Carbon electron counting

An electron count shows (6x11) electrons contributed by gold, four from carbon deducting two for the positive charge, leaves 68 electrons, which just occupy the four bonding MOs plus the gold 5d orbitals [191]. 

The molecule has one nitrogen atom p ). Interpreting the parentheses in the formula reveals that there are four carbon atoms p ) and eleven hydrogen atoms (5 ). You should be able to verify that the valence electron count is 32 e. ... 

Because the inner atoms are electron-deficient in the provisional structure, we should pay particular attention to their electron count in the final stmcture. Each of the carbon atoms has an octet composed of four bonds, in accordance with line stmcture mles. 

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. 

Williams [1] has given an excellent review on Early Carboranes and Their Structural Legacy and he defines carboranes as follows Carboranes are mixed hydrides of carbon and boron in which atoms of both elements feature in the electron-deficient polyhedral molecular skeleton . According to the electron counting rules [2] for closo- (2n + 2 SE), nido- (2n + 4 SE) and arachno-clusters (2n + 6 SE SE = skeletal electrons, n = number of framework atoms) and the An + 2 n electron Hiickel rule, small compounds with skeletal carbon and boron atoms may have an electron count for carboranes and for aromatics (see Chapters 1.1.2 and 1.1.3). 

At first glance, electron count in these clusters gives a remarkable analogy to the carbaboranes. Each aluminum atom contributes two electrons to the cluster, three electrons are from each carbon atom, and one electron is added by the bridging hydrogen atom. A total count of 32 electrons or 16 electron pairs results, which in accordance with the Wade rules [6] gives an arachno-type 13-vertex cluster. Indeed,... 

Wade electron counting rules borane-like cluster nomenclature. On initially studying compounds such as boranes (boron hydrides) and carboranes (or carbaboranes boron—carbon hydrides), Wade (1976) proposed a number of rules which have then been extended to several compounds and which relate the number of skeletal electrons with the structure of deltahedral clusters. A polyhedron which has only A-shaped, that is triangular, faces is also called a deltahedron. 

Part IV of this article outlines a description of the processes involved in cleavage of a single metal-carbon bond, and puts forward a theory of the influences of electron count, and of configuration interaction, on metal-carbon bond stability. Finally, in Part V, I attempt a critical appraisal of the method adopted and the results obtained. 

Triatomic fragments involving boron and carbon, such as those found in many borocarbide structures,16 show typical valence electron counts ranging between 12 and 16 electrons. These counts are unusual for isolated triatomic molecules, which usually have 16 or more electrons, unless one of the terminal atoms is FI (12 electrons).17 Within the boron-carbon family, four of six possible arrangements for the trimeric fragments have been observed (1) C—C-C, (2) C-B-C, (3) B-B—C, and (4) B-C-B. Coincident-ally, only those with D,h point symmetry occur in metal borocarbides. They include (1) CCC units in Sc3C4 18 (2) CBC units in Si BCj 19,20... 

Cb) C=N Nitrogen, as before, is electrically neutral. A neutral carbon has 4 valence electrons, and so carbon in this species, with an electron count of 5, has a unit negative charge. The species is cyanide anion its net charge is -1. 

An electron count of 5 is one more than that for a neutral carbon atom. The formal charge on carbon is -1, as is the net charge on this species. 

Two unshared electrons contribute 2 / to die electron count of carbon. 

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