nm have been... 

Considerable work has already been carried out using ab initio calculations to predict the photodissociation dynamics of gas-phase metal carbonyls (45). This is a fertile area for computational work, given the extensive experimental results available, which include the use of ultrafast methods to characterize the short time behavior in photoexcited states. There is considerable evidence that surface crossings, especially of a spin-forbidden nature, play a considerable part in the dynamics. Much of the theoretical work so far has focused on reduced-dimensionality models of the PESs, which have been used in quantum mechanical smdies of the nonadiabatic nuclear dynamics, in which spin-forbidden transitions are frequently observed (45). Here, too, the potential benefits to be derived from a proper understanding of the spin-state chemistry are considerable, due to the importance of light-induced processes in organometallic and bioinorganic systems. 

The formation of addition compounds from photodissociated metal carbonyls and acetylenes has been postulated from flash spectroscopic studies of acetylene/02 explosions in the presence of metal carbonyls 144>. 

Chemiluminescence and photoluminescence in diatomic iron oxide, Rb2, and alkali-metal dimers with halogen atoms and metal vapour-oxidant flames,202 203 lifetime measurements of selectively excited states of diatomic hydrides,204 photodissociation of alkali-metal halide vapours,206 spin-orbit relaxation of the HTe ( 2IIi) radical,20 the photodecomposition of metal carbonyl anions such as [Mn(C04)] in the vapour phase,207 and the fluorescence of Rhodamine 6G in the vapour phase 208 have been studied in recent reports. In the last study it was concluded that an insufficient concentration of the fluorescing dye could be maintained in the vapour phase to permit laser action to occur. 

Ptiotoeliiiiiiiation of CO and CO2 from Organometallic Compounds - There have been several reviews covering various aspects of the photodissociation of metal carbonyls. These include quantum chemistry and photodynamics, photo-... 

Metal Carbonyl facilitated chemical transformations occur through thermo or photodissociation of one of the carbonyl group from 18-electron transition metal carbonyl, leads to coordinatively unsaturated product, which gives them the capacity to behave as a catalyst for chemical reactions [1], Polynuclear metal carbonyl clusters have their advantages in multisite interaction between the substrate molecules. Transformation of substrate... 

Lammertsma, /. Am. Chem. Soc., 125, 3558 (2003). Photodissociation of the Phosphine-Substituted Transition Metal Carbonyl Complexes Cr(CO)5L and Fe(CO)4L A Theoretical Study. 

Goumans TPM, Ehiers AW, van Hemert MC, Rosa A, Baerends EJ, Lammertsma K (2003) Photodissociation of the phosphine-substituted transition metal carbonyl complexes CrfCOlsL and Fe(CO)4L a theoretical study. J Am Chem Soc 125 3558-3567... 

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