Transition metal complex solids

Table 4 Selected transition metal complex solids claimed to be metal composed of single component...

Chapter 10, in relation to classical bond orders and bond energies. In the meantime, Chapter 9 deals with the molecular orbital separation, conjugated systems, non-localizable tt molecular orbitals and resonance. In Chapter 11 a brief extension of molecular orbital theory is made to include three categories of systems fullerenes, transition metal complexes, solid aggregates (and band theory). Finally, Chapter 12 mainly illustrates the direct relations between orbitals and chemical reactivity and between orbitals and spectroscopy, with emphasis on electronic transitions and on spectral parameters in NMR spectroscopy. 

Possible alternatives to cross-linked polymer supports are soluble and colloidal polymers. They would require large scale ultrafiltration for industrial use. Although ultrafiltration is not yet economical for desalination of seawater, it might be for a separation of a more expensive product. One example is the catalytic partial hydrogenation of soybean oil (361 with soluble polymer-bound transition metal complexes. Solid inorganic supports such as silica gel and alumina are usually not subject to these physical attrition and filtration problems. 

The dominant features which control the stoichiometry of transition-metal complexes relate to the relative sizes of the metal ions and the ligands, rather than the niceties of electronic configuration. You will recall that the structures of simple ionic solids may be predicted with reasonable accuracy on the basis of radius-ratio rules in which the relative ionic sizes of the cations and anions in the lattice determine the structure adopted. Similar effects are important in determining coordination numbers in transition-metal compounds. In short, it is possible to pack more small ligands than large ligands about a metal ion of a given size. 

The finding of preparatively available iminoboranes RB = NR some years ago opened exciting new possibilities not only in B—N chemistry, but also in coordination chemistry. The first examples of iminoborane-transition-metal complexes have now been published. The structurally completely characterized t-BuB = NBu-t adds, like its alkyne analog, to the 03(00)5 fragment as a bridging ligand. When Co2(CO)g and t-BuB = NBu-t are dissolved in pentane at 0°C, warming to RT and evaporation of unreacted iminoborane yields (t-BuBNBu-t)Co2(CO)5 (86%) as a black solid, which can be recrystallized from ether-nitromethane (1 3) ... 

Table 19. Results of calorimetric and DSC measurements on spin-state transitions in solid transition-metal complexes... 

In preliminary studies various metal salts such as PdCl2 were attached to the ligands and the complexes characterized by NMR spectroscopy and X-ray crystallography. Typical examples are the palladium complexes (32)—(44) (Scheme 9).66 The catalytic profiles of these and other transition metal complexes prepared by this method still need to be defined. Moreover, solid-phase synthetic versions would be advantageous, and this may well be possible via the (protected) phenol function. 

While these spectroscopic and redox properties alone would be sufficient for direct use of transition metal complexes in solution-phase ECDs, polymeric systems based on coordination complex monomer units, which have potential use in all-solid-state systems, have also been investigated. 

The most important structural parameters of the transition metal complexes are summarized in Table XXV and Figs. 35-37 display the solid state structures of three selected complexes. 

Another type of DOUBLE ENDOR, called special TRIPLE , has been introduced by Dinse et al.90 to study proton hf interactions of free radicals in solution. In a special TRIPLE experiment two rf fields with frequencies vp + Av and vp — Av are swept simultaneously. For systems with Tln < T,i this leads to a considerable signal-to-noise improvement and to TRIPLE line intensities which are directly proportional to the number of nuclei with the same hf coupling constant. It should be remembered, however, that in transition metal complexes in the solid state the resonance frequencies are not, in general, symmetrically placed about the free proton frequency vp and that the condition Tln < Tj,i is not always fulfilled. 

The ENDOR techniques, of course, are not confined to studies of transition metal complexes. A fast growing interest on electron nuclear double and multiple resonance experiments is also noticed in other fields of natural sciences, such as radical, radiation and polymer chemistry, solid state physics, biophysics and mineralogy. 

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