Ralph G.Wilkins

Kinetics and Mechanism of Reactions of Transition Metal Complexes

2nd Thoroughly Revised Edition

VCH Weinheim • New York • Basel • Cambridge

Contents

Ligands and Complexes XIII

1 The Determination of the Rate Law 1

1.1 The Rate of a Reaction and the Rate Law 1 1.2 The Rate Law Directly from Rate Measurements 2 1.2.1 Initial-Rate Method 3 1.2.2 Critique of the Initial-Rate Method 4 1.2.3 Steady-State Approach 5 1.3 Integrated Forms of the Rate Expression 5 1.4 Monophasic Unidirectional Reactions 7 1.4.1 Zero-Order Dependence 7 1.4.2 First-Order Dependence 8 1.4.3 Second-Order Dependence 10 1.4.4 Conversion of Pseudo to Real Rate Constants 11 1.5 Monophasic Reversible Reactions 13 1.5.1 Conversion of Reversible to Unidirectional Reactions 15 1.6 Multiphasic Unidirectional Reactions 16 1.6.1 Concurrent Reactions 17 1.6.2 Consecutive Reactions with no Elements of Reversibility 18 1.6.3 Two-Step Reactions with an Element of Reversibility 23 1.6.4 Reaction Schemes Associated with (1.98) 24 1.6.5 Two-Step Reactions with Total Reversibility 27 1.7 Recapitulation 31 1.8 Relaxation Kinetics 32 1.8.1 Single-Step Reactions 32 1.8.2 Multistep Reactions 33 1.9 Exchange Kinetics 38 1.10 The Inclusion of [H+] Terms in the Rate Law 41 1.10.1 One Monoprotic Reactant, One Acid-Base Equilibrium 41 1.10.2 Two Acid-Base Equilibria 43 1.10.3 The Effect of High Acid Concentration 47 1.11 Kinetics and Thermodynamics 49 1.12 Concluding Remarks 50 References 50 Problems 56

VIII Contents

2 The Deduction of Mechanism 65

2.1 The Rate Law and Mechanism 65 2.1.1 First-Order Reactions 67 2.1.2 Second-Order Reactions 68 2.1.3 Third-Order Reactions 69 2.1.4 Even Higher-Order Reactions 72 2.1.5 Negative-Order Reactions 72 2.1.6 Fractional-Order Reactions 73 2.1.7 The Inclusion of [H+] Terms in the Rate Law 75 2.2 Further Checks of Mechanism 80 2.2.1 The Detection and Study of Intermediates 80 2.2.2 The Determination of Bond Cleavage 84 2.3 Activation Parameters, Thermodynamic Functions and Mechanism . . . . 87 2.3.1 The Effect of Temperature on the Rate of a Reaction 87 2.3.2 The Variation of Ea or A//* with Temperature 89 2.3.3 The Effect of Pressure on the Rate of a Reaction 90 2.3.4 The Variation of AK* with Pressure 91 2.3.5 Activation Parameters and Concentration Units 91 2.3.6 Reaction Profiles 91 2.4 Free Energy of Activation and Mechanism 93 2.5 Linear Free-Energy Relationships - AG* and AG 96 2.5.1 Hammett Relationship 99 2.5.2 Taft Relationship 101 2.5.3 Bransted Relationship 101 2.5.4 Swain-Scott Relationship 103 2.6 Enthalpy of Activation and Mechanism 104 2.7 Entropy of Activation and Mechanism 105 2.7.1 AS* and the Charge of the Reactants 105 2.8 Volume of Activation and Mechanism 106 2.8.1 AH- and AS* Values - The Isokinetic Relationship 108 2.8.2 AS* and A K* Values 109 2.8.3 Use of All Parameters 110 2.9 Medium Effects on the Rate 110 2.9.1 The Effect of Electrolytes 110 2.9.2 The Effect of Electolytes — Medium or Mechanistic? 115 2.9.3 The Solvent Effect and Mechanism 116 References 118 Problems 123

