Nido species

By no means do all metallocarboranes have the metal atoms occupying vertices of the basic polyhedra. Apart from many derivatives in which o-bonded metal residues occupy exo sites attached to particular skeletal atoms, several metalloboranes and -carboranes are known in which the metal occupies an edge-bridging site, effectively replacing a bridging hydrogen atom of the parent borane. Many are nido species related to BeHio, for example, the /x-silyl and /i-germyl carboranes. 

Fundamental Closo species Nido species Arachno species... 

Fundamental polyhedron h Closo species (0 = 6-1) Nido species (0 = 6-2) Arachno species (0 = 6-3)... 

Reduction of cZoso-dicarbaboranes, CaBn-zHn, to the dianionic nido species, CaB 2Hn , which are more susceptible to rearrangement reactions, also provides a route to isomers of the starting materials in which the carbon atoms have moved to different polyhedron vertices, e.g. 185,186),... 

In related reactions, units, such as Cr(CO)3, Mn(CO)3", Fe(CO)8 , Co(C5H5) +, Mc3N, Be +, or T1+, that can supply 1 skeletal atom and three vacant AO s (no skeletal electrons) for cluster bonding can be used to fill in the vacant vertices of nido or arachno species, thereby converting them into closo or nido species, e.g. 184,194),... 

Nonicosahedral structures should be anticipated for seven- and nine-vertex nido species. 

It would appear that rule 1 correctly predicts the structures of all nido species (see Fig. 1) except perhaps the structures of the eight-vertex nido species that, at least in the two cases known, tentatively seem to assume the arachno configuration. Candidate structures for C4B4Hg are... 

In the terminology used in Wade s rules1" , the 60-electron tetrahedral cluster [HFeRu3(CO)i3] is considered a closo species (XXIX) and the 62-electron open butterfly structure of [HFe4(CO)13] is considered a nido species (XXX). The structural difference is related to the fact that one of the CO groups is a 2-electron donor in the former and a 4-electron donor in the latter. 

This is basically the same as the one in procedure A but using the closo species instead of the nido one.l8, It is far simpler than the former one since the nido species does not need to be synthesized. For [(rf-NC4H4)Co(C2B,Hu)] the procedure is as simple as the reaction of o-carborane with KCNCJHJ and CoCl2 in the ratio 1 12 5 in dme yields after working up [(rf-NC4H4)Co(C2B,Hll)] in 76% yield. The reaction is shown in Figure 3. 

This is a 15-electron species with three vacant positions, isolobal with CH. This is a 16-electron species with one vacant position, isolobal with CH3 ". This is a 15-electron species with three vacant positions, isolobal with CH. B]iHi3 is derived from BnHi/ a nido species. 

C3B3H7 is equivalent to BgHio, derived from BgH5 , a nido species. C2B5H7 is equivalent to B7H9, derived from B7H7, a closo species. 

Entries in the second column of Figure 3.1 and in Figures 3.3 and 3.4 show the deltahedral fragment skeletal shapes most commonly encountered among boranes and carboranes of formulae B H +4, CB iH +3, C2B 2H +2, and so on, all of which can be regarded as derived from hypothetical anions [B H ]4 by protonation and/or isoelectronic replacement of B by C. Their molecular formulae are sometimes written as CxB H +4, where x + y = n, the total number of skeletal atoms. The skeletal structures of these nido species. 

As an alternative to the reductive generation of a nido borane or carborane cluster from a closo parent, one can regard a nido species as the product of decapitation or deboronation of a closo parent. Formal removal of a B kf unit, normally from a high connectivity site in a closo borane or carborane, followed by protonation of the anionic residue around the open face generated, leaves a neutral nido residue. 

The structural patterns illustrated in Figure 3.1, and the shapes shown in column 3 for arachno species, are essentially those first suggested by Williams in 1971,2 and have stood the test of time remarkably well. However, the shapes shown should be regarded as illustrative rather than definitive. As with the nido species, alternative shapes to those shown in Figure 3.1 are possible for arachno species, with different sites left vacant on the parent deltahedron. We have already noted that many nido boranes and carboranes with eight skeletal atoms show deviations in their atom connectivities from those shown in Figure Nevertheless, apart from these particular exceptions,... 

Number of Skeletal Electron Pairs" Parent Deltahedron (Symmetry) Closo Species CjB,H,+2 (H ) Nido Species C,B,H,44 (H"), Arachno Species C3,H + (H ),... 

Note that both descriptions of the Fe4 residue, whether as an arachno species based on an octahedron, or as a nido species based on a trigonal bipyramid, are compatible with a butterfly shape. However, the dihedral angle between the wing planes, and the M-C-M angles at the core (or equatorial) carbon atom, will differ markedly according to which description is used. A... 

E13.14 The formula BjHg belongs to a class of borohydrides having the formula B H,r+4, which is characteristic of a nido species. There are five BH units, which contribute 5> 2 = 10 electrons (assuming one B-H bond per B atom), and the four additional H atoms, which contribute a further four electrons, giving 14 skeleton electrons in total, or seven skeleton electron pairs, which is n+2 with n = 5. This is characteristic of nido clusters. 

E13.16 (a) The formula BjoH belongs to a class of borohydrides having the formula B H,h4, which is characteristic of a nido species. 

B5H8 is derived from B5H5, a nido species. 

There are also examples of clusters where a hydrogen atom is bonded to from three (as in 22.6) to six metal atoms. In addition the dellahedron may be cither complete (a closo species) as in 22.1-22.10 or may have one vertex missing (a nido species) as in 22.11-22.13, two vertices missing (an arachno species) as in 22.14,22.1 5, or perhaps more. In many of these cases the reader may be used to viewing these molecules, not as the remnants of a cage, but in a completely different way, perhaps. 

---