The syntheses of rhodium(III) and iridium(III) half sandwich complexes containing tertiary arsine and C NMR spectroscopy, mass spectrometry and single crystal X-ray diffraction. DFT calculations show the formation of the complexes from (Cp*MCl 2 ) 2 and EPh 3 (E = P, As, Sb) to be highly exothermic, although the enthalpic driving force is decreasing in the expected sequence P > As > Sb.