| Popis: |
A MNDO and 300-MHz 1H NMR study of some trigonal-bipyramidal (TBP) five-coördinated phosphorus (Pv) compounds is reported. It is shown by the MNDO calculations that, in the oxaphosphole Pv TBP compounds 5a-c, the electron distribution in the axial bonds of the TBP is affected by the electronegativity of the substituent at C4 of the oxaphosphole ring. With increasing electronegativity of the substituent at C4, the electron density on the axial exocyclic oxygen atom O1 decreases whereas the electron density on the axial endocyclic atom O1 increases. This is supported by a 1H NMR conformational analysis of the C1[sbnd]C2 bond of the oxaphosphole Pv TBP compounds 6–11. The gauche(-) rotamer fraction (O1 and O1 trans situated) of these compounds, which is correlated to the electron density on O1, is reduced to 30% as compared to the absolute axial g-rotamer fraction (59%) of the dioxaphosphole Pv TBP compound 13, most likely because of the presence of the carbonyl group at C4 of the oxaphosphole ring. So, both the 1H NMR and MNDO study show that electron withdrawing substituents on the oxaphosphole ring of Pv TBP compounds reverse the electron transfer in the axial P[sbnd]O bonds of the TBP (as compared to dioxaphosphole compounds), from exocyclic O1 towards endocyclic O1 |
