Bonding Analysis of the Tetrylynes HEEH E C - Pb

The results of the quantum chemical calculations and the experimental observations are a challenge to chemical bonding theory to explain the unusual findings in terms of bonding models. Four questions are addressed in this section, which will be dealt with in a didactical way. We refer to the original literature, which is dted below for technical details. The questions are  

Why is the triply bonded, linear structure of acetylene HC=CH not an energy minimum for HE=EH (E = Si-Pb) but an energetically, high-lying saddle point on the PES  

Why do the substituted homologues E2R2 have trans-bent equilibrium geometries Do they possess E-E triple bonds  

The key to answering the questions above lies in the electronic state of the fragments EH, which are engaged in E-E bonding in E2H2. Because the vinylidene isomer C has a different atomic connectivity where the hydrogen atoms are bonded to only one atom E, we do not include it in our discussion which is, thus, restricted to isomers A, B, Dl, D2, and E. 

The situation looks very different for the heavier homologues E H. Table 2.3 shows that the net energy gain of — 2 x AE in the case of sihcon is only 44.5 kcal, which is much less than the typical value for a Si- Si single bond (75 kcal mol ) [17]. 

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