Coordination chemistry chelate effect

Phosphacyclic diphosphines (73a) and (73b) with wide natural bite angles were synthesized and the effect of the phosphacyclic moieties on the coordination chemistry in the [(diphosphine) Rh(CO)2H] complexes was studied. Both NMR and IR spectroscopy showed that the phosphacyclic xantphos ligands exhibit an enhanced preference for diequatorial chelation compared to the diphenylphosphino-substituted parent compound. In the hydroformylation of 1-octene the introduction of the phosphacyclic moieties leads to higher reaction rates. The dibenzophospholyl- and phenoxaphosphino-substituted xantphos ligands exhibit a high activity and selectivity in the hydroformylation of trans-2- and 4-octene to linear nonanal. CO dissociation rates from the... 

At this stage, very little is known about the stabilities of various isomers in coordination chemistry and obviously much less about the reasons for them. It is not clear in many instances whether the chelate conformations have any great influence on isomer stability, or how the influence is exerted in the few instances where it has been established. It is conceivable that the conformation exerts its influence through the entropy term rather than as a AH effect. On the other hand, the conformers may have quite different effects on the solvation energies of the complex isomers, and the stability difference may be reflected through this term. 

Chapter 12 Coordination Chemistry Structure 472 Coordination Number 472 Coordination Number 2 473 Coordination Number 3 474 Coordination Number 4 474 Coordination Number 5 479 Coordination Number 6 488 Coordination Number 7 503 Coordination Number 8 507 Higher Coordination Numbers 509 Generalizations about Coordination Numbers 511 Linkage Isomerism 513 Other Types of Isomensm 52] The Chelate Effect 522... 

The chelate effect is well known in coordination chemistry and relates to the observation that metal complexes of bidentate ligands (such as 1,2-diaminoethane, en) are significantly more stable than closely... 

Other theoretical considerations for synthetic coordination chemistry (isolobal and isoelectronic analogies, chelate, cis and trans effects, factors affecting the acid/ base properties of coordination compounds, bond theories, etc.) are covered in detail in excellent recent monographs [3,34b,104,106] and, for this reason, are not presented in this book. 

Many of the complexes prepared through subcomponent self-assembly underwent clean substitution chemistry, which may operate both at covalent and coordinative levels. As discussed below, driving forces for such substitutions included the relief of steric encumbrance, the substitution of an electron-poor subcomponent for an electron-rich one, the use of pK i differentials, and the chelate effect. 

The importance of the chelate effect combined with the construction of multidentate ligands is well known in lanthanide chemistry. This is expressed in the rich coordination chemistry of / -diketonates [88] or complexes with Schiff bases [89] and macrocyclic polyethers [90] where lanthanide cations achieve steric saturation by high coordination numbers. Entrapment of the cation in a macrocyclic cavity results in greater complex stability. However, simply functionalized ligands such as ethanolamines can also supply a suitable ligand sphere [91-93],... 

The chelating effect of the surface appears to be the driving force for the formation of the surface cis-octahedral complex. Equation 17, which is the sum of eqs 15 and 16, is accompanied by an entropy increase with the release of water molecules and the disappearance of charged species leading to a subsequent disordering of nearby solvent molecules. This aspect is well documented in solution coordination chemistry [68, 69]. 

Central to the stability and chemistry of complexes formed by mixed donor ligands are two key concepts of coordination chemistry. The first is the chelate effect, which applies to all polydentate ligands, and reflects the increase in stability of a type of complex as monodentate donor molecnles are replaced by polydentates with donors linked by chelate rings. The hard-soft acid-base theory is particnlarly relevant to mixed donor ligands where donors of distinctly different character may bind to a central metal ion. The like prefers like concept means hard nonpolarizable donor atoms (N and O, for example) bond preferentially to hard nonpolarizable metal... 

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