Tetrahedral metal clusters, valence electron

In both [Fe4X4(N0)4](X = S or Se) and [Fe4S2(NO)4(NCMe3)2] the total valence electron count is 60. This is the number characteristic of tetrahedral tetranuclear metal clusters, such as [Ir4(CO)12], in the Wade and Mingos skeletal-electron counting schemes (76, 77) and, furthermore, each iron atom in these clusters obeys the 18-electron rule, provided that it forms single Fe-Fe bonds to each of the other iron atoms in the tetrahedron. 

All elements of the group form Zinti compounds with electropositive metals. Continuous networks of covenantally bonded atoms are generally found, rather than the clusters common with group 14. For example, NaAl and NaTl have tetrahedral diamond-like networks of A1 or Tl, which can be understood on the basis that A1 and Tl- have the same valence electron count as carbon. 

Predicted structures of transition metal-carbonyl complexes using Wade s rules are often, but not always, accurate.33 For example, the clusters M4(CO)12 (M = Co, Rh, Ir) have 60 valence electrons and are predicted to be nido complexes (14 + 4 valence electrons). A nido structure would be a trigonal bipyramid (the parent structure) with one position vacant. X-ray crystallographic studies, however, have shown these complexes to have tetrahedral metal cores.34... 

In a salt-like description of these compounds, AE, the alkali metal A transfers its valence electron to the tetrel , and - in agreement with the Zintl-Klemm-Busmann concept [9] - the tetrel elements form P4-analogous tetrahedral clusters [ 4] . These highly charged clusters [ 4] are retained in a liquid ammonia solution of RbPb, as shown by Korber et al. via the crystallization of the ammoniate Rb4Pb4(NH3)2 [10]. The isotypic compounds A4Sii4(N 143)2 were obtained for A = Rb and Cs from reactions of the element Sn, as well as from Cs with Sn(C6Hs)4 in liquid ammonia [10]. 

Bonding in species with the unit ()U3-M)E3, e.g. those derived from P4 and AS4, can be regarded for the purpose of electron-counting either as complexes in which the triphosphorous cycle P3 behaves as a tri-hapto three-electron donor towards the transition metal moiety or as a tetrahedral cluster in which one phosphor in P4 has been substituted by a three-electron donor. Counting 30 valence electrons corresponds to an electron-precise tetrahedron with six two-electron/two center bonds. In such kind of compound with stoichiometries ME3 and M2E3 (see Figs. 3.13 and 3.14) the E-E distances are shorter than those in the pure element E4. This has been attributed to electron delocalization toward... 

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