Lobal characteristics

Fig. 1.4), or indeed in ferrocene, Fe(C5H5)2. This is because such units as CH and Fe(CO)3 or MiiiCDf have very similar frontier orbital characteristics with which to bond to other units. Though not isoelectronic, they are isolobal/" " " Their frontier orbitals have similar energies, extensions in space, and lobal characteristics, which enables them to participate in the same types of bonding (Fig. 1.21). 

A further feature of the cluster bonding MOs worth commenting on concerns the way their lobal characteristics can be classified irrespective of the point group to which a particular cluster belongs, by labeling them as S, P, D, and so on according to how their lobal characteristics would match those of... 

A third approach notes the relationship between the anion [(q -CsHs) Fe(q5-C2B9Hii)]2 and the parent carborane from which it is derived, C2B10H12. An anionic [(q5-C5H5)Fe] unit in the former replaces a neutral BH unit in the latter. The capacities of these two units, [(CsHsjFe] and BH, to participate in cluster bonding are similar. Each can function as a source of two electrons, and three AOs, for use in skeletal bonding. Not only are the numbers of their frontier orbitals the same, but so are their lobal characteristics they are isolobal (Fig. 3.21).The metal unit can effectively supply a radially oriented Pzdz. hybrid orbital and two pd p.xL i,P-,d., ) hybrid orbitals to bond to the nido... 

Theoretical justification for the electron bