Higher Electron Counts

There are examples with even more electrons around the central atom, such as IF7 (14 electrons), [TaFg] (16 electrons), and [XeFg] (18 electrons). There are rarely more than 18 electrons (2 for s, 6 for p, and 10 for d orbitals) around a single atom in the top half of the periodic table, and crowding of the outer atoms usually keeps the number below this, even for much heavier atoms that have /orbitals energetically available. 

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Transformations to the [ZnS] structure type considerably lessen the strength of these unfavorable Fe-Fe interactions. In the earlier members of the MN series (ScN, TiN, VN,. ..), the Fermi level lies low in the M-M COHP curve, not yet sampling so many antibonding states. For these electron counts, the sodium chloride structure, with much shorter M-M contacts, is favored. For higher electron counts, switching to the [ZnS] structure type reduces the antibonding M-M interactions by a significant amount. It also can be shown, numerically, that the Fe-N bond for the tetrahedral coordination is more covalent than for the octahedral coordination (see also below). 

On the C- and N-covered surfaces, the disruption is overcompensated by the formation of strong adsorbate-Ni bonds and by new Ni-Ni surface bonds resulting from the clock reconstruction. When O is forced into a coplanar site, however, both the higher electron count and increased electronegativity of the O atoms lead to severe disruption of the surface bonding and the formation of weak Ni-0 bonds. When O atoms sit above the surface, they form more polar Ni-0 bonds, contribute less electron density to the Ni surface bands, and cause less disruption to the Ni-Ni surface bonds. These results suggest that similar to the organometallic clusters, the... 

The many higher boranes such as B5H9 and BgH 2 are similarly electron deficient and cannot be described by a single Lewis structure. They can often be described in terms of a combination of two- and three-center bonds. Alternatively, their structures can be rationalized by electron-counting schemes such as those proposed by Wade. Analysis of the electron density of these molecules by the AIM method shows that there are bond paths between all adjacent pairs of atoms. So from the point of view of the AIM theory there are bonds between each adjacent pair of atoms, but these cannot all be regarded as Lewis two-center, two-electron bonds as is the case in B2H6. 

The components of polar intermetallics generally include an active metal from the group 1 or 2 or the rare-earth series plus, sometimes, a late-transition metal, and a metal from the p-block. Because of the presence of an electron-poorer late transition metal, polar intermetallics generally have lower e/a values (about 2.0-4.0) than classic Zintl phases (>4.0) [45], Note these values are traditionally calculated over only electronegative atoms [45], in contrast to those of Hume-Rothery phases (<2.0) [45] and QC/ACs (2.0 0.3) [25], for which electron counts are considered to be distributed over all atoms. The former two higher values are decreased to about 1.5-2.5 and >2.5, respectively, when counted over all atoms (but with omission of any dw shells). For comparison purposes, Fig. 3 sketches the distribution of all these intermetallic phases according to e/a counted over all atoms, as we will use hereafter. 

Ruthenium and osmium carbene complexes possess metal centers that are formally in the +2 oxidation state, have an electron count of 16 and are penta-coordinated. Ruthenium complexes exhibit a higher catalytic activity when an imidazole carbene ligand is coordinated to the ruthenium metal center (21). 

Higher nuclearity systems such as the cubic M8 and icosahedral M12 rEx undergo reduction/oxidation reactions owing to their open shell nature and resulting flexibility in electron count [Eqs. (261)-(263)].330331 The reduction... 

Similar reaction conditions for manganese, iron and cobalt complexes lead to more stable arene systems, presumably due in part to a higher valence electron count for these metal centres. Reduction of [3,5- Pr2-2,6-Trip2C6HMX]ra (M = Mn, X = I, n= 1 M = Fe, X = C1, n= 1 M = Co, X = C1, n — 2) gives rise to the Mn(I) inverted sandwich complex [3,5- Pr2-... 

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