σ-π Continuum in Indole-Palladium(II) Complexes.

The intrinsic features of (hetero-arene)–metal interactions have been elusive mainly because the systematic structure analysis of non-anchored hetero-arene–metal complexes has been hampered by their labile nature. We report successful isolation and systematic structure analysis of a series of non-anchored indole–palladium(II) complexes. It was revealed that there is a σ–π continuum for the indole–metal interaction, while it has been thought that the dominant coordination mode of indole to a metal center is the Wheland-intermediate-type σ-mode in light of the seemingly strong electron-donating ability of indole. Several factors which affect the σ- or π-character of indole–metal interactions are discussed.