Rules eighteen-electron rule

In many respects, the successes of this model are remarkable. Iron(O) possesses a total of eight electrons in its valence shell. To satisfy the eighteen-electron rule, five two-electron donors are needed, and compounds such as [Fe(CO)5] are formed. These molecules also obey simple VSEPR precepts, and [Fe(CO)s] adopts a trigonal bipyramidal geometry. Conversely, the use of two five-electron donor ligands such as the strong r-acceptor cyclopentadienyl, Cp, gives the well-known compound ferrocene (9.3). 

In a similar vein, we observe nickel(O), possessing ten electrons in its valence shell, to require four carbonyl ligands to satisfy the eighteen electron rule and form [Ni(CO)4l, whilst chromium(O), with six electrons in its valence shell forms [Cr(CO)6]. These latter compounds are tetrahedral and octahedral respectively. 

When using the eighteen electron rule, we need to remember that square-planar complexes of centers are associated with a 16 electron configuration in the valence shell. If each ligand in a square-planar complex of a metal ion is a two-electron donor, the 16 electron configuration is a natural consequence. The interconversion of 16-electron and 18-electron complexes is the basis for the mode of action of many organometallic catalysts. One of the key steps is the reaction of a 16 electron complex (which is coordinatively unsaturated) with a two electron donor substrate to give an 18-electron complex. 

Models, paper, A40-A43 Molar susceptibility, 461 Molecular orbital (MO) theory, 139. 153-182.413-433 and ir bonding. 420-425 and eighteen-electron rule. 624-625... 

Eighteen Electron Rule rule noting that coordination complexes with eighteen electrons are stable electrons from both metal species and ligand donor species are counted... 

For case (1) complexes, examples of which include many first transition series compounds (Table 8.8.3), the lt2g orbitals are essentially nonbonding and A0 is small. In other words, the 2eg orbitals are only slightly antibonding and they may be occupied without much energy cost. Hence, there is little or no restriction on the number of d electrons and the eighteen-electron rule has no influence on these complexes. 

Table 8.3 3. Three types of complexes in relation to the eighteen-electron rule...

Eighteen-electron rule — An electron-counting rule to which an overwhelming majority of stable diamagnetic transition metal complexes adhere. The number of non-bonded electrons at the metal plus the number of electrons in the metal-ligand bonds should be 18. The 18-electron rule in transition metal chemistry is a full analogue of the Lewis octet rule . 

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