Transition metal contractions

Basch Available for Sc through Cu(15.v8/ 5d) with several contraction schemes. The transition metal set yields slightly higher energy than Wachters set. 

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

Kinetic observations for decomposition of some representative transition metal sulphides are summarized in Table 13. Several instances of an advancing interface [contracting volume, eqn. (7), n = 3] rate process have been identified and the rate may be diminished by the presence of sulphur. Diffusion control is, however, believed to be important in the reactions of two lower sulphides (Ni0.9sS. [687] and Cu1-8S [688]). These solids have attracted particular interest since both are commercially valuable ores and pyrolysis constitutes one possible initial step in metal extraction. 

The covalency contraction parameter, Rv, which measures the volume of a transition metal compound MmX relative to the volume of MgmXn, is proportional to the electronegativity of X and thus decreases as the covalence of the M—X bond increases. 

There is an approximately linear relationship between Rv and spin transfer coefficients determined from electron and nuclear magnetic resonance and neutron diffraction, i.e., a contraction of the unit cell accompanies the transfer of spin from transition metal to the ligands. 

One of the consequences of the lanthanide contraction is that some of the +3 lanthanide ions are very similar in size to some of the similarly charged ions of the second-row transition metals. For example, the radius of Y3+ is about 88 pm, which is approximately the same as the radius of Ho3+ or Er3 +. As shown in Figure 11.8, the heats of hydration of the +3 ions show clear indication of the effect of the lanthanide contraction. 

Quantum chemists have developed considerable experience over the years in inventing new molecules by quantum chemical methods, which in some cases have been subsequently characterized by experimentalists (see, for example, Refs. 3 and 4). The general philosophy is to explore the Periodic Table and to attempt to understand the analogies between the behavior of different elements. It is known that for first row atoms chemical bonding usually follows the octet rule. In transition metals, this rule is replaced by the 18-electron rule. Upon going to lanthanides and actinides, the valence f shells are expected to play a role. In lanthanide chemistry, the 4f shell is contracted and usually does not directly participate in the chemical bonding. In actinide chemistry, on the other hand, the 5f shell is more diffuse and participates actively in the bonding. 

Undoubtedly one of the most used LFER in transition metal chemistry involves electron transfer rate constants and associated equilibrium constants in outer sphere redox reactions. These are an unusual class of reactions in chemistry since bonds are only stretched or contracted in the formation of the activated complex. They therefore lend themselves well to theoretical treatment. We shall have more to say about these reactions in Chap. 5. It is sufficient here to state the simple form of the LFER with an example (Fig. 2.8). For the reaction... 

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