Isoelectronic cluster structure

X-ray diffraction, which showed that the structure of the [Tc2Cl8]2 ions is analogous to that of the isoelectronic cluster rhenium and molybdenum chlorides and bromides with a quadruple M-M bond [1,89,90] (Fig. la, Table 1). 

If there is only one atom shared between two clusters (an unknown situation for B) then o = l. Thus, [(R2C2B4H4)2E], E = Ga , Ge, (Figure 2.19) exhibits a structure consisting of two pentagonal pyramids fused at one apical vertex occupied by the atom E. There are 16 pairs in the isoelectronic clusters (2 R2C2B4H4 fragments = 14 pairs + E = 2 pairs) and m + + o = 2 + 13- -l = 16. The need for parameter o is discussed in the Jemmis article in the reading list. 

The structures of [Mo(CO)4(AuPPh3)5]+ " and [Mo(CO)3(AuPPh3)7]+ are shown in Fig. 5. The structure of [Mo(CO)5(AuPPh3)3]+ was not determined. It can, however, be assumed that it has the same structure as the isoelectronic cluster V(CO)5(AuPPh3)3.f Flomonuclear gold cluster compounds are always formed as by-products, but the amount can be reduced by using an excess of the metal carbonyl complex. 

Ms " clusters have 12 framework bonding electrons as has [BsHs]- (p. 161) the anions are also isoelectronic with the well-known cation [Bis]. Similarly, the alloy NaSn. 2.23 reacts with cryptand in ethylenediamine to give dark-red crystals of [Na(ciypt)]4 [Sng] the anion is the first example of a C41, unicapped Archi-median antiprism (Fig. 10. lOc) and differs from the >3/, structure of the isoelectronic cation [Bis] + which, in the salt Bi+[Bi9] +[HfCl6]5 (p. 591), features a tricapped trigonal prism, as in [BgHg] " (p. 153). The emerald green species [Pb9] , which is stable in liquid NH3 solution, has not so far proved amenable to isolation via ciyptand-complexed cations. 

Figure 13.25 (a) Structure of the anion As7 , isoelectronic with As4Se3 (p. 581). The sequence of As-As distances (base>cap>side) is typical for such cluster anions but this alters to the sequence base >side>cap for neutral species such as As7(SiMe3)3 shown in (b). 

Ga3R3]2- (R = bulky aryl group) [24], isoelectronic with the aromatic cyclo-propenium cation. In addition, tin clusters exhibit a variety of interesting cage structures (see Chapter 2.5.4) [25]. 

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