Bonding electron counting rules

While sharing of electrons, i.e., covalent bonding, is the major component of the cohesive force in intermetallics, rationalization of their structure formation based on such chemical bonding is not trivial, because of the failure of the common electron counting rules that chemists have developed over the years from the studies of covalent compounds. The origin of the problem is the well-delo-... 

B8C18 has a dodecahedral Bg c/o.vo-skclclon with 2n = 16 electrons. In this case, the Wade rule neither can be applied, nor can it be interpreted as an electron precise cluster nor as a cluster with 3c2e bonds. B4(BF2)6 has a tetrahedral B4 skeleton with a radially bonded BF2 ligand at each vertex, but it has two more BF2 groups bonded to two tetrahedron edges. In such cases the simple electron counting rules fail. 

This chapter summarizes recent developments in the expanding field of electron-deficient compounds having from three up to 13 skeletal boron and carbon atoms. In particular, the focus will be on the transition of classical organoboranes into non-classical compounds. Therefore, we first want to briefly review electron counting rules and bonding characteristics of these classes. For a more thorough discussion see Chapter 1 by King and Schleyer. 

REMARKS ON THE CHEMICAL BOND FACTOR AND VALENCE-ELECTRON COUNTING RULES... 

Whereas in ligand bridged dinuclear complexes, removal or addition of two electrons makes or breaks one metal-metal bond (15) this does not seem to be the case for clusters, presumably because of their delocalized bonding. At least for one case, however, two-electron reduction can induce a significant change in cluster shape (18,42) the 84-electron cluster Os6(CO),g with framework 1 is easily reduced to the 86-electron anion Os6(CO) g with framework 2, in accordance with skeletal electron counting rules. 

The electron counting rules of Wade (S3), Williams (117), and Rudolph (118) can serve as a useful concept to explain structure and bonding in a variety of systems which at first glance are very different Zintl phases, boranes and carboranes, transition metal n complexes and carbonyl clusters, nonclassical carbocations, and also n complexes of main-group elements. According to... 

First some simple calculational examples are presented to illustrate the changes in skeletal bonding as protons are moved about as well as to make connections with the electron counting rules. This is followed by a contemporary example illustrating how attempts to reconcile differences between molecules that are isoelectronic only in the sense of the electron count-... 

Eighteen-electron rule — An electron-counting rule to which an overwhelming majority of stable diamagnetic transition metal complexes adhere. The number of non-bonded electrons at the metal plus the number of electrons in the metal-ligand bonds should be 18. The 18-electron rule in transition metal chemistry is a full analogue of the Lewis octet rule . 

There is a connection between an orbital description of electronic structure and the more elementary bonding discussions such as those reviewed in the Appendix. In this section we describe the connection of the 8- and 18-electron rules in order to provide a basis for understanding how the cluster electron-counting rules emerge from and are connected to molecular orbital descriptions of cluster bonding. 

The electronic structures of borane clusters were first successfully described using localized three-center- and two-center-two-electron bonds. These treatments have been replaced by the cluster electron-counting rule based on MO methods hence, why bother with the three-center bond model in a book about clusters Let s consider why there is value in a more localized approach. 

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