3 The Experimental Determination of the Rate of Reaction 131

3.1 Essential Preliminaries 131 3.1.1 Reactant Species in Solution 131 3.1.2 Stoichiometry of Reaction 133 3.1.3 The Nature of the Products 133

Contents IX

3.1.4 The Influence of Impurities 134 3.1.5 The Control of Experimental Conditions 134 3.2 The Methods of Initiating Reaction and their Time Ranges 135 3.3 Flow Methods 136 3.3.1 Continuous Flow 137 3.3.2 Quenched Flow 138 3.3.3 StoppedFlow 139 3.4 Relaxation Methods 140 3.4.1 Temperature Jump 141 3.4.2 Pressure Jump 141 3.4.3 Electric-Field Jump 143 3.4.4 Ultrasonic Absorption 144 3.5 Large Perturbations 145 3.5.1 Flash or Laser Photolysis 145 3.5.2 Pulse Radiolysis 148 3.5.3 Comparison of Large Perturbation Methods 151 3.6 Competition Methods 151 3.7 Accessible Rate Constants Using Rapid Reaction Methods 151 3.8 The Methods of Monitoring the Progress of a Reaction 153 3.9 Spectrophotometry 154 3.9.1 Ultraviolet and Visible Regions 154 3.9.2 Infrared Region 158 3.9.3 Fluorescence 159 3.9.4 Polarimetry 160 3.9.5 Nmr Region 161 3.9.6 Nmr Line Broadening 163 3.9.7 Epr Region 169 3.9.8 Epr Line Broadening 170 3.10 Non-Spectrophotometric Methods 171 3.10.1 [H+] Changes 171 3.10.2 Cationic and Anionic Probes 173 3.10.3 Conductivity 173 3.10.4 Thermal Changes 174 3.10.5 Pressure Changes 174 3.10.6 Electrochemical Methods 174 3.11 Batch Methods 175 3.12 Competition Methods 176 3.12.1 Stern-Volmer Relationship 177 3.12.2 Isotope Fractionation 178 3.13 The Study of Transients 178 3.13.1 Spectral Properties of Transients 179 3.13.2 Thermodynamic Properties of Transients 180 3.13.3 Chemical Reactivity of Transients 180 References 181 Problems 192

X Contents

4 Substitution Reactions 199

4.1 The Characteristics of Substitution Reactions 199 4.1.1 Solvated Metal Ions 200 4.1.2 Representation of Substitution Mechanisms 201 4.2 Substitution in Octahedral Complexes 201 4.2.1 Solvent Exchange with Metal Ions 202 4.2.2 The Interchange of Different Unidentate Ligands 205 4.2.3 Outer Sphere Complexes 206 4.2.4 Characteristics of the Various Mechanisms 207 4.2.5 The Limiting First-Order Rate Constant 208 4.2.6 Second-Order Rate Constants 210 4.2.7 Summary 211 4.3 Accelerated Substitution of Unidentate Ligands 212 4.3.1 H+-Assisted Removal 212 4.3.2 Metal lon-Assisted Removal 213 4.3.3 LigandAssisted Removal 214 4.3.4 BaseAssisted Removal 215 4.3.5 The Quest for Five Coordinate Intermediates 217 4.4 Replacement Reactions Involving Multidentate Ligands 219 4.4.1 The Formation of Chelates 219 4.4.2 Effect of [H + ] on the Rates of Substitution in Chelate Complexes 221 4.4.3 Metal IonAssisted Dechelation 222 4.4.4 LigandAssisted Dechelation 223 4.5 Replacement Reactions Involving Macrocycles 224 4.5.1 Azamacrocycles 226 4.5.2 Crown-Ethers and Cryptands 227 4.5.3 Porphyrins 229 4.6 Substitution in Square-Planar Complexes 232 4.6.1 The Kinetics of Replacement Involving Unidentate Ligands 232 4.6.2 Activation Parameters 235 4.7 Ligand Effects on the Rate 236 4.7.1 Effect of Entering Ligand 236 4.7.2 Effect of Leaving Group 237 4.7.3 Effect of Ligands Already Present 237 4.7.4 Effect of Solvent 238 4.7.5 Reaction Pathways 238 4.7.6 Chelation in Square-Planar Complexes 240 4.8 Substitution in Tetrahedral Complexes 242 4.9 Substitution in Five-Coordinate Complexes 243 4.10 Substitution in Organized Surfactant Systems 244 4.11 Substitution in Metalloproteins 245 References 246 Problems 252

