THE SOLUTION STABILITIES OF CHELATE COMPOUNDS

245 Pages·12.612 MB·English

by  VAN UITERT, LEGRAND GERARD

The Pennsylvania State College
The School of Chemistry and Physics
Department of Chemistry
The Solution Stabilities of
Chelate Compounds
A Dissertation
by
LeG-rand Gerard Van Uitert
Submitted in partial fulfillment
of the requirements
for the degree of
Doctor of Philosophy
June 1952
Approved:
Head of the Department
Acknowledgements
I wish to express my appreciation to Dr. W . 0.
Fernelius for his encouragement and. for his ahle
d.irection of this research problem; to Dr. G. G-. Haas
for his contributions to tb.e theoretical considerations
involved in the text; to Dr. B. E. Douglas for his
careful criticism of the manuscript; to the above per­
sons and Drs. R. W. Taft, Jr., H. D. Zook, and R. P.
Seward for the many helpful discussions I have enjoyed,
with, them and assistance in other forms; to the research
group for their cooperation in many ways ; and to the
Atomic Energy Commission and the Union Carbide and
Carbon Corporation for their financial support of this
research project.
I especially wish to acknowledge the many long
hours that my wife has contributed to the completion
of this dissertation.
Ttie Solution Stabilities
Cbelat© Compounds-
Abstract
The experimental work contained herein has been
designed to elucidate the basic relationships that
exist for the reversible chelate compound, formation
equilibria between be^a-diketones and metal ions in
solution.
In the past, pH titrations in partially non-aqueou
solvents have been made in order to determine the sta­
bility of coordination compounds. The interpretation
of such data has been reconsidered with, the object of
obtaining thermodynamic stability constants.
The variation of the pKd values for several beta-
diketones with, changes in composition of water-dioxane
solutions has been found t-o follow the pattern for
simple acids above a mole fraction of dioxane (ng) of
0.10. Shifts in the keto-enol equilibrium causes devia
tions from linearity below = 0.10.
The variation of the logarithms of the formation
constants (log. Kf values) of the metal chelate com­
pounds with, structurally similar beta-diketones h.as
been found to be an essentially linear function of the
negative logarithms of the acid dissociation constants
of the beta-diketones.
The stabilities of the metal chelate complexes of
a common ch.elat.irg agent, in general, increase with the
electronegativities of the metal ions (Xrn values)
involved wit.b.in a common 1)011(3 hybrid (B^) classifi­
cation. When the log. values for the metal ions
are plotted as a function of Xm , separate, approxi­
mately straight, lines are obtained for the mono­
valent, the divalent, and the trivalent ions. The
slopes of these lines stand in the ratios of the rela­
tive stabilities for the probable bond hybrid states
about the cation. On this basis, one may write
log. Kfq - a(Xm*E>h.) - Const,
for the exchange constants representing, chelate com­
pound. formation in solution.
It can be shown that the above equation can be
expected from considerations of molecular orbita.1
.
theory
The coordinating abilities of nitrogen and oxygen
have also been found to be consistent with molecular
orbital theory. It is shown in Section Six that
basicity should decrease in the following orders for
nitrogen and oxygen
, > . =N~ , -N
and. — O , ^0 , -=0
* / *
as found in amines, ring nitrogen, the ^C-N- group,
and cyanides for nitrogen and the enolat.e ion, ethers,
and carbonyl groups for- oxygen. Experimental evaluations
indicate the following order of group coordinating
abilities
-N-N- > > COO" ^ )c > -0
Th.e relationships between chelate compound forma­
tion constants in different solvent mixtures have
been demonstrated for water-dioxane mixtures and
ethanol. The true ratio of the first to the second
formation constants Kf]_/Kf2 for the chelate complex­
es of beta-diketones formed in aqueous solutions is
obscured by the leveling effects of salt anions and
aquation upon the metal ions. As these factors are
reduced, the apparent ratio increases far beyond the
limits that can be accounted for statistically.
Measurements in ethanol demonstrate that the ratio of
the two constants is close to 1 x 10^. Data for beta-
ketoesters and malonates in ethanol are also included.
Table of Contents
I. Introduction ....... 1
II. Source of materials ....................... 4
III. Experimental ............................... 8
IV. Definitions of symbols....... ...... 15
V. Section One - The determination of thermo­
dynamic formation constants in the
water-dioxane system....... .............. 16
VI. Section two - Chelating agent dissociation
constants ................................ 35
VII. Section Three - Chelate compound formation
constants for t.he divalent, met,al ions .. 46
A. Calculation procedure ................. 46
B. The formation constants ................51
C. Beta-diketones vs. chloride salts .... 52
D. Beta-diketones vs. nitrate ........... 61
E. Beta-diketones vs. perchlorate ...... 64
F.- The salt anion effect ................ 66
G-. The solvent effect • ............. 74
H. Chelate compound species .......... 81
Table of Contents
VIII. Section Four - Additional chelate
compound formation constants ............. 91
A. Monovalent ions .............. 91
B. T^ivalent ions and tborium(IV) ...... 93
C. Mercury(ll) ............ 94
D. Palladium( II) .......................... 95
IT. The uranyl i o n ........................ 95
F. Formation constants for cerlum(lll) .. 96
IX. Section Five - Measurements in absolute
ethanol ................................... 102
A. Experimental ....................... 102
B. Nickel vs. acetylacetone ............. 104
C. Nickel vs. bef.a-ketoesters ........... 105
D. Nickel vs. malonic esters ............ 107
X. Section Six - G-eneral consideration con­
cerning chelate compound stability ....... 112
A*. General ................................ 112
B. The application of electronegativity
values to solution data ............... 117
C. The role of the metal ion in the
chelation process ........ 120
D. The role of the ligand. ............... 128
E. A comparison of ligands .............. 130
F. Experimental evaluations ............. 133
Table of Contents
X I . Appendices ................................ 141
A. Appendix to Section One ............... 141
B. Appendix to Section two ............... 14-7
C. Data for divalent perchlorate salts .. 153
D. Lead(Il) data .......................... 166
E. Data for divalent nitrate salts ...... 169
F. Data for divalent chloride salts ..... 177
G . Tabulated, stability constants ........ 193
1 . pK^ values in n2 = 0.380 ........ 194
2 . MCI 2 stability constants ......... 195
3. M(£10-5)2 stability constants ..... 199
4. M( C104)2 stabilit3r constants .... 201
H. Appendix to Section Four .............. 203
1. Mercury(ll) ........... 204
2. Palladium( II) ..................... 206
3. Uranyl nitrate .................... 209
4. Thorium(IV) ......... 210
5. Scandium(III) and aluminum(III)... 211
6 . Cerium(lll) ..... 212
7. Monovalent ions ..... 215
I. Additional data ....................... 220
1. Half chelation titrations ....... 221
2 . Salt anions complexation of
zinc(II) ........................... 223
Table of Contents
3. Data in ethanol ......... 225
4. Carbon hydrogen analyses .............229
XII. References ............... 232