Ralph G. Pearson

Hard and soft acids and bases ― the evolution of a chemical concept

The present paper will deal mainly with the more recent developments of the concept of chemical hardness. However, a necessary summary of the early work will be given

Chemical hardness and density functional theory

The concept of chemical hardness is reviewed from a personal point of view.

The HSAB Principle — more quantitative aspects

Abstract The HSAB Principle is reviewed, with special emphasis on common misconceptions of its meaning. The most quantitative way to use it is discussed, along with an analysis of why it sometimes seems to fail. Recent developments in the concept of chemical hardness are also reviewed. It is concluded that these have not yet helped much in estimating bond energies, but that there is hope for the future.

The second-order Jahn-Teller effect

Abstract The second-order Jahn-Teller effect is an example of reactions proceeding by an interaction between the HOMO and the LUMO within the same molecule. The consequences can be decomposition of the molecule or a structural change. First the general theory is given, and then a number of examples are presented.

SYMMETRY RULES FOR CHEMICAL REACTIONS

Abstract The symmetry properties of molecular orbitals and of reaction coordinates can be used to decide on the feasibility of selected chemical reaction mechanisms. Some reaction paths are shown to have a large energy barrier and are said to be “forbidden by orbital symmetry.” The reactions of molecules with no symmetry can also be analyzed by being compared to related symmetric molecules, where the molecular orbitals are topological identical.

The electronic chemical potential and chemical hardness

Abstract Two quantities, derived from density functional theory, that help define any chemical system are the electronic chemical potential and the chemical hardness. Some of the properties and applications of these fundamental quantities are described.

Symmetry Rule for Predicting the Structures of Molecules. II. Structure of X2Yn Molecules

Using the second‐order Jahn–Teller effect as a criterion, molecular shapes are predicted for molecules with the formulas X2Y2, X2Y4, and X2Y6. The method is remarkably successful, the only failure being the inability to predict the correct angle of rotation about single bonds. However, this is a necessary failure.

Acid catalysis of the hydrolysis of acetato-pentammine complexes of cobalt(III), rhodium(III) and iridium(III)

Abstract The syntheses of [M(NH3)5(RCOO)](ClO4)2 where M = Rh(III) or Ir(III) and RCOO− = CH3COO−, (CH3)3CCOO− or CF3COO− are described. The rates of hydrolysis of these complexes as well as the corresponding Co(III) systems were determined in acid, [H+] = 0·01 – 0·1 M. In all cases the rates of hydrolysis increase with increase of acid concentration. The mechanism of acid hydrolysis of these systems is discussed.

Electron Paramagnetic Relaxation in Ionic Solutions

The interaction of paramagnetic cations with paramagnetic anions in solution is considered. The resultant broadening of the paramagnetic resonance lines of one of the species can be used to measure rates of formation of ion‐pairs in cases in which the interaction is strong. Experimental results are given for a number of ions in water and compared to theoretical rates of diffusion controlled reactions between ions of opposite charge. The effect of the presence of inert, diamagnetic ions in the solution is considered.

Base hydrolysis of some chloroammine-platinum(IV) complexes☆

Abstract The rates of base hydrolysis of [Pt(NH 3 ) 5 Cl] 3+ , cis and trans -[Pt(NH 3 ) 4 Cl 2 ] 2+ , trans [Pt(NH 3 ) 3 Cl 3 ] + , and trans -[Pt(en) 2 Cl 2 ] 2+ were measured. The acid dissociation constants of these complexes were also measured. These data show that in the basic solutions used for the kinetic studies amido complexes (e.g. [Pt(NH 3 ) 4 NH 2 Cl] 2+ ) were the predominant species. Under such basic reaction conditions considerable reduction accompanied hydrolysis of those compounds which contained trans chloro groups. Probable mechanisms of base hydrolysis and of reduction are discussed.