Electron-rich clusters

Figure 4.27. Valence electron count and vertex count in main group clusters. Notice, according to McGrady (2004), that the different classes of clusters (electron-rich, electron-precise, etc.) simply occupy different domains in a continuum defined by the two variables (electron and vertex counts).

Forming a hydrogen bond between (i )-homocitrate and His 442 could effectively release electron density into the cluster. Studies on structurally defined N2 complexes have shown that the binding of N2 to a metal site and its ability to be protonated are favored by electron-rich sites. Thus, it is postulated that the electron-richness of the... 

Ligand Preferences. The LVC s are electron rich and in order to exist in stable compounds they require ligands, such as CO, that are weak donors and good tt acceptors. On the other hand the HVC clusters are not attractive to such ligands nor do they require them for stability. In this respect, there is a direct parallel to mononuclear compounds where M° prefers CO, RNC or similarly ir-acidic ligands while the Mn+ (n = 2-4) ions do not generally form CO complexes. 

The 90 electron trigonal prismatic cluster, [Rh C(CO) ], is electron rich and seems to behave as though there is a pair of electrons pointing out of each triangular face. Each metal atom is surrounded by five groups (1 terminal CO, 3 bridging CO s and the interstitial carbide) in an essentially square pyramidal... 

Only recently [42a, 46, 47] it was found that the presence of an electron rich transition metal such as Ir in melts of BiX3 (X = Cl, Br) and Bi metal leads to the formation of large isolated and weakly coordinating cluster anions of type [IrBi Xn]-and [IrBi Xn]2- that stabilize the smaller hitherto unknown Bis+ and Bi 2+ in the interstices of the anion lattice (Eq. 5). 

Classical aromatics like the electron-rich, cyclobutadiene dianion A or cydo-pentadienyl anion B and electron-precise hydrocarbons (e.g., benzene C, Figure 3.2-1) have pure n multicenter bonds and therefore are generally not regarded as clusters. 

Electron-precise, electron-deficient and electron-rich clusters. A cluster classification often adopted in several books, and related to the rules previously presented, corresponds to a subdivision into three categories electron-precise, electron-deficient and electron-rich types. The electron-precise clusters may be considered as reference structures. 

Wade expanded the 1971 hypothesis to incorporate metal hydrocarbon 7T complexes, electron-rich aromatic ring systems, and aspects of transition metal cluster compounds [a parallel that had previously been noted by Corbett 19) for cationic bismuth clusters]. Rudolph and Pretzer chose to emphasize the redox nature of the closo, nido, and arachno interconversions within a given size framework, and based the attendant opening of the deltahedron after reduction (diagonally downward from left to right in Fig. 1) on first- and second-order Jahn-Teller distortions 115, 123). Rudolph and Pretzer have also successfully utilized the author s approach to predict the most stable configuration of SB9H9 (1-25) 115) and other thiaboranes. 

The electron-rich trinudear Au3 (bzim) 3 and Au3 (carb) 3 react with F6 to yield the sandwich clusters [n Au3(bzim)3 2](PF6) 29 and [n Au3(carb)3 2](PF6) 30 [54]. The cation of both compounds contains atom which interacts in a distorted trigonal prismatic coordination with six Au(I) atoms from two cyclic Au3C3N3 moieties at Au- distances ranging from 2.971(1) to 3.107(1) A, indicative of appretiable metal-metal interaction. Two Au(I) atoms on each planar trinudear unit are involved in intermolecular aurophilic bonding interactions [Au-Au 3.109(1)/3.066(1) A in 29 3.059(1)/3.052(1) A in 30], thus resulting in an infinite columnar chain with a BBABBA- pattern (Figure 4.11). 

This mechanism, involving sequential hydronation of N2 initially at the nitrogen atom remote from the metal(s), followed by cleavage of the N—N bond, then hydronation of the resulting nitride, is, in essence, the same mechanism that has been developed on electron-rich, low-oxidation-state Mo phosphane complexes (Section III). The only real difference is that in this case the N2 is bound to four Fe sites in a cluster. 

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