Isoelectronic cluster borane

Another important concept is the notion of stabilization by means of coordination, A classic example is the. stabilization of the fugitive species cyclobutadiene, C4H4I by coordination to (Fe(CO)3l (p. 936). As the C atom is isoclectronic with (BH], so (C4H4] is isoelectronic with the borane fragment (BaH which is similarly stabilized by coordination to (Fe(CO)3l or the isoelectronic (Cofr/ -CsHs)) (see Panel on p. 174), Indeed it is a general feature of metallaboranc chemistry that such clusters are often much more stable than are the parent boranes themselves. 

The next development in the understanding of structure and bonding in the Zintl ions recognized their relationship to the polyhedral boranes and the isoelectronic carboranes. Then the Wade-Mingos rules [13-16], which were developed to understand the structure and bonding in polyhedral boranes, could be extended to isovalent Zintl ions and related post-transition element clusters. 

A separate series of closo-carboranes, isoelectronic with the BnH2 dianions and the neutral QB 2H compounds, consists of the monocarbon anions CB iH (Table 5-3). The preparative routes to these species vary widely, but a commonly used method for incorporating a single carbon atom into a borane cage involves the conversion of cyanoboranes to amino derivatives the cyano carbon atom satisfies its valence requirements via absorption into the cluster framework. 

H3BNH3, which is isoelectronic and isostructural with ethane. However, there is also a rapidly developing interest in boron-nitrogen clusters that are structural and electronic counterparts of boranes rather than classical organic molecules.29 Figure... 

The electron counting schemes described for boranes can be extended to isoelec-tronic species such as carboranes,22 clusters containing both carbon and boron as framework atoms. The CH+ unit is isoelectronic with BH many compounds are known in which one or more BH groups have been formally replaced by CH+ (or by C, which has the same number of electrons as BH). For example, the replacement of two BH groups in closo-B6H62 with CH+ yields closo-CjB. ... 

Finally, the conceptual isoelectronic replacement of C-C by B-N can be applied to carba-boranes, thus leading (by appropriate synthetic routes) to azaboranes in which one or more of the cluster vertices of the borane is occupied by an N atom. So far, the following species have been characterized, the relevant cluster geometries and numbering schemes being given by the indicated structures on pp. 153-85 arachno-... 

Boranes form three types of clusters closo (closed), nido (nestlike), and arachno (weblike). Boranes of the general formula B H and isoelectronic carboranes such as CB iH and C2B 2H have closo structures n skeletal atoms lie at the vertices of a polyhedron composed of triangles (Figure 3.2). 

We have seen what happens in a borane cluster if there are not enough BH fragments to fill the vertices we get a nido structure with an empty vertex. The same is true for transition metal clusters, for example, in Fe5(CO)i5C, the carbon atom, which is not considered as a vertex atom, is encapsulated within the cluster and gives all its four valence electrons to the cluster. The Fe(CO)3 fragment contributes two cluster electrons as it is isoelectronic with Os(CO)3. The total count is therefore (5 x 2) + 4 = 14, and the number of vertices is -1 = 6. This requires the structure shown as 13.18, as is observed for this and the analogous Ru and Os species. 

Borane clusters are divided into four categorise depending on the structure they adopt. The relationship between polyhedra in each class are shown in Pig. 22. Clusters with the general formula and the isoelectronic... 

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