Triple bond, electronic structure length

Recrystallization of 76 (R=H) from CH2CI2 provided crystals adequate for X-ray structural determination. The molecule was found to be saddle shaped with a phenyl ring at each vertex and nadir. The alkynyl bonds were found to be essentially linear and to possess a mean length of 1.194 A, typical for the length of triple bonds in free butadiyne. Although 76 is a dehydrobenzoannulene possessing a 4n TT-electron circuit, the nonplanarity of the macrocycle alleviated much of the strain associated with a flat structure and thus precluded the possibility of anti-aromatic ring currents. 

The production of two moles of carbon monoxide and the 18-electron rule lead us to predict that the acetylene molecule is acting as a four-electron donor. In fact this is just one of many complexes in which alkynes bind in this fashion.81 For example, the structure of the diphenylacetylene complex in Fig. 15.26 shows that the positions of the two rhodium atoms are such as to allow overlap with both tr orbitals in the carbon-carbon triple bond.82 The extent of back donation into the antibondirg orbitals determines the lengthening of the C—C bond and the extent to which the C—H bonds are bent away from the complex. Bond length values vary greatly from system to... 

The structure of the molecule is shown in Fig. 19.2.5(a). The electronic configuration is cr2n4 2 2. The bond valence is 3, indicating a ResRe triple bond, and the bond length is 223.2 pm. 

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