Methods suitable for determining the step-stability constants of very stable complexes of transition metals with 1,lO-phenanthroline and its analogues are reviewed. Partition coefficients for 2,2'-bipyridyl and l , 10-phenanthroline between aqueous buffers of ionic strength 0 . 1~ and chloroform, carbon tetrachloride, and n-hexane have been measured at 25'.From measurements of the distribution of the two ligand bases between these solvents and aqueous buffers containing known concentrations of various cations, step-stability constants, valid for 25" and 0. 1M-potassium chloride (or nitrate) have been determined for bivalent ions of manganese, iron, cobalt, nickel, copper, zinc, and cadmium.The stabilities of the 1 : 1 complexes follow the Irving-Williams order in each case and they increase in the order Mn < Cd < Fe < Zn < Co < Ni < Cu. For the 3 : 1 complexes, however, the order becomes Mn < Cd < Zn < Co < Cu < Fe < Ni. The anomalous stability of the ferrous triscomplex is shown to arise because K , is greater than both K , and K,, owing to formation of a spin-paired complex.THIS series of papers form part of an extensive study of the quantitative aspects of steric hindrance to chelation with the immediate goal of seeking an interpretation of the I' singular " reactions 1 between certain heterocyclic bases and ferrous or cuprous ions which form the basis of the " ferroin '' and the ' I cuproin " reaction. To this end the stability constants of complexes of zinc, cadmium, and the transition metals manganese, iron, cobalt, nickel, and copper with 2,2'-bipyridyl, 1,lO-phenanthroline, 2-methyl-, 5-methyl-, and 2,9-dimethyl-phenanthroline have been determined. Most of this work was completed by 19M2 but publication was delayed in order to include values for 5-methylphenanthroline. The present paper (Part I) deals with complexes of 2,2'-bipyridyl and 1,lO-phenanthroline. Part I1 reports work with the ligands 2-methyl-and 2,9-dimethyl-l,lO-phenanthroline which differ from the parent base in carrying one or more substituents adjacent to the nitrogen atoms and thus presenting steric hindrance to co-ordination. In Part I11 the theory of the method is elaborated somewhat and results are reported for 5-methyl-1,lO-phenanthroline in which the electron-repelling substituent is so located as to cause no steric hindrance to co-ordination. In Part IV some calorimetric measurements will be reported. Tris-complexes of 2,2'-bipyridyl (bipy) and 1,lO-phenanthroline (phenan) with ferrous iron are known to be remarkably stable, with overall formation constants of about lo1' and 1021, respectively. The " robust " character of such transition-metal complexes is further exemplified by the considerable optical stability of the antipodes of the triscomplexes of nickel and ferrous iron.3 If the Irving-Williams order of stabilities

The dissociation constant of the acid conjugate to 2,9-dimethyl-l, 10-phenanthroline is found to be 105-85 in WlM-potassium chloride a t 25'. The distribution coefficients for 2-methyl-and 2,g-dimethyl-1,lO-phenanthroline between aqueous buffers and n-hexane, chloroform, or carbon tetrachloride have been measured, both alone and in the presence of known quantities of certain bivalent cations ; thence the step-stability constants have been computed for the metal complexes formed in the aqueous solutions. All complexes formed by 2-methyl-1,lO-phenanthroline are appreciably less stable than those with 1.1 0-phenanthroline, but the stability order Mn < Fe < Co < Ni < Cu > Zn and Cd is obeyed for both the 1 : 1 and the 2 : 1 complexes.There is no anomaly for K , with iron(I1); and copper(I1) does not form a 3 : 1 complex.Complexes with 2,9-dimethyl-1, 10-phenanthroline are still less stable, owing to the increased steric hindrance, and there is reasonable evidence that for the 2 : 1 copper complex K , < K2. The implications of these measurements when designing a specific reagent for copper are discussed.MANY results published since 1953 have confirmed the validity of the generalisation that the stability of complexes formed by the bivalent transition metals follows the order Mn < Fe < Co < Ni < Cu > Zn and Cd, irrespectively of the nature of the ligand.In the paper in which the evidence then available was summarised Irving and Williams pointed to two possible factors that might operate to alter the stability sequence from its normal order. The first of these, " orbital stabilisation " ( k , a change from the normal ground state of the ion to a spin-paired state), was shown to be likely to occur with compounds of ferrous iron with certain ligands and should lead to complexes of unusual stability, so that the normal order Mn < Fe < Co would be replaced by Mn < Co < Fe. The * Part I, preceding paper.

Although 2-methyl-1 : 10-phenanthroline (pK = 5-42) is a stronger base than 1 : 10-phenanthroline (pK = 4-96) it forms weaker complexes with ferrous ions (log K , = 4-2, log K , = 3.7, and log K , = 2-9). This difference in behaviour is due to steric hindrance to co-ordination which increases with further substitution to such an extent that 2 : 9-dimethyl-1 : lo-phenanthroline will not form complexes with FeII although it does so with CuI.Tris-complexes predominate in the strongly red solutions formed by equivalent amounts of Fe++ and 1 : 10-phenanthroline. Corresponding solutions of Fe++ and 2-methylphenanthroline are feebly yellow and contain principally the bis-complex. That perchlorate ions precipitate tris-complexes from either solution can be explained quantitatively from considerations of the stability constants of the respective complex ions and the measured solubility products of their perchlorates.1 : 10-PHENANTHROLINE (" phenan " ; I ; R = R' = H) with ferrous salts gives a series of compounds containing the stable red ion Fe(phenan),++ which is of great analytical importance. The diamagnetism of its salts indicates that the organic components are held by octahedral d2sp3 hybrid orbitals. The coplanar resonating system formed by the three aromatic rings of each phenanthroline molecule and the five-membered ring which it forms with the central iron atom is thus repeated in three atomic planes at right angles, and the asymmetry of the resulting structure has recently been confirmed by a successful resolution (Dwyer and Gyarfas, J . PYOC. Roy. SOC. N.S.W., 1949, 83, 263). The considerable optical stability of the enantiomorphous tris-1 : 10-phenanthroline ferrous ions (t& = 121 min. at 15"; Dwyer, personal communication) is in keeping with the slow rate of exchange with the radio-nuclide 59Fe (it = 100 min.; Ruben, Kamen, Allen, and Nahinsky, J . Amer. Chem. SOC., 1942, 64, 2297) and the high over-all stability constant, since K3 = [Fe phenan,i+]/pei+][phenan]3 = 2 x 1O2I (Kolthoff, Lee, and Leussing, ibid., 1948, 70, 2348).On the other hand, Pfeiffer and Christeleit ( J . pr. Chem. 1938, 151, 127) commented on the fact that 2-methyl-1 : 10-phenanthroline (" mephenan " ; I ; R = Me, R' = H) forms sparingly soluble complex salts pe(mephenan)J (ClO,), and *-\ [Fe(mephenan)JS0,,12H20, which are only orange-coloured in contrast to

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