The first enantiomerically pure secondary chloro- and iodoarsines based on the 2,2’-bis(methylene)-1,1’-binaphthyl group have been prepared. The seven-membered (aR)-iodoarsepine has been prepared by the disproportionation of the corresponding enantiomerically pure (aR)-phenylarsepine with one equivalent of triiodoarsine in boiling toluene. The (aR)-chloroarsepine was prepared from the iodoarsepine by metathesis with silver chloride in dichloromethane. The isolation of these novel compounds facilitated an investigation of the interactions between chiral arsenium ions and symmetrical and chiral tertiary phosphines with the view to developing a new asymmetric synthesis of tertiary phosphines by deracemisation. Crystal structure determination on several phosphine-stabilised arsenium salts derived from the (aR)-iodoarsepine indicated orthogonal coordination of the phosphine to the trigonal plane of the AsC2 core of the six-electron arsenium ion. The 1H NMR spectra of the phosphine-stabilised salts derived from the (aR)-iodoarsepine indicate rapid exchange of the phosphine at 25oC, but the rate of exchange is reduced when there is a 2-methoxylmethylphenyl substituent on the phosphine because of a moderately strong interaction of this substituent with the phosphorus and arsenic atoms in the complex.