Cyclobutadiene complexes bonding

A schematic interpretation of the bonding m cyclobutadiene complexes can be given within the framework outlined in the preceding sections and this is illustrated in Fig. 19.26. 

Figure 19.26 Orbitals u.sed in descnbing the bonding in metal-ij - cyclobutadiene complexe.s. The sign convention and axes arc as in Fig. 19.23...

For the cyclotrimerization of alkynes, several mechanisms have been proposed. The most plausible ones are a concerted fusion of three ir-bonded alkyne molecules, and stepwise processes involving a cyclobutadiene complex or a five-membered metallocyclic intermediate (98). In the case of the cyclotrimerization of a-alkynes it is possible to discriminate between a reaction pathway via a cyclobutadiene complex and the other reaction pathways, by analysis of the products. If cyclotrimerization proceeds via a cyclobutadiene complex and if steric factors do not affect the reaction,... 

The CpCo-stabilized ethynylated cyclobutadienes are considerably more robust, and the parent 76 can be isolated as a yellow crystalline material, stable at ambient temperature for several hours. At 0°C 76 decomposes in the course of several days, which is indicated by darkening of the formerly brillant-yellow needles. The stability of 76 made in X-ray analysis feasible and the bond angles/distances obtained are in good agreement with reported values for ethynylated cyclobutadiene complexes already described [35,36]. 

If the compounds shown in Figures 4.1 and 4.2 are regarded as mixed metal-carbon clusters, the pyramidal shapes of their MC skeletons will be seen to be those appropriate for nido systems in which (n -1-1) skeletal atoms are formally held together by (n -1- 3) skeletal bond pairs. This number includes the n pairs of electrons in the ring carbon-carbon o bonds as well as the three pairs of electrons in their K-systems. For example, in the cyclobutadiene complex (r " -C4H4)Fe(CO)3, 5 each CH unit can contribute three electrons, and the Fe(CO)3 unit can contribute two electrons for skeletal bonding, making... 

We discuss in this section the principal bonding characteristics shown in the metallocene and bis-benzene series and the way in which these trends influence the splittings within the set of 3d-orbitals and consequently determine the electronic ground states of these complexes. A brief account of the bonding of the recently discovered symmetrical bis-cyclobutadiene complex NiCbj will also be summarised. 

Compounds 33 and 34 are readily formed from 31 by direct reaction with CpCo(CO)2. A possible reaction sequence is formation of the triene, 31, from alkyne dimerization followed by reaction with the cobalt species to give the three complexes. Reaction of 34 with alkynes yielded only cyclobutadiene complexes alkyne metathesis was not observed, probably since the carbon-to-metal bonds are too strong. 

Some of the cyclobutadiene complexes [e.g., (XIII) and (XXVIII)] appear to be inert to the action of donor ligands such as phosphines, pyridine, etc. However, a number form adducts. These have not been fully described, but triphenylphosphine adducts of tetramethylcyclobutadienenickel chloride (XIV) (28) and tetraphenylcyclobutadienenickel bromide (XXXIII) (41) and an o-phenanthroline complex (29) of the former have been reported. Other reactions with donor molecules in which the cyclobutadiene-metal bond remains intact are mentioned in Section VI, G. 

Some reports of failures to obtain cyclobutadiene complexes have also appeared (10,40). Chatt et al. (21) also noted that biphenylene (II) was very reluctant to form a metal complex. The only ones which could be made were molybdenum tricarbonyl complexes where the Mo(CO)3 group was bonded to the benzene, rather than to the four-membered ring. 

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