LX “half-arrow” electron-counting

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

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... 

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...

Mo Mo /A Mayer bond index M Mo-Mo configuration and calculated bond order electron count bond order [p-LX] half-arrow method electron count bond order half-electron method... 

An analysis of the bonding in compounds with bridging hydrogen ligands demonstrates that, for every 3-center-2-electron M-H-M interaction, the half-electron method over counts the M-M bond order by one from the value predicted by the p,-LX half-arrow method. At one level, this is not really an issue once it... 

In contrast, the p,-LX half-arrow method requires one to consider the structure of the molecule prior to determining the M-M bond order. The p,-LX half-arrow method thus provides a more complete picture of the bonding by clearly distinguishing between (1) the number of 3-center-2-electron M-H-M interactions and (2) the number of direct 2-center-2-electron M-M interactions. A significant advantage of the p,-LX half-arrow method is that, by not forcing an 18-electron count on... 

In addition to the half-arrow p,-LX description of the bonding for evaluating M-M bond orders, an alternative method merely apportions the electron associated with each hydride ligand equally to both metals (the half-electron method) [24-27]. Specifically, the M-M bond order is obtained by application of the formula w = (18n - A )/2, where m is the number of 2-center-2-electron M-M bonds, n is the number of M atoms, and N is the total electron count assuming that each bridging hydride ligand contributes one electron. For example, the half-electron method predicts an Re = Re double bond for [(CO)4Re]2(p--H)2 [25]. 

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