.eta.2-Cyclobutadienoid transition metal complexes. Preparation and characterization of a binuclear complex possessing a bridging cyclobutadiene ligand

Abstract

so the extent to which configuration is lost during dimerization of 1 to 2, will depend on the relative activation energies of the two "forbidden" processes 6 -* 7 and 6 -*• 8. Since both have very low activation energies, the difference between them must be correspondingly small. The conversion of 1 to 2, or of 2 to 1, should therefore take place with partial retention, but much loss, of configuration. This is the case. 16 The loss of configuration in these reactions is due to the formation first of the "wrong" intermediate (6 in the case of 1 -* 2) which has to undergo a "forbidden" conversion to the "right" one (7). If the latter were formed directly, it should collapse to cyclobutane with little or no loss of configuration. Bartlett and Porter16 have indeed found that photolysis of 10 gives 12 with little of the trans isomer; in this case loss of N2 from 10 gives the biradicaloid 11 analogous to 7. Mex ,-, zMe h Mex >-i zMe Et*N=N*Et Et^O ¿Net 10 11Organometal. Chem., 65, C57 (1974).(14) D. J. Ehnholt, unpublished results.

Normal and anomalous ring opening of 1-3-.eta.-pentaarylcyclobutenylpalladium complexes

“…3 At least one other ring opening of a cyclic C4 organometallic, 6 -*• 7, has been described which can also be viewed as proceeding in a conrotatory manner. The opposite process, the opening of a cyclobutenyl complex, has been demonstrated by Powell et al for the endo-alkoxytetraphenylcyclobutenylpalladium /3-diketonate complex (4) to the -butadienyl complex (5). 3 This ring opening occurs Four different modes (a-d) of ring opening for an fi-cyclobutenyl-metal complex may be envisaged, if the reasonable assumption is made that this proceeds via an 771-( -) cyclobutenyl complex.…”

Triple-decker transition metal complexes bridged by a single carbocyclic ring

“…4 [40]. The structure in which a symmetrical g 4 :g 4 coordination mode is adopted is likely to be Jahn-Teller unstable [30].…”

Synthesis and reactivity of saturated hydrocarbon-bridged dinuclear complexes

“…Conversion of 41 to 40 is proposed to occur through Sn2' attack of Na[CpFe(CO)2] at the carboncarbon double bond from the same side as chlorine. 33,34 It is also possible that iron aids the loss of Cl" from 41 to form an r/2-cyclobutadiene intermediate that is subsequently attacked by CpFe(CO)2~t o produce 40. When 40 is refluxed in toluene, cyclobutene ring opening occurs to give 1,4-diiron butadiene complex 42.35 Oxidation of 40 affords the interesting dinuclear dicationic complex 43.33 Reaction of 1,2-dichlorobenzocyclobutene with Na-[CpFe(CO)2] produces the diiron benzocyclobutene complex 44 which was the first complex having two metal centers linked by a two-carbon bridge.…”