Transition metal carbonyls bonding 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...

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. 

The stabilities of the metal-carbon bond formed from oxidative additions are as varied as their mechanistic pathways. Metal-carbon bond strengths increase going down a triad in an isostructural series of complexes. Alkyl migration to CO ligands on the metal to form acyl derivatives is more facile in first-row transition metals because of their lower metal-carbon bond energies. The thermal stability of alkyls vs. acyls does not follow any pattern, except that the availability of a sixth coordination site in ML (acyl) complexes favors the alkyl carbonyl isomer. The corresponding acyl, which can be made by running the reaction of the alkyl or aryl halide in CO (at 1-3 atm), is more stable by... 

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). 

Accurate Density Functional calculations make possible today the systematic investigation of periodic trends in the bond energies of transition metal complexes. Computational results are presented for metal-metal bonds in dimers of the group 6 transition metals, metal-ligand bonds in early and late transition metal complexes, and metal-carbonyl bonds in hexa- penta- and tetra-carbonyl complexes. 

Ribeiro da Silva, M. A. V., Reis, A. M. M. V., Thermochemistry of the Metal-Carbon Bond. Critical Review of Bond Energy Data of Organometallic Compounds and of Transition Metal Carbonyls, Rev. Port. Quim. 20 [1978] 47/62. 

The metal-olefin bond energy data are scarce. Such energies for group 8 transition metals are usually about 20-30% lower than bond energies of terminal carbonyl groups. " The metal-olefin bond energies for some iron, nickel, and rhodium complexes are given in Table 6.12. 

Our group has studied another class of transition metal carbonyl complexes, namely, the positive ions M(CO)+ (M = Cu, Ag, Au n = l-4).ii Table 11 shows the optimized M+—CO bond lengths at the HF/II and MP2/II levels of theory. The calculated and experimental M+—CO first dissociation energies of the carbonyl ligand are also shown. The experimental values have been taken from the recent compilation of observed Dq(0 K) values by Armentrout using... 

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