Complexes of the type (Ph3M)2RhH(SiR3)X [M = P, R = Cl, OEt(X = Cl, Br, l), R3= Cl2Me, Cl2Et, ClMe2, ClEt2,Et3(X = Cl, Br), R3= Me3, Ph3(X = Cl). M = As, Sb, R = Cl, OEt(X = Cl)] are obtained by direct reaction between the appropriate silane and (Ph3M)3RhX. The stability of the phosphine complexes to thermal decomposition and to dissociation in solution varies widely from R = Cl or OEt (very stable) to R = alkyl or phenyl (completely dissociated in solution). The presence of a Rh—H bond is demonstrated by infrared and 1H n.m.r. spectroscopy (νRh–H 2020–2130 cm.–1, JRh–H 21–27 c./sec., JP–Rh–H 13–15 c./sec.), and a monomeric, five-co-ordinate structure is proposed. The complex (Ph3P)2RhH(SiCl3)Cl reacts with CO or PF3 to give SiHCl3 and (Ph3P)2Rh(L)Cl (L = CO, PF3), and evidence for the formation of the very unstable complex (Ph3P)2Rh(CO)Cl(H)SiCl3 is presented. The complex (Ph3P)2RhH[Si(OEt)3]Cl reacts with anhydrous hydrogen chloride to give SiH(OEt)3 and (Ph3P)2RhHCl2. The catalysis of the addition of R3SiH (R = Ph, Et, Cl) to hex-1-ene by (Ph3P)3RhCl and by (Ph3P)2RhH(SiCl2Et)Cl is discussed.