Double bond Effective atomic number

The effective atomic number (EAN) rule is useful for interpreting how ligands with more than one double bond are attached to the metal. Essentially, each double bond that is coordinated to the metal functions as an electron pair donor. Among the most interesting olefin complexes are those that also contain CO as ligands. Metal olefin complexes are frequently prepared from metal carbonyls that undergo substitution reactions. 

The cht ligand bonds in different ways in the complexes Ni(CO)3(cht), Fe(CO)3(cht), and Cr(CO)3(cht). Nickel has 28 electrons and gains six from the three CO ligands. Thus, it needs only two additional electrons from cht to obey the effective atomic number rule, and only one double bond in cht will be coordinated in Ni(CO)3(cht). 

A soluble dimer [Ru(cod)Cl2]2 (71), having the same formula as the insoluble polymer, has been isolated as a product from the reductive complexation of cod to ruthenium by Zn in EtOH/THF. The compound has been structurally characterized, and the Ru-Ru distance of 2.791 A in this compound is in the range found for Ru-Ru single bonds, whereas a double bond would be required to satisly the effective atomic number rule see Effective Atomic Number Rule) ... 

Although unsaturation is a rather uncommon phenomenon there are some examples of such compounds, for instance, the clusters (/t2-H)20s3(CO)io and Fe4(CO)ii (/t4-PR)2 which contain a total of 46 and 62 electrons respectively i.e. clusters with two electrons less than those required by the effective atomic number rule. As mentioned earlier in Sect. 2.3, in these cases metal-metal double bonds are supposed to be present. This class of clusters can add several different donor ligands to form electronic saturated species with 48 and 64 electrons respectively. 

Three donor carbonyl groups each contribute two electrons to the iron atom. Fach of the double bonds also contribute two electrons to iron, making a total of ten ligand electrons donated. Zero-valent iron possesses 26 electrons of its own, giving iron a total of 36 electrons. Thus the effective atomic number (FAN) of iron in cyclobutadieneirontricarbonyl equals the atomic... 

The conclusions on the mechanism of the double bond hydrogenation on metallic catalysts can be summarized as follows (1) with respect to structure effects on rate, all transition metals behave similarly (2) the reactivity of the unsaturated compounds is governed mostly by the number and size of the substituents on the carbon atoms of the double bond through their influence on adsorptivity (3) the electronic nature of the substituents plays a minor if any role. 

Cyclic olefins and diolefins form much more aerosol than 1-alkenes that have the same number of carbon atoms (for example, cyclohexene 1-hexene, and 1,7-octadiene 1-octene). The same effect of chain length and double-bond position is observed for diolefins (1,7-octadiene > 1,6-heptadiene > 1,5-hexadiene, and 1,7-octadiene 2,6-octadiene). Heavier unsaturated cyclic compounds, such as indene and terpenes, form even more aerosol. 

A number of examples involving nitrile oxide cycloadditions to cyclic cis-disubstituted olefinic dipolarophiles was presented in the first edition of this treatise, notably to cyclobutene, cyclopentene, and to 2,5-dihydrofuran derivatives (15). The more recent examples discussed here also show, that the selectivity of the cycloaddition to 1,2-cis-disubstituted cyclobutenes depends on the type of substituent group present (Table 6.8 Scheme 6.41). The differences found can be explained in terms of the nonplanarity (i. e., pyramidalization) of the double bond in the transition state (15) and steric effects. In the cycloaddition to cis-3,4-diacetyl-(197) and cis-3,4-dichlorocyclobutene (198), the syn-pyramidalization of the carbon atoms of the double bond and the more facile anti deformability of the olefinic hydrogens have been invoked to rationalize the anti selectivity observed. 

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