Carbyne 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. 

The same dichotomy of bonding models is also found for carbyne complexes that have a formal triple bond M=CR. There are metal-carbyne bonds that belong to the donor-acceptor type (the Fischer car-... 

A similar dichotomy of bonding models exists for other classes ofTM compounds where the chemical bond can be discussed either in terms of electron-sharing interactions TM-R or as donor-acceptor bond TM-L. Examples are TM compounds with carbenes CR2 or carbynes CR as ligands, which can be considered as either Fischer-type complexes or as Schrock-type alkylidenes and alkylidynes [31, 32). We want to emphasize that the two bonding models should be considered as sketches of two extreme situations, whereas the electronic situation of real molecules has components of both forms. The value of such dichotomic models lies in the fact that they establish an ordering scheme, which is very useful for describing the physical and chemical properties of molecules. 

Metal carbyne complexes MsCR also have Fischer and Schrock extreme bonding formulations, although the distinction i less marked than for carbenes. In one bonding model, the free carbyne can be considered as doublet for Fischer and quartet for Schrock forms (Fig. A doublet carbene is a 2e donor via its... 

Examples of carbyne-metal bonds in organometallic complexes, the adequate model being the framed one, a robust cluster... 

Kreis from 16 and BI3) [43] that the seminal paper about the synthesis and characterization of the first carbyne metal complex was submitted to Angewandte Chemie. When Fischer went to Stockholm in December 1973, he had a ball-and-stick model of Cr(CCH3)(CO)4l in his luggage, demonstrating to the audience of his Nobel Lecture that a transition metal compound with a metal-carbon triple bond is not a fantasy, but really exists. 

---