Electron counting methods

The descriptor hypercloso is used here for clusters whose shapes are closed triangulated polyhedra, but which have one less electron pair available for skeletal bonding than do conventional closo species, according to traditional electron counting methods. 

A justification for the skeletal electron-counting method has resulted from a Wolfsberg-Helmholz-MO calculation on the very symmetrical anion COg(CO)j4 ... 

The electron counting method of choice is a matter of individual preference. Method A includes the formal oxidation state of the metal Method B does not. Method B may be simpler to use for ligands having extended tt systems for example, ligands have an electron count of 5, ligands an electron count of 3, and so on. Because neither description describes the bonding in any real sense, these methods should, like the Lewis electron-dot approach in main group chemistry, be considered primarily electron... 

Although these electron-counting methods in NO complexes are useful, they do not describe how NO actually bonds to metals. The use of NO, NO, or NO does not necessarily imply degrees of ionic or covalent character in coordinated NO these labels are simply convenient means of counting electrons. 

Hoffmann uses electron-counting method A, in which chloride is considered a negatively... 

K. Mizuno, K. Kumaki, S. Hata, Y. Nishii, and S. Tomioka, Estimation of drug stability based on extra-weak chemiluminescence. I. Construction of apparatus for measuring of extra-weak chemiluminescence using photo-electron counting method [in Japanese], Yakugaku Zasshi 98, 513-519(1978). 

Hoffmann uses electron-counting method A, in which chloride is considered a negatively charged, 2-electron donor. tr) -C5H5 is considered the 6-electron donor... 

The effectiveness of this electron counting method can be illustrated by looking at Scheme 2.6, which shows carbon and two of its other 4A family members, silicon (Si) and germanium (Ge). As can be seen, their total electron counts are very different (6,14, and 32), but nevertheless, all the three atoms have four electrons in their valence shells. Since the valence shell in main group elements can contain a maximum of eight electrons, the connectivity of all these atoms will be four. 

Application of the p,-LX Half-Arrow Electron-Counting Method to Other... 

Fig. 9 Different descriptions of the metal-metal bond orders in some dinuclear complexes with bridging hydride ligands according to the electron-counting method. The half-electron method does not explicitly take into account the 3-center-2-electron nature of the M-H-M interaction and thus results in a greater M-M bond order than would be predicted theoretictilly. In contrast, the half-arrow method treats the 3-center-2-electron nature of the M-H-M interactions explicitly and thereby predicts an M-M bond order which is in accord with theory...

Table 4 Comparison of calculated bond orders for a series of dinuclear molybdenum compounds with those predicted by two alternative electron-counting methods. In each case, the bond order predicted by the [p.-LX] half-arrow method corresponds exactly to that derived by consideration of the orbital occupations, whereas the half-electron method fails for compounds with bridging hydride and methyl ligands. Data ttiken from reference 6...

Consideration of both the Mayer bond order analysis and the molecular orbital analysis indicates that the Mo-Mo bond order predicted by the p,-LX half-arrow method for [CpMo(p,-02CH)]2(p,-PH2)(p,-H) is much more in accord with the theoretical calculations than is the bond order of two predicted by the half-electron method. A variety of other studies support the use of the p,-LX half-arrow electron-counting method for predicting M-M bond orders in dinuclear compounds [52-59]. In particular, Hoffmann and Albright have used MO theory to analyze the... 

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