Bis(di‐<i>tert</i>‐butylphosphanyl)methan‐Komplexe des Rhodiums: Geometrie, Elektronenstruktur und Derivate des 14‐Elektronenteilchens [Rh(dtbpm)Cl]. Molekülstruktur von Rh(dtbpm)Cl(PMe₃)

Abstract

Bis(di‐ tert ‐butylphosphanyl)methane Complexes of Rhodium: Geometry, Electronic Structure, and Derivatives of the 14‐Electron Fragment [Rh(dtbpm)Cl]. Molecular Structure of Rh(dtbpm)Cl(PMe 3 ) 14‐Electron fragments [M(PR 3 ) 2 X] (M = Rh, Ir, X = halogen etc.) are considered to be an important class of highly reactive, coordinatively unsaturated intermediates in many metal‐induced stoichiometric or catalytic transformations of organic substrates. As available theoretical data suggest a slightly preferred T‐shaped groundstate geometry with a less symmetric cis rather than the usually implied trans phosphane arrangement for such tricoordinate d 8 ‐ML 3 ‐type systems with monodentate phosphanes PR 3 , the chemistry of η 2 ‐diphosphanylmethane complexes of rhodium with four‐membered RhPCP‐chelate rings and thus with enforced cis phosphane coordination and anomalously small cis P—Rh—P angles has been studied by theory and by experiment. MO calculations (EH) have been performed both for the model 14‐electron system [Rh(dhpm)Cl] (dhpm = diphosphanylmethane, H 2 P—CH 2 —PH 2 ) and for the experimentally accessible fragment [Rh(dhbpm)Cl], where dtbpm is bis(di‐ tert ‐butylphosphanyl)‐methane, ( t Bu) 2 P—CH 2 —P( t Bu) 2 . The electronic and geometric structure of these species is described. Employing the unusual ligand dtbpm, tailor‐made for stabilizing mononuclear η 2 ‐ and destabilizing dinuclear μ‐diphosphanylmethane coordination, the chloro‐bridged dimer [Rh(dtbpm)Cl] 2 , has been synthesized. In agreement with steric and electronic considerations, its chemistry is dominated by a facile dissociation to monomeric (presumably solvent coordinated) fragments [Rh(dtbpm)Cl], even in benzene, as suggested by molecular mass determinations. Accordingly, by using [Rh(dtbpm)Cl] 2 as a starting material, a series of sterically very congested but nevertheless mononuclear, square‐planar complexes Rh(dtbpm)Cl(L) (L = CO, PMe 3 , PPh 3 , PCy 3 , pyridine, acrylonitrile) with chelating dtbpm could be readily prepared and fully characterized. The relative stability of these potential alternative precursors of a [Rh(dtbpm)Cl] intermediate towards dissociation of ligands L is reported. The molecular structure of Rh(dtbpm)Cl(PMe 3 ) as the first representative of this class of compounds has been determined by X‐ray crystallography.