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. [ABSTRACT FROM AUTHOR]