Abstract A new class of phosphinine/rhodium catalysts for the hydroformylation of terminal and internal alkenes is presented in this study. A series of phosphabenzenes 1–14 has been prepared by condensation of phosphane or tris (trimethylsilyl) phosphane with the corresponding pyrylium salt. Trans-[(phosphabenzene) 2 RhCl (CO)] complexes 21–25 have been prepared and studied spectroscopically and by X-ray crystal-structure analysis. ...