Metals valence electrons

Trunsition-MetnlHydrides, Tiansition-metal hydiides, ie, inteistitial metal hydrides, have metalhc properties, conduct electricity, and ate less dense than the parent metal. Metal valence electrons are involved in both the hydrogen and metal bonds. Compositions can vary within limits and stoichiometry may not always be a simple numerical proportion. These hydrides are much harder and more brittie than the parent metal, and most have catalytic activity. 

For many species the effective atomic number (FAN) or 18- electron rule is helpful. Low spin transition-metal complexes having the FAN of the next noble gas (Table 5), which have 18 valence electrons, are usually inert, and normally react by dissociation. Fach normal donor is considered to contribute two electrons the remainder are metal valence electrons. Sixteen-electron complexes are often inert, if these are low spin and square-planar, but can undergo associative substitution and oxidative-addition reactions. 

In tier (1) of the diagram (for the electronic structure of iron(III)), only the total energy of the five metal valence electrons in the potential of the nucleus is considered. Electron-electron repulsion in tier (2) yields the free-ion terms (Russel-Saunders terms) that are usually labeled by term ° symbols (The numbers given in brackets at the energy states indicate the spin- and orbital-multiplicities of these states.)... 

In metals, valence electrons are conduction electrons, so they are free to move along the solid. On the contrary, valence electrons in insulators are located around fixed sites for instance, in an ionic solid they are bound to specific ions. Semiconductors can be regarded as an intermediate case between metals and insulators valence electrons can be of both types, free or bound. 

Framework dectrcxK (F) apn] tbc number of metal valence electrons (M) plus the number of electrons donated by kgands (L) minus twelve (F = M + L — 12). 

The probability of C-O bond scission within CO or CH, 0 is probably related to the adsorption geometry of these species. Whereas CO adsorbs perpendicularly on Pd(l 1 1), the C-O bond in CHO (and CH2O) species is tilted with respect to the palladium substrate (378). The tilted arrangement may allow for a better overlap between the CH O orbitals and the metal valence electron density, thus weakening the C-O bond. 

For binary metal carbonyl compounds, the 18-electron mle is a very useful concept. Stable metal complexes will be formed when the metal has 18 electrons in its valence shell (metal valence electrons -H 2 electrons from each CO ligand). Since Tc(0) has 7 valence electrons, the neutral monomeric species Tc(CO) cannot be stable, but ions like [Tc(CO)6]" or [Tc(CO)5] attain a total of 18 electrons. In the neutral molecule, it will dimerize to Tc2(CO)io in order to obey the 18-electron rule. The formation of a Tc-Tc bond adds an electron on each Tc atom. This 18-electron mle is quite useful to predict the stmctures of the metal binary carbonyl compounds. 

When an olefin coordinates to a transition metal, the olefin n bond donates electrons to an empty metal orbital (donor bond) and the olefin n orbital accepts metal valence electrons from a filled metal atomic orbital (back-bond). Two molecular orbitals can describe the conventional representation of the metal-olefin bond as originally proposed by Dewar and modified by Chatt 7). 

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