Effects of nitrogen substitution in poly(pyrazolyl)borato ligands: From orbital energy levels to C-H center dot center dot center dot O hydrogen bonding nork Janiak, C, Scharmann, T, Green, J, Parkin, R, Kolm, M, Riedel, E, Mickler, W, Elguero, J, Claramunt, R, Sanz, D