Bond distances cyclobutadiene

High level molecular orbital calculations of cyclobutadiene itself and experimen tally measured bond distances of a stable highly substituted derivative both reveal a pat tern of alternating short and long bonds characteristic of a rectangular rather than square geometry... 

Crystal structure data15 indicate that in the vast majority of (cyclobutadiene)metal complexes (4) the cyclobutadiene ligand is approximately square-planar with nearly equal C—C bond distances (ca 1.46 A) and bond angles of ca 90°. Within a given complex the cyclobutadiene carbon-to-metal distances are roughly equal. 

By means of X-ray crystallography, NMR studies and quantum-chemical studies, an insight into the problem of aromaticity of the tetrakis(trimethylsilyl)cyclobutadiene dianion 142a of Sekiguchi could be gained (Figure 18) . The cyclic dianion, which should be aromatic due to Huckel s Rule, shows non-alternating C—C bond distances in the solid... 

The bond length variation in the substituted benzodicyclobutadiene shown in Fig. 14.3.6(d), as determined from X-ray analysis, can be fairly well accounted for by resonance between the canonical formulas la and lb. The central six-membered ring contains a pair of extremely long C(sp2)-C(sp2) bonds at 154.0(5) pm, which significantly exceed the reference bond distance of 149.0(3) pm observed for tris(benzocyclobutadieno)benzene [Fig. 14.3.6(e)], The bond length pattern in the latter compound indicates that its structure is better described by the radiallene formula Ha in preference to formula lib, which contains anti-aromatic cyclobutadiene moieties. 

Structural studies of cyclobutadiene and some of its derivatives reveal a pattern of alternating single and double bonds and a rectangular, rather than a square, shape. Bond distances in a stable, highly substituted derivative of cyclobutadiene illustrate this pattern of alternating short and long ring bonds. 

The carbon-carbon bond distance in cyclobutadiene as 1.4 A, given and shown that it is a paramagnetic molecule by considering its p-electrons as freely moving particles in a two dimensional box ... 

Cyclobutadiene is electrons. The single bond distance of 1.4 A indicates delocalization of p electron... 

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]. 

Fig. 9 illustrates that the two acetylenic systems become nearly parallel at C1-C6 distances close to 3 A where the constructive overlap of the re-orbital with one of the re -nodes is compensated by a destructive overlap with the other rc -node (Fig. 9, bottom). From a conceptual point of view, the properties of the in-plane re-system at the 3 A threshold bear a striking resemblance to the interaction of the two re-bonds in D2h cyclobutadiene where the re-re interaction is zero and the re-re repulsion is considerable, thus accounting for the extreme instability of this antiaromatic molecule.41 Even more relevant is a comparison with the TS of the symmetry forbidden thermal [2S + 2S] cycloaddition (Fig. 10) which prompted us to call this region antiaromatic .42... 

The iron complex has a square-planar cyclobutadiene ring with C—C distances averaging 1.46 A and internal bond angles 90°+ 1°. The substituent phenyl groups are bent back out of the plane of the cyclobutadiene ring away from the iron by an average of 11 ° and are all twisted about the... 

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