Reversible Öffnung von Metall-Metall-Bindungen: Quantitative Untersuchung des Ringöffnungsgleichgewichts von [Cp′(CO)2Mn]2SeAryl+ / Reversible Opening of Metal Metal Bonds: Quantitative Analysis of the Ring Opening Equilibrium for [Cp′(CO)2Mn]2SeAry1+

Abstract

The compounds [Cp′(CO)2Mn]2SeR+ are isoelectronic analogues of the well known “inidene”-species [LnM]ER (E = P, As, Sb, Bi). While these latter “inidene” compounds exist in an open form with no metal-metal-interaction and the main group atom in a trigonal planar environment allowing for a three-center-4π-system M E M, their isoelectronic selenium analogues generally show equilibria between the “inidene”-type metal metal non bonded forms and their metal metal bond closed cyclic isomers. In a valence bond picture the four electrons, delocalized in the three-center-4π-system of the open isomers, are localized in the closed ones: two in the metal metal bond and two as a lone pair at a pyramidally coordinated selenium center. The corresponding equilibria are quantitatively analyzed for [Cp′(CO)2Mn]2SeR+ (R = PCF3-C6H4 (1), PCH3-C6H4 (PToI) (2), °CH3—C6H4 (°Tol) (3)) by UV/VIS spectroscopy of their solutions at different temperatures. The ring opening process which corresponds to the reversible opening of a metal metal bond is found to be entropically driven with reaction-entropies of around 25 [JΚ-1mol-1]. Correspondingly, imposing steric hindrance on R, [Cp′(CO)2Mn]2SeMes+ (4) yields a stable bond opened “inidene”-type compound. Reducing steric hindrance in [Cp′(CO)2Mn]2Se′Pr+ (5) gives the bond closed form as the only detectable isomer.