The products, resulting from addition under mild conditions of the gaseous molecules CO, O2, H2, and HCl, to some rhodium(I)–bis(ditertiaryphosphine) complexes, are described; the [Formula: see text] phosphines used were Ph2P(CH2)nPPh2, n = 1–4, and (+)-diop (diop = 2,3-O-isopropylidene 2,3-dihydroxy-1,4-bis(diphenylphosphino)butane). Monocarbonyls were isolated from addition of CO to [Formula: see text] complexes (n = 1 and 3), the reactions being reversible; there was no reaction with the n = 2 system, while the n = 4 and diop systems absorbed > 1 mol of CO per Rh to give mixtures of products. The same cation precursors add O2 to yield cis-[Formula: see text] adducts (n = 1–3), the binding being reversible for the n = 2 system; pure products were not isolated with the n = 4 cation which absorbs 3 molO2/Rh; the diop system was unreactive toward O2. Reversible additions of 1 mol H2 yield isolable cis-[Formula: see text] complexes with the n = 3 and diop systems; the n = 1 and 2 [Formula: see text] cations are unreactive toward H2, while a mixture of hydrides is formed from the n = 4species. The chlorohydrido cations [Formula: see text] (A = Cl, BF4, PF6, SbF6), n = 1–3, were prepared via routes involving addition of gaseous HCl to precursor [Formula: see text] complexes; however, the reactions with the n = 4 and diop systems did not yield pure products.The solution structures of all the isolated five- and six-coordinate addition products have been determined using 1H and variable temperature 31P nmr. The five-coordinate carbonyl with n = 1 shows equivalent phosphines from −50 to 25 °C, while the n = 3 system shows inequivalent phosphines resulting from a trigonal bipyramidal structure with an equatorial carbonyl. The [Formula: see text] cation, n = 3, has cis geometry, while trans-structures are preferred for the n = 1 and 2 species.