Valence electron rules fragments

When the number of metal atoms in a cluster increases, the geometries of the clusters become more complex, and some are often structurally better described in terms of capped or decapped polyhedra and condensed polyhedra. For example, the first and second clusters listed in Table 19.4.3 are a capped octahedron and a bicapped octahedron, respectively. Consequently, capping or decapping with a transition-metal fragment to a deltapolyhedral cluster leads to an increase or decrease in the cluster valence electron count of 12. When a transition-metal atom caps a triangular face of the cluster, it forms three M-M bonds with the vertex atoms, so according to the 18-electron rule, the cluster needs an additional 18 - 6 = 12 electrons. The parent octahedron of [Os6(CO)is]2- has g = 86, the monocapped octahedron Osy(CO)2i has g = 98, and the bicapped octahedron [Oss(CO)22]2- hasg = 110. 

It seems to be a sort of a rule in these Zintl phases that whenever there is a trigonal prismatic cation coordination then the silicon atoms form planar fragments and vice versa. It is not yet understood why this is so. To none of the Si-Si-bonds in SrMgSi2 a i-bonding contribution can be assigned neither in terms of bond distances nor by valence electron numbers and counting rules. 

Metallaboranes and metallacarboranes can be classified structurally by a procedure similar to that for boranes and their main group derivatives.30 In this scheme, the valence electron count of the metal-containing fragment is first determined and then compared with the requirements of the 18-electron rule. This fragment can then be considered equivalent to a BH fragment needing the same number of electrons to satisfy the octet rule. For example, a 15-electron fragment... 

Metals with d-valence atomic orbitals that extend in space can be expected to strongly interact with adsorbate orbitals. When they also have a high d-valence electron occupation, it will direct adsorption intermediates to specific surface coordination sites. For instance, on Pt [38] or Ru surfaces, the NHj [39] molecule will adsorb atop. The NH2 fragment will adsorb in twofold coordination and NH in threefold coordination [40]. On these metals, atomic oxygen will adsorb twofold and OH onefold [38]. Similar coordination rules are found for CHj, CH2, and CH. The C atom prefers coordination in a fivefold coordination as available on the (100) surface of a face-centered cubic (fee) metal, with four metal atom neighbors in the plane and one below. 

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