Bond lengths acceptor-substituted

The planarity of the tricoordinate phosphorus and thus the aromaticity of 4 can be influenced by substituents. It turns out from ab initio calculations that 7T-acceptor groups either at phosphorus or at the neighboring carbon have a planarizing effect. The barrier to planarity decreases to 1.56 kcal/mol (MP2/ 6-31G(d)) as a result of —BH2 substitution at phosphorus. The bond length alternation in the planar form of this substituted derivative increases in the... 

Depopulation or population of MOs 9, 11 and 16, but not the 1 a 2 MO 29, leads to opposing changes in vicinal and distal bond lengths. Accordingly, one can expect charac-teristicchanges in the geometry of 1 upon substitution bya7t/cr- acceptor/donor substituent (Table 14). 

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