Phosphine oxide and arsine oxide complexation of alkali metal cations

• Triphenylphosphine oxide complexes of Li + , Na + , K + , and Rb + are reported. • The first crystal structure of a Rb + –phosphine oxide complex is described. • The first structural characterization of arsine oxide complexes of alkali metals. Pnictine oxides are moderately strong Lewis bases but have a hardness that predisposes them for interacting favorably with hard metal ions. Here, we investigate the interactions of prototypical pnictine oxides Ph 3 PO and Ph 3 AsO with the alkali metal cations Li + , Na + , K + , and Rb + . Although the interactions of phosphine oxides with Li + , Na + , and K + are well-precedented, this work describes the first extension to include the structural characterization of a phosphine oxide coordinated to Rb + . The presently described compounds in which Ph 3 AsO coordinates to Li + , Na + , K + , or Rb + are the first structurally characterized examples of any arsine oxide binding to any alkali metal cation. These data allow for a systematic comparison of the interactions and suggest that, for a given pnictine oxide, the strength of the R 3 PnO–M + bond does not appreciably change across the series of alkali metals. For each alkali metal, however, the data clearly show that the arsine oxide binds more strongly than the phosphine oxide.