Configuration interaction, xviii

Esterification of the 18-hydroxyl followed by an acid-catalyzed equilibration (at C-3, cf. isoreserpine reserpine) resulted in the formation and isolation of the inverted equivalent of reserpine (XVIII). The conversion into the reserpine configuration requires a comment, since it is probably facilitated in this case by an adverse interaction in the trans-trans-cis form between the C-9 proton and the methylene group at C-14. Congeners of reserpine were also subjected to this sequence of... 

When heated at 150 C in xylene for 24 h with an excess of phospholes, Santini showed that complexes of type XVI gave sandwich complexes of type XVIII with a head-to-tail configuration which were fully characterized including an X-ray crystal structure analysis. Depending on the starting materials, a number of mixed homo- and hetero-bimetallic sandwiches could be obtained. On the basis of structural and NMR data, it appears that there is a strong through-space interaction between the non-bonded iron and phosphorus atoms in these complexes. 

A final topic concerns the axial ligand orientations and an attempt to understand the reasons for the small imidazole complexes. The data in Table XVIII is taken to clearly indicate a preferred orientation of imidazole ligand(s) that are sterically unfavorable. A series of charge iterative Huckel theory calculations for a broadly representative series of complexes reveals a k bonding effect that favors eclipsed orientations of the ligand(s). The n bond is dominated by the metal pji-imidazole pit interaction. The results provide an explanation of why the orientation effect of imidazole complexes seems insensitive to metal d configuration, spin state, oxidation state, and the presence or absence of a sixth axial ligand. 

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