Carbene complexes bonding models

BONDING MODELS AND REACTIVITY PATTERNS FOR TRANSITION METAL CARBENE AND CARBYNE COMPLEXES... 

The wealth of empirical information collected for transition metal carbene and carbyne complexes may be best interpreted within the framework of sound theoretical models for these compounds. Perhaps the most significant contribution made by the theoretical studies of carbene and carbyne complexes concerns an understanding of the reactivity patterns they display. In this section the relationship between bonding and reactivity is examined, with particular emphasis being given to the ways in which studies of Ru, Os, and Ir compounds have helped unify the bonding models applied to seemingly diverse types of carbene and carbyne complexes. 

In view of the similarities between the bonding models for carbene and carbyne complexes it is not surprising that similar patterns of reactivity should be observed for these compounds. Thus nucleophilic and electrophilic additions to the metal-carbon triple bond are anticipated under appropriate circumstances, and both orbital and electrostatic considerations will be expected to play a role. 

The similarity between the bonding models for transition metal carbene and carbyne complexes was noted in Section II. That the reactivity of the metal-carbon double and triple bonds in isoelectronic carbene and carbyne complexes should be comparable, then, is not surprising. In this section, the familiar relationship between metal-carbon bond reactivity and metal electron density is examined for Ru and Os carbyne complexes. 

In scrutinizing the various proposed reaction sequences in Eq. (26), one may classify the behavior of carbene complexes toward olefins according to four intimately related considerations (a) relative reactivities of various types of olefins (b) the polar nature of the metal-carbene bond (c) the option of prior coordination of olefin to the transition metal, or direct interaction with the carbene carbon and (d) steric factors, including effects arising from ligands on the transition metal as well as substituents on the olefinic and carbene carbons. Information related to these various influences is by no means exhaustive at this point. Consequently, some apparent contradictions exist which seem to cast doubt on the relevance of various model compound studies to conventional catalysis of the metathesis reaction, a process which unfortunately involves species which elude direct structural determination. 

The nature of the bonding in silylene-metal complexes, as compared with the better known metal-carbene complexes, is a question of considerable interest. MO calculations on H2Si=Mo(CO)5 indicate that the Si—Mo bond consists of a cr-donor and --backbond component, like the carbon-metal complexes. The -component is, however, weaker than for metal carbenes251. Infrared C=0 frequencies for the base-free silylene metal complexes support this model. Theoretical considerations of the bonding in silylene-metal complexes are treated more fully in Section IV.E. 

Hegedus et al. discovered that irradiation of chromium-carbene complexes resulted in a photoinsertion of CO into the Cr-carbene bond to form Cr-ketene complexes [96, 97]. This opened novel routes to the preparation of valuable compounds via Cr-ketene chemistry. Among them, the reaction of metallated ketenes with imines was intensively explored [98-100]. Within this context, the reaction between several model Cr-ketenes (120) and imines was explored at the B3LYP/6-31G ECP level of theory [101, 102], The mechanisms thus obtained are reported in Scheme 31. 

Molecular models are available for all the reactions and intermediates invoked in these mechanisms. For example, diazoalkanes have been known to generate metal-carbene complexes, with the cyclopropanation of metal-metal double bonds under smooth conditions (compare Section 3.1.7) being of particular relevance to the chemistry of metal surfaces, cf. eq. (10) [8 a, 23],... 

The results of the EDA for the Fischer-type carbene complexes (CO)5W-E(OH)2 are given in Table 13.24. Chemical experience shows that the substituent R must be a TT-donor group in order that a Fischer-type complex becomes stable enough to be isolated. We included the model compound (CO)sW-CH2 in our study in order to analyze the influence of the ir-donor group OH on the bonding situation. 

The links between formation of an iron-alkyl complex and irreversible destruction of the heme moiety have not been forged, but model studies with diaryl- and carbethoxy-substituted carbene complexes suggest that the halogenated carbenes may shift to form a bond with a nitrogen of the porphyrin. The resulting A -haloaIkyl adduct are likely to undergo water-dependent hydrolysis and might therefore not be detected by the methods used to isolate other A -alkyl porphyrins. However, the formation of alternative reactive species that attack the protein or the heme cannot be ruled out. 

---