Rate constants have been measured for the reaction M + P -+ MP, where M is one of Fe( II), Co( 11), Ni( 11), or Cu( 11) and E' is the neutral hematoporphyrin IX molecule. T h e r-lative rates of reaction were found to be in the order CU > Co > > Fe > > Xi, and the relative thermodynamic stabilities of the complexes are predicted to be in the order Xi > Cu -Co > Fe according to the ratio of intensities of the main bands in the visible spectrum. The activation parameters for metalloporphyrin formation are reported. The biological implications of the relative rates of formation and stabilities of the coniplexes are discussed.

McBryde, Brisbin, and I r v h g . 52451004. The stability of Metal Complexes of 1,10-Phenanthroline and its Analogues. Part I I P 5-Methyl-1,IO-phenanthroline.The Bransted acid dissociation constant of 5-methyl-1, LO-phenanthroline (pK, 5-28} has been obtained spectrophotometrically and there is evidence that the base accepts a second proton. The stability constants of its complexes with bivalent ions of the metals manganese, iron, cobalt, nickel, copper, zinc, and cadmium have been obtained by a partition method. All results apply to 25" and an ionic strength of 0.1111 (KC1 or KNO,). The advantages of using mixtures of organic solvents to cover the range of partition measurements has been demonstrated.For 1 : 1 complexes the order of increasing stability is Mn < Cd < Fe < Zn < Co < Ni < Cu. The red ferrous tris-complex is diamagnetic and so stable that for 1 : 3 complexes the stability order becomes Mn < Cd < Zn < Cu < Co < Fe < Ni.Stability constants for complexes of iron, manganese, and cobalt with 1,lO-phenanthroline have been redetermined.

The stepwise formation constants for the complexes formed by 5,6-dimethyl-and 4,7-dimethyl-1,lO-phenanthroline with bivalent iron, cobalt, nickel, copper, and zinc were determined by a partition method. The measurements were made a t 25' C and in aqueous solutions having an ionic strength maintained a t 0.1. The enhanced basicity of the ligands compared to the parent phenanthroline is paralleled by increased stability of the metallic complexes. The abnormally, high formation constants of the cobalt complexes suggest oxidation to cobalt (111).A number of studies t o determine the stability of metal complexes of the phenanthrolines have been published (1-8), among the most complete of which are those of Irving and co-workers (2, 4). In an extension of these studies the present investigation has sought formation curves, and hence formation constants, for the complexes formed by 5,6-and 4,7-dimethyl-1,lO-phenanthroline with a selected group of ions of the first transition series.The inductive effect of substituent methyl groups in increasing the basic strength of phenanthroline is apparent from the data of the following table: From these data it is seen that the basic strength of these compounds is enhanced by the presence of each methyl group in the conjugated ring system, and that the extent of this enhancement is related to the propinquity of the methyl groups t o the nitrogen atoms. In agreement with expectations, the increased basicity of the compounds investigated is paralleled by an increase in stability of the complexes investigated except, as noted elsewhere, when steric hindrance t o coordination about the metal atom occurs (5). The experimental method used was based on partition of the ligand between an aqueous solution and a solvent immiscible with water. The principle of the method is old, but its extension to the study of formation of complexes involving stepwise equilibria was first suggested by Kolthoff et al. (9), and subsequently extended and refined by Irving and co-workers (2). A measured volume of a solution of the ligand of known concentration in a selected solvent was shaken with a measured volume of aqueous solution containing

The Micro-Determination of Porphyrins An integrated laboratory experiment Many universities in recent years have become interested in integrating the various chemistry divisions with the desire to eliminate the artificial boundaries of chemistry. This laboratory we believe successfully combines inorganic, analytical, and biological chemistry. It serves as a useful vehicle for teaching some coordination chemistry with a biological bent, as well as analytical techniques in a non-aqueous solvent.It is suggested for use for the more senior student, at the third or fourth year level.Use is made of the distinct spectra and large formation constants of some of the metalloporphyrins for the microdeterminations of porphyrins using standard solutions of some first transition metal cations as titrants. The method as related here has the advantage of not requiring the knowledge of the molar extinction coefficients of the porphyrins, which are often difficult to obtain either in published data, or experimentally, because of the necessity of a pure crystalline compound. Most porphyrin determinations require the molar extinction value. The method can also be used for the micro-determination of some first transition metals, using a standard porphyrin solution as the titrant. but has less practical merit, since there are more convenient and accurate methods available.

Notes lowed by quick freezing to -190°y ielded only the spectrum of CIO2 in a rigid lattice,16,17 Figure 4.