Coordination compounds factors affecting stability

Pcs are one class of such compounds with their delocalized two-dimensional 18 7t-electron systems and exceptional stabilities, which make them suitable candidates for NLO applications. From this point of view, Pcs, subphthalocyanines, and related compounds have been investigated extensively in recent years [14]. Besides the excellent 18 Tt-electron systems, the substituents at the benzene rings, coordinative metal, and axial coordination ligands of the Pcs affect the degree of the NLO properties significantly. However, correlation between these factors has not been fully established yet. 

A Lewis acid e.g. aluminium tert-butoxide, boron trifluoride, neutral alumina) is considered to coordinate with both the 17-OH and 20-keto functions, bringing them into a cfs-relationship and effectively locking the side chain in one conformation. Product formation is then determined by the relative ease of migration of the Cps)--C(i ) and C(i6>-C(i7) bonds towards the electron-deficient C<20). The structures of the resulting ketones show that the C i3)-C(i ) bond migrates in the i7j -hydroxy compound (i), and the C(i6)-"C(i7) bond in the i7a-hydroxy isomer (2). The reason for this difference has been the subject of much speculation, and is still not clear. The factors which have been considered [202] as affecting the stability of respective transition states include ... 

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