Contents XI

5 Oxidation-Reduction Reactions 257

5.1 General Characteristics 257 5.2 Classification of Redox Reactions 258 5.3 Characterization of Mechanism 259 5.4 Outer Sphere Reactions 262 5.4.1 The Applications of the Marcus Expression 269 5.5 Inner Sphere Redox Reactions 269 5.6 The Bridging Ligand in Inner-Sphere Redox Reactions 270 5.7 Some Other Features of Redox Reactions 272 5.7.1 Mixed Outer- and Inner-Sphere Reactions 275 5.7.2 Two-Electron Transfer 276 5.7.3 Redox Catalyzed Substitution 276 5.7.4 Stereoselectivity 277 5.8 Intramolecular Electron Transfer 279 5.8.1 Between Two Metal Centers 279 5.8.2 Metal Complexes with Reducible Ligands 282 5.8.3 Induced Electron Transfer 284 5.9 Electron Transfer in Proteins 285 5.10 The Future 288 References 288 Problems 292

6 The Modification of Ligand Reactivity by Complex Formation 299

6.1 The Metal as a Collecting Point Reactant 299 6.1.1 Neighboring Group Effects 300 6.1.2 Template Chemistry 301 6.1.3 Collecting Reactant Molecules 303 6.2 Promotion of Reaction within the Metal-Bound Ligand 305 6.3 Hydrolysis of Coordinated Ligands 308 6.3.1 Carboxylate Esters: - C O z R -> - C O z H 308 6.3.2 Amides and Peptides: -CONHR -> - C 0 2 H 311 6.3.3 Nitriles: - C N -• - C O N H 2 313 6.3.4 Phosphate Esters: ROPOf" -> P0 4

3 - 314 6.3.5 Other Groups 317 6.4 The Acidity of Coordinated Ligands 317 6.4.1 Coordinated Water 318 6.4.2 Coordinated Ammonia and Amines 320 6.4.3 Other Coordinated Ligands 320 6.5 Electrophilic Substitution in Metal Complexes 322 6.6 Masking Effects 322 6.7 Disturbance of Reaction Stoichiometry 323 6.8 Molecular Strain Alterations 323 6.9 Function of the Ligand 324 6.10 Conclusions 325

XII Contents

References 325 Problems 328

7 Isomerism and Stereochemical Change 337

7.1 Conformational Isomerism 334 7.2 Configurational Isomerism 335 7.2.1 Planar, Tetrahedral 336 7.2.2 Planar, Square Pyramidal 336 7.2.3 Planar, Octahedral 337 7.2.4 Tetrahedral, Octahedral 338 7.3 Spin Equilibria in Octahedral Complexes 339 7.4 Linkage Isomerism 340 7.4.1 Rearrangement Studies 341 7.5 Geometrical and Optical Isomerism 343 7.6 Octahedral Complexes 343 7.6.1 Complexes of the Type M(AA)3 343 7.6.2 Complexes of the type M(AA,)3 348 7.6.3 Complexes of the Type M(L,)(L2) 351 7.6.4 Complexes of the Type M(AA)2X2 and M(AA)2XY 351 7.6.5 Isomeric Forms As Biological Probes 354 7.7 Four-Coordinated Complexes 355 7.7.1 Optical Isomerism in Tetrahedral Complexes 355 7.7.2 Geometrical Isomerism in Square Planar Complexes 356 7.8 Five-, Seven- and Eight-Coordinated Complexes 359 7.9 Inversion and Proton Exchange at Asymmetrie Nitrogen 360 References 365 Problems 368

8 A Survey of the Transition Elements 373

References 424 Problems 433

Problems - Hints to Solutions 443

Index 455