Coordination Behavior and Coligand‐Dependent <i>cis</i>/<i>trans</i> Isomerism of Calcium Bis(diphenylphosphanides)

Abstract

Abstract The salt metathesis reaction of KPPh 2 with CaI 2 and SrI 2 in THF yields the corresponding complexes [(THF) 4 Ae(PPh 2 ) 2 ] [Ae = Ca ( 1 ), Sr]. Depending on the crystallization conditions, cis ‐ or trans ‐[(THF) 4 Ca(PPh 2 ) 2 ] can be isolated. In solution, a fast equilibrium converts these isomers into each other leading to a single set of resonances in the NMR spectra. The THF ligands can be replaced by stronger Lewis bases, such as N ‐methylimidazole (MeIm), which leads to the formation of [(MeIm) 4 Ae(PPh 2 ) 2 ] [Ae = Ca ( 2a ), Sr ( 2b )]. The use of tetradentate hexamethyltriethylenetetramine (hmteta) fixes a cis configuration yielding [(hmteta)Ca(PPh 2 ) 2 ] ( 3 ). All compounds are very air and moisture sensitive, therefore a few crystals of the partially oxidized product [(hmteta)Ca(PPh 2 )(OPPh 2 )] ( 4 ) were also obtained.