Transition elements complexes

The X-ray crystal structures of many of these complexes have now been determined representative examples are. shown in Fig. 4.11 from which it is clear that, at least for the larger cations, coordinative saturation and bond rhrectionality are far less significant factors than in many transition element complexes. Further interest in these ligands stems from their use in biochemical modelling since they sometimes mimic the behaviour of naturally occurring, neutral, macrocydic antibiotics such as valinomycin, monactin, nonactin, nigericin... 

For a threefold degenerate MO occupied by three electrons, a configuration of a triradical having a quartet multiplicity is required hy Hund s rule. In organic chemistry, systems of this type occur rarely but with transition element complexes, quartets, quintets, and sextets, they are common. 

Theory has not yet reached a stage where energies or even the ordering of states can be calculated reliably for transition element complexes. Therefore, complementary contributions and cooperation between experiment and theory are needed in the unravelling of energy states and the description of bonding for these important systems. 

Finally, many complexes that participate in homogeneous catalytic reactions have electron counts less than 16. This is especially true for high-oxidation-state early-transition-metal complexes such as (C2H5)TiCl3, Ti(OPr )4, etc. Cat-alytically active, late-transition-element complexes with electron counts less than sixteen are also known. An important example is RhCl(PPh3)2, a 14-electron complex that plays a crucial role in homogeneous hydrogenation reactions (see Section 7.3.1). 

Photochemistry of Metal-Metal-Bonded Transition Element Complexes... 

Whereas each of these above-described new compositions offered significant advantages over existing CVD sonrces, each also snffered some limitation that prohibited widespread exploration in the preparation of thin films (Table 15). In a radical departure from more traditional rontes, the preparation of a new class of ligands, namely the jS-diketo ethers, and their ntilization in the formation of novel families of both main gronp and transition element complexes, has been investigated. ... 

Octahedral complexes of Cr (d ), and of the low-spin d species Co , Rh , Ii, and Pt, are numerous and well studied. They are kinetically inert, that is, they undergo ligand-substitution reactions relatively slowly, and the bulk of kinetic and mechanistic stndies on transition element complexes have been performed... 

The types of isomerism possible in transition element complexes... 

When considering the structures of coordination compounds it is worth noting that transition element complexes are usually formed from reactions between their salts and Bronsted bases in solution. However, the structures of the compounds formed are usually determined in the solid state using samples crystallized from solution. While it may usually be assumed that the solid state structures are similar to the solution structures, this may not always be so. and some complexes may adopt different structures in solution and the solid state. 

After reading this chapter you should have gained an understanding of the origins of the electronic spectra and magnetism of transition element complexes and a knowledge of ... 

Electronic Spectra and Magnetism of Transition Element Complexes 131... 

For the higher numbers of valence-shell pairs, 7, 8 and 9, there are only a few non-transition element complexes known. In the case of IF7 the structure appears to be a pentagonal bipyramid, contrary to the entry in Table 4-1. However, with these higher numbers, the predictions of preferred arrangements necessarily become less certain because the repulsive energy of the set of electron pairs does not have a pronounced minimum for any one configuration and atom-atom interactions assume greater importance. 

The force constant analysis of 1 indicates that the H2 ligand here is further activated toward OA than may have been previously thought. It has been a paradox that the dHH in 1 or any of the group 6 complexes in Table 8.1 (0.85-0.89 A, solid state NMR) are not as stretched as some of those found in later transition element complexes (1.0-1.5 A), yet the H-H bond in 1 undergoes equilibrium cleavage in solution. Thus the observed dm may not always reflect the degree of readiness to break, i.e., a very late transition state may exist. At the other end of the spectrum, for W(CO)5(H2) and other complexes with extremely weakly bound H2, the T-shaped entity pictured above with one internal coordinate, the H-H stretch, may be a more appropriate model for vibrational analysis. However, it is difficult to determine which analysis should be applied because some minimal BD and incipient... 

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