Variety in Coordination Modes of Ligands in Metal Complexes, Softcover reprint of the original 1st ed. 1988 Coll. Inorganic Chemistry Concepts, Vol. 11

Metal complexes play important roles as catalysts or other participants in synthetic and biological reactions. Substrates and sometimes attacking reagents also are activated through coordination with metal atoms or ions. In these events the natures not only of the central metals but also of ancillary ligands exert important influences on the stability and reactivity of the coordinated substrates. A ligand in general can adopt various coordination modes depending on its chemical environment, thus functioning as a probe. The number of coordination modes increases with increasing complexity of the ligand. In this book it is shown that even the simplest mono- and diatomic ligands such as H, CO, and N2 exhibit a variety of coordination modes, which are related to their reactions. The thiocyanate anion is taken up as a representative of the triatomic ambidentate ligands, and factors influencing the preferences for N- und S-bonding are summarized. Coordination chemistry of ß-dicarbonyl compounds is a highlight of this book. Acetylacetone, one of the most familiar Werner ligands, is shown to favor -carbon and n-allylic bonding in many instances. Its versatile behaviour in changing coordination modes is revealed.

1 Introduction.- 1.1 Classification of Ligands.- 1.2 Linkage Isomerism.- 2 Monoatomic Ligands.- 2.1 Coordination Modes for the Hydride Ligand.- 2.1.1 Terminal Hydride Ligands.- 2.1.1.1 Preparative Methods.- 2.1.1.2 Characterization.- 2.1.1.3 Tricapped Trigonal Prism Structure.- 2.1.1.4 Octahedral Structure.- 2.1.1.5 Pentagonal-Bipyramid Structure.- 2.1.1.6 Trans Influence of the Hydride Ligand.- 2.1.2 Unsupported M—H—M Linkage.- 2.1.3 Dinuclear M(µ-H)nM and Mixed-bridged Systems.- 2.1.3.1 Preparative Methods.- 2.1.3.2 Characterization.- 2.1.3.3 M(µ -H)2M Systems.- 2.1.3.4 M(µ-H)3M Systems.- 2.1.3.5 M(µ-H)4M Systems.- 2.1.3.6 M(µ-H)(µ-X)n Systems.- 2.1.4 Edge-bridging (µ2) and Face-bridging (µ3) Hydride Ligands in Metal Clusters.- 2.1.5 Systems with an Interstitial Hydrogen Atom.- 2.1.6 Metal Clusters Including an Interstitial Light Atom other than Hydrogen.- 2.2 Chemical Reactions of Hydride Ligands.- 2.2.1 Reactions with Acids.- 2.2.2 Reactions with Bases.- 2.2.3 Reactions with Halogens and Organic Halides.- 2.2.4 Intramolecular Migratory Insertion.- 2.2.5 Reductive Elimination Reactions.- 2.3 Role of Rhodium Hydride Complexes in the Catalytic Hydrogenation of Olefins.- 2.3.1 Homogeneous Activation of Dihydrogen by Metal Complexes in Solution.- 2.3.1.1 Three Modes of H2 Activation.- 2.3.1.2 Dihydride Formation by Oxidative Addition of H2.- 2.3.1.3 Dihydrogen Complex.- 2.3.2 Reaction Pathway for the Hydrogenation of Olefins Catalyzed by RhCl(PPh3)3.- 2.3.3 Hydrogenation of Olefins Catalyzed by Cationic Rh(I) Complexes, [Rh(PP)(S)2]+.- 2.3.3.1 Formation of Cationic Rh (I) Complexes.- 2.3.3.2 Mechanism of Olefin Hydrogenation.- 2.3.3.3 Asymmetric Hydrogenation.- 3 Diatomic Ligands.- 3.1 Coordination Modes for Carbon Monoxide.- 3.1.1 Terminal Bonding and µ(C) Bridging.- 3.1.2 Mx—CO—M? Bridging.- 3.1.2.1 M—CO—M? Systems.- 3.1.2.2 M2—CO—M? Systems.- 3.1.2.3 M3—CO—M? Systems.- 3.1.3 CO Bridge Involving the ?2 (side-on) Linkage.- 3.1.3.1 µ(?1,?2) Mode.- 3.1.3.2 µ3(?1,2?2) Mode.- 3.1.3.3 µ4(3?1,?2) Mode.- 3.2 CO Cleavage and Reduction.- 3.3 Coordination Modes for Dinitrogen.- 3.3.1 End-on Unidentate Coordination.- 3.3.2 End-on Bridging.- 3.3.3 End-on µ3-Bridging.- 3.3.4 Side-on Coordination.- 3.3.5 Side-on Bridging.- 3.3.6 End-on: side-on µ3-Bridging.- 3.4 Protonation of the Coordinated Dinitrogen.- 4 Triatomic Ligands.- 4.1 Coordination Modes for the Thiocyanate Ion.- 4.1.1 One-end Bridging.- 4.1.1.1 µ(N) Bridging.- 4.1.1.2 µ(S) Bridging.- 4.1.1.3 µ3 (S) Bridging.- 4.1.2 End-to-end Bridging.- 4.1.2.1 Single µ(S,N) Bridging.- 4.1.2.2 Double µ(S,N) Bridging.- 4.1.2.3 µ3(2S,N) Bridging.- 4.1.2.4 µ4(3S,N) Bridging.- 4.2 Infrared Spectroscopy for Determining the Coordination Modes of the Thiocyanate Ligand.- 4.3 Factors Influencing the Relative Stabilities of the N-bonded and S-bonded Thiocyanate Complexes.- 4.3.1 Principle of Hard and Soft Acids and Bases (HSAB).- 4.3.2 Electronic Effects of Ancillary Ligands.- 4.3.3 Steric Effects of Ancillary Ligands.- 4.3.4 Solvent and Counterion Effects.- 5 Polyatomic Ligands: ?-Dicarbonyl Compounds.- 5.1 Three Coordination Modes for Neutral Molecules.- 5.1.1 Keto-enol Tautomerism and Structures of Enol Molecules.- 5.1.2 Metal Complexes of Neutral Molecules.- 5.1.3 O,O?-Chelation of the Keto Molecules.- 5.1.4 O-Unidentate Coordination of Enol.- 5.1.5 ?2(C,C?) Coordination of Enol.- 5.2 Coordination Modes for Monoanions.- 5.2.1 O,O?-Chelation and Bridging.- 5.2.2 Central Carbon Bonding and C,O,O?-Bridging.- 5.2.3 Outer-Sphere Coordination.- 5.2.4 O-Unidentate Linkage.- 5.2.5 ?Allylic Coordination.- 5.2.6 Terminal Carbon Bonding.- 5.3 Coordination Modes for Dianions.- 5.3.1 Central Carbon Bonding.- 5.3.2 Chelation through Terminal Carbons.- 5.3.3 Dienediolate Chelation.- 5.3.4 C,O,O?-Bridging.- 5.3.5 ?-Allylic Coordination.- 5.3.6 C,O-Chelation.- 5.3.7 ?3,O,O?-Bridging.- 5.4 ?3,C,O-Bridging of the Acetylacetonate Trianion.- 5.5 Concluding Remarks.- 6 Abbreviations.- 7 References.