New complexes of the types [RuX2(CO)L3], [RuXY(CO)L3], [RuX2(CO)2L2], and [Ru2X3L6][anion](X = Cl, Br, or I; Y = H, Cl, Br, I, NO3, acetate, propionate, or benzoate; L = PR3 or AsR3) are described. The hydrido-complexes [RuHX(CO)(PR3)3], configuration (III), react with nitric acid or carboxylic acids to give complexes of the type [RuXY(CO)(PR3)3](Y = NO3 or carboxylate) but with halogen acids (HY) mixtures are formed. An arsine hydrido-complex [RuHCl(CO)(AsMe2Ph)3] is described. The ruthenium–chlorine stretching frequencies for these complexes are recorded and discussed. Tertiary phosphines cause much weakening of a ruthenium–chlorine bond in trans-position, and carbon monoxide is a poorly trans-bond weakening ligand but causes more weakening than chlorine as trans-ligand. These bond-weakening effects are discussed. There is good correlation between a low ruthenium–chlorine stretching frequency and a high lability of the chlorine towards nucleophilic displacement; inert chlorines show high ruthenium–chlorine stretching frequencies. The stepwise formation of [RulCl(CO)(PMe2Ph)3] and [Rul2(CO)(PMe2Ph)3] from [RuCl2(CO)(PMe2Ph)3], configuration (II) by nucleophilic displacement of chloride by iodide, has been followed by an n.m.r. method. N.m.r. data and values for νCO are given.