Transition metal carbonyls energies

The development of comprehensive models for transition metal carbonyl photochemistry requires that three types of data be obtained. First, information on the dynamics of the photochemical event is needed. Which reactant electronic states are involved What is the role of radiationless transitions Second, what are the primary photoproducts Are they stable with respect to unimolecular decay Can the unsaturated species produced by photolysis be spectroscopically characterized in the absence of solvent Finally, we require thermochemical and kinetic data i.e. metal-ligand bond dissociation energies and association rate constants. We describe below how such data is being obtained in our laboratory. 

Lewis et al,16 DH° 40 Kcal/mole, but is nevertheless consistent with trends found in other transition metal carbonyls, i.e. first bond dissociation energies are typically greater than second bond dissociation energies. See Table I. Note that the DH° for... 

Table L Bond Dissociation Energies for Transition Metal Carbonyls...

Conclusions. Time-resolved CO laser absorption spectroscopy can provide information useful in characterizing the primary photochemical channels in gas-phase transition metal carbonyls. We have found that product vibrational energy distributions indicate that W(CO)g and Cr(CO>6 dissociate via different... 

Pensak and McKinney (28) [PM], using this method, have recently reported a systematic study of first-row transition metal carbonyl complexes for which experimental bond distances and angles were reliably reproduced, along with key bond dissociation energies. 

A rather different route to short-lived intermediates is to irradiate a suitable precursor with radiation of an appropriate energy. Most common is UV-visible photolysis. Section 5 discusses some of the rich photochemistry displayed by transition metal carbonyls, these being very amenable to UV-visible photolysis. Thus unsaturated carbonyl fragments may be produced ... 

The infrared spectra of transition metal carbonyl anions provide an excellent illustration of the effect on the charge of the n acceptor ability of CO as expected, the more negative the charge, the stronger the n acceptance and the lower the energy of the C-0 stretching vibrations (Table 4-2). 

Earlier ab-initio calculations on metal carbonyls were contradictory on the question of whether 7r-bonding or cr-bonding was dominant. However the latest, and best, calculations using DFT with relativistic corrections show clearly that TT-bonding is the more important. There is a wealth of information available on bond strengths and stabilities of transition metal carbonyls. Since we are dealing with orbitals on CO and the metal that are rather constant, we can check to see if Equation (3.1) is consistent with the bond energy data for various metals. 

Photochemistry of transition metal carbonyl complexes as the borderline between organic and inorganic chemistry is mentioned in Section 6.3.9. Since the dissociation energy of a common metal carbonyl oxide bond is usually low, photodecarbonylation, that is, release of the CO molecule, is the most common photoprocess observed (entry 8). 

The following work describes our investigation into some unexpected chemistry resulting from our interest in unsaturated metal clusters. We study such systems primarily by mass spectrometry, and to assist these investigations we developed the new data presentation technique of energy-dependent electrospray mass spectrometry (EDESI-MS). One of the first systems studied was that of alkoxylated transition metal carbonyl clusters, which display loss of formaldehyde in their electrospray ionisation fragmentation process. We have been able to correlate this behaviour with macroscopic chemical properties of these clusters. 

Relativistic corrections to density functional (DFT) calculations by means of DFT have successfully been evaluated by van Wiillen [94], with application e.g. to transition metal carbonyls, unfortunately only at the level of the lowest order relativistic correction 0 c ) to the energy (i.e. slightly better than with the Breit-Pauli approximation). For a comparison of this method with a combination of DFT and ZORA see Ref. [95],... 

Transition metal carbonyl is important in laser chemistry as sources of metal atoms or as precursors for chemical vapor deposition (CVD) [104]. Ni(CO)4 shows a typical photofragmentation reaction initiated by the XeCl laser (308 nm, 4.03 eV), and the knowledge on the mechanism is valuable for the design and control of the laser-induced CVD. The SAC-Cl method was applied to the excitation spectrum and the potential energy curves relevant to the photofragmentation reaction [105]. 

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