Electronic, counterion and medium effects on the bonding mode of the seleno-cyanate ion in trans-hydridoselenocyanatotetrakispiperidineiriduium(III) complexes

Abstract

The results of infrared spectral studies of the complexes trans-[Ir(pip)₄(H)CNSe]Y (pip = piperidine; CNSe⁻ = selenocyanate ion, bonding mode unspecified; Y = NCSe⁻, B(C₆H₅)⁻₄, and ClO⁻₄), both in the solid state and in solution, having shown them to be ideal systems for the investigation of ambidentate ligand bonding mode controls. The greater stability of the N-bound selenocyanate, with SeCN⁻ as the counterion, both in the solid state and in solution (irrespective of solvent dielectric), serves to further confirm the existence of antisymbiosis in an octahedral complex of a class (b) metal involving a strong trans-director (H⁻) with no π-bonding capability. The greater stability of the Se-bound selenocyanate, with B(C₆H₅)⁻₄ as the counterion, in the solid state is a clear-cut example of counterion bonding mode control. The selenocyanato—tetraphenylborate complex was found to be subject to a KBr-assisted isomerization in the solid state, and all three complexes were observed to undergo a heterogeneous KBr-catalyzed ionization of the coordinated SeCN⁻ ion in DMSO solution, when studied in KBr cells. In CH₂Cl₂ solution, where minimal KBr catalysis is observed, hydride transfer to the solvent, with a concomitant loss of NCSe⁻ to give a five-coordinate intermediate, which recombines with NCSe⁻ to form trans-[Ir(pip)₄(SeCN)₂]⁺, occurs instead over an extended time period. The vCN frequency frequency of the non-coordinated NCSe⁻ is very sensitive to the solvent dielectric, suggesting the existence of H-bonding to the amine proton of the piperidine ligands in the low dielectric solvents. The complexes trans-[Ir(pip)₄(H)X] ClO₄ (X = NCS⁻, CN⁻) were included in the study, for purposes of comparison. The vIrH frequency exhibited by the cyanide complex was found to be very low, due to the strong trans-influence of the cyanide group.