Covalent bonds, between transition

Only covalent bonds between Li and Na and transition metals are known, there being none with the lower group-IA metals or with inner transition elements. Bonding is inferred from metal-metal distances as well as calculations. ... 

In the last 20 years a great deal of effort has been focused towards the immobilization of chiral catalysts [2] and disparate results have been obtained. In order to ensure the retention of the valuable chiral hgand, the most commonly used immobihzation method has been the creation of a covalent bond between the ligand and the support, which is usually a solid, hi many cases this strategy requires additional functionalization of the chiral hgand, and this change - together with the presence of the very bulky support - may produce unpredictable effects on the conformational preferences of the catalytic complex. This in turn affects the transition-state structures and thus the enantioselectivity of the process. 

Since the first complex having a covalent bond between silicon and iron, (CH3)jSiFe(CO)2( j -C5H5), was prepared in 1956, the chemistry, of compounds of group IV3 metals bonded to transition metals has expanded very rapidly. Up to now, a few hundred compounds of this type are known, and some excellent reviews have been published dealing with their synthesis and properties " . ... 

Coordination bonds, between transition metal atoms and ligating atoms, 27 72-76 Coordination chemistry, of oligopyridines, see Oligopyridines, coordination chemistry Coordination complexes covalent binding in, 5 13-15 fluorocyclic derivatives, 15 346-368 Coordination compounds... 

The term metallomesogen is often used when a (transition) metal is complexed (by coordinative bonds between the crown heteroatoms and the metal center) by a crown ether. These compounds were discussed in the previous sections as complexed crown ethers (instead of metallomesogens ) together with their uncomplexed precursors. In this section, mesogens with a covalent bond between... 

For efficient regeneration, the catalyst should form only labile intermediates with the substrate. This concept can be realized using transition metal complexes because metal-ligand bonds are generally weaker than covalent bonds. The transition metals often exist in different oxidation states with only moderate differences in their oxidation potentials, thus offering the possibility of switching reversibly between the different oxidation states by redox reactions. 

It has struck many workers in the field that the transition metals and near-transition metals are the best catalysts for many gas reactions (78), and it was suggested (79) that probably the covalent bonds between the chemisorbed atoms and the metal were formed by sharing of an electron of the atom with a d electron of the metal. It is, indeed, not improbable that d electrons of the metals play an important role in these chemisorption phenomena. It is known from their role in the bond strengths of complex chemical compounds that pairing of other electrons with d electrons leads to strong bonds. An incorporation of d elec-... 

The reverse kinetic isotope effect (82Br reacts faster than 77Br ) of about 0.5-5% would imply that the covalent bond between aromatic ring carbon and bromine is practically formed in the transition state of the bromodestannylation reaction. 

In the case of covalent bonding in transition-metal complexes, the qualitative correlation between the observed fine structures and the calculated... 

The rate and extent of the dissociation are dependent upon the ligands of the transition metal compound (see Fig. 11 and Table II). This can be understood only if there is a covalent bond between the N atoms and the metal.11... 

In this case, ionization means the transition of a covalent bond between the atoms of a certain compound in solution to an ionic bond. 

The electronic state calculations of transition metal clusters have been carried out to study the basic electronic properties of these elements by the use of DV-Xa molecular orbital method. It is found that the covalent bonding between neighboring atoms, namely the short range chemical interaction is very important to determine the valence band structure of transition element. The spin polarization in the transition metal cluster has been investigated and the mechanism of the magnetic interaction between the atomic spins has been interpreted by means of the spin polarized molecular orbital description. For heavy elements like 5d transition metals, the relativistic effects are found to be very important even in the valence electronic state. 

The resonance between the covalent and ionic bond structures of a molecule produces, by the superposition of the electron clouds of the ionic bond and of the covalent bond, a transitional electron cloud. This is discussed below in terms of wave mechanics. The electron cloud of the bond, however, will of course be continuous and the splitting into component parts, which this method of treatment has incurred, is the direct result of the attempt to describe a complex chemical bond in terms of two simpler types of bonds which may be represented by classical structural symbols. 

---