D. 117. MARGERUM
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Vol. 79
his research group for many helpful discussions during the course of this work. The fJcilities of thv Institute for -1tomic Research of Iowa S t a t c Cuilege were Used ( i u r i n ~this investigation.
be reported in detail a t a later date. Other substitution reactions which might proceed by similar mechanisms will warrant investigation. Acknowledgments.-The author is greatly indebted to Professor R. E. Rundle and members of
AXES, IOWA
~ C O N T R I B U T I O SFROM DEPARTMEKT O F CHI3MISTRI., P E R D U E t : S I V E R S I T Y ]
Kinetics of 1,lO-Phenanthroline Chelation. 11. Effect of Hydroxide Ion on the Dissociation Rate of Tris-( 1,lo-phenanthro1ine)-iron(I1) '3'
B\- D.
JIARGERUBI
RECEIVED DECEMBER 10, 19-56 Hydroxide ion greatly increases the rate of dissociation of tris-( l,lO-plieiiaiitiirolincj-iroii(II)despite the fact that therc are no acidic protons in the complex. The value of ko, first-order dissociation rate constant with respect t o the complex. is kl[OH-] 4- ko[OH-]* ky[OH-Ia. The kinetics may be exdependent on the hydroxide ion concentration: ko = k d plained by a n S Nmechanism ~ with nucleophilic attack by water or hydroxide ion to form reaction intermediates, ~vhichare in turn attacked by water or hydroxide ion, or by an Ss l mechanism with similar intermediates.
+
+
M HCl), but is greater than the dissociation rate. It has been suggested that racemization of FePhS++ proceeds both by a dissociation mechanism and by an intramolecular racemization mechanism. Much more work has been done on the mechanism of substitution reactions with cobalt(II1) complexes than with other coordination compounds. When weak nucleophilic groups react with cobalt(II1) complexes, the reactions proceed by S N ~kinetics, More powerful nucleophilic substitutions may be interpreted as S N or~ SN~ lCB9t1O reactions. The SNlCB reaction is onc where the conjugate base of the complex is formed prior to a slow first-order step in the reaction mechanism. I n support of the SNlCB kinetics it has been pointed out that "complexes containing no acidic protons release their replaceable ligands a t :I rate independent of the pH of the solutioii."'" FePhf+ Fe++ f Ph This generalization may be applicable to the coFast FePh++ + Ph 2FePh*++ l)alt(III) complexes but it does not fit all colripleses. Tris - ( 1 , l O - phenanthroline) - irori(II'1 conFeP1i2'.'' + P h FePh3+I- Slow tains no acidic protons but its dissociation rate i l l kd where h d = 0.0045 niin.-'. Only the addition or basic solution is highly dependent upon the fiH of removal of the third 1 10-phenanthroline molecule the solutioii. This paper is concerned with tlli. to iron(I1) proceeds a t a measurable rate. Because nature and significance of the hydroxide ion effect of the stability of the mono and tris complexes, the on the rate of dissociation of FePh3++. bis-(1,lO-phenanthrolinej-iron(I1) ion, FePhz++, Experimental does not exist in any appreciable concentration in A stock solution of F e P h 3 was prepared using 3 sliglit. solutions of varying iron(I1) and 1,lO-phenanthro- excess of 1,lO-phenanthroline so that the concentration of I'ePh3++ was 4.87 X 1 0 - 4 ;id atid the excess 1,IO-phenanline c ~ n c e n t r a t i o n . ~ Lliroline was 2 . S X lo-' M . Ten-ml. aliquot.;
