Other Ligand Systems

A number of zinc halide complexes, including bridging halides, have already been mentioned in the context of the other ligands systems discussed in this Chapter. Examples will be presented of catalytic systems and solution speciation, particularly in the presence of other coordinating ligands. There are now also a few zinc fluoride species that have been well characterized. 

Heterocyclic nitrogen donors and their adducts with zinc chloride have been studied.623,624 A large number of other ligand systems have also been characterized, for example, zinc halide adducts of 2,2-dimethylpropane-1,3-diamine and hexamethylphosphoramide have been studied.625,626 The formation of mixed ligand complexes with chloride and substituted pyridines has been studied.627 The zinc tris(pyridyl) chloride anion has also been structurally characterized.628 Manganese(II) ions have been used to probe the stereochemistry in reactions of zinc halides with pyrazine.629... 

The zinc complex of a tripodal N2S20 pentadentate ligand undergoes amide alcoholysis of a coordinated amide group.893 Examples of amide hydrolysis are known for other ligand systems.894... 

The distribution of the d electrons between these various orbitals is determined by the differences in energy level between them. The energy difference is directly proportional to the electric field strength of the ligand systems taking part in complex formation. Since the transition-metal ions do not exist in isolation, they must be evaluated in association with counter-ions or solvents such as water or other ligand systems present. 

One recent, important development has been (136) the electrolytic preparation from [Ni"(CN)4]2- of [Ni CN H ]-, which has a half-life of 11 minutes in acidic solution at 25°C and serves as a precursor for the incorporation of nickel(III) in other ligand system. The complex is tetragonally elongated from its EPR spectrum and can add other anions axially. The bis aquo species has an absorption... 

Among the O-containing heterocyclic ligands, tetrahydrofuran (THF) 376 is the subject of permanent interest [13]. As a result of its interaction (3.18) with metal salts, the molecular solvent complexes of the type 298 (Sec. 2.2.4.4) and 577 are formed, and can be used as precursors to obtain coordination compounds with other ligand systems ... 

There are many other heterocycles containing not only oxygen but N and/or S. Crown-type groupings can also be attached to other ligand systems to give different types of binding sites, and in lariat ethers, which are crowns with side chains. 

Further attempts to replace alkoxy groups with other ligand systems, like cyclopentadienyl, in order to reduce the Lewis acidity of the metal atom have been unsuccessful."... 

Complexes of the 4 type catalyze the hydroboration of various olefins with catecholborane at ambient temperature [173], The proposed mechanism of the hydroboration reaction - although not within the scope of this book - parallels that of the hydrogenation and hydrosilylation reactions. The architecture of both olefins (terminal > terminal disubstituted > internal disubstituted > trisubstituted) and organolanthanides (TOF(La) 10 TOF(Sm) TOF(5) = 4 TOF(4) affects the rate of hydroboration, which for 4(La CH(SiMc3)2) and 1-hexene is TOF = 200 h , for example. The observed high regioselectivities are exclusively anti-Markovnikov. For smaller metal centers (Y, Zr, Ti) and other ligand systems (bis(cyclopentadienyl), bis(benzamidinato)) inactivation of the catalyst by catecholborane or Lewis base-metal complex induced disproportionation of catecholborane appeared to compete effectively with the catalytic conversion [174]. 

A comprehensive review of early transition metal hydrides including those of Nb and Ta is available.447 Hydrido-metallocene systems are discussed in detail there. The present account concentrates on other ligand systems, although mono- or polynuclear metallocene systems with interesting structural features are covered. Polyhydride systems have been reviewed from a theoretical standpoint.448... 

Obviously, basic research is needed to provide a more comprehensive insight into the dependence of enantioselectivities on ligand electronics which is only poorly understood compared to steric effects. Nevertheless the concept of electronic asymmetry to enhance enantio selectivity in other ligand systems is very appealing and should be considered to a greater extent for future ligand optimization studies. 

In the meantime, many other catalytic systems have been described for the hydroformylation of fatty compounds, especially by Fell s group [49-51], They used water-soluble catalysts, for instance consisting of Rh4(CO)12 and surface-active sul-fobetaine derivatives of tris(2-pyridyl)phosphine [49]. Other ligand systems are the sodium salt of TPPTS in combination with detergents [50] and the lithium salt of triphenylphosphinemonosulfonic acidTPPMS [51]. 

The reported experimental data, DFT-derived structural electronic and energetic information, and mechanistic interpretations, lead to a consistent picture, which is in fine with observations with other ligand systems, although there is some disagreement in terms of possible interpretations. However, there are a number of open questions (e.g., regarding the spin state of the ferryl species, which for both the tetra- and pentadentate ligand-based systems is predicted by DFT to be close to the spin cross-over limit) (175). Therefore, some of the current interpretations might be revised in the future. 

Preliminary experiments and ideas based on published work with other ligand systems exist, but are too incomplete to be reported here in detail. These include complexes and their properties with novel bispidine hgands (see Section... 

Many of these new developments are related to the very specific properties of bispidine ligands, others might also have been observed with other ligand systems. New bispidine ligands (Section 11) whose coordination chemistry has not yet been developed in detail, novel bispidine chromophores (Section... 

Those transition metals that can react with dinitrogen do so via a concerted donor-acceptor interaction. Whether such an interaction is possible for a given system depends on the electronic configuration of the metal and on the influence of other ligands. Systems in which nitrogen reduction is promoted by transition metal compounds may be classified according to whether the reaction is carried out in a protic or an aprotic medium. 

The s)mthetic methodology can be readily extended to other ligand systems, as evidenced by the isolation of an analogous tetranuclear cluster... 

Ethylene polymerizations for rapid screening purposes have been described above. These polymerizations can be catalyzed by a-diimine-derived catalysts or by a host of other ligand systems. This section will focus on polymerizations run under more controlled conditions utilizing isolated complexes of Ni and Pd bearing a-diimine ligands because these are the most thoroughly described in the literature at present. These catalysts are among the most active, and they display features that will be used as reference standards in subsequent discussions. 

The generation of dendrimeric coordination polymers has also been an area of significant activity. Imaginative routes to novel Ru- or Pt-polypyridyl systems (eg, 65) (197) have been reported and many other ligand systems have been exploited (117,118,198-201). These materials are of interest with respect to their photophysical and electrochemical properties and possibly their catalytic behavior. 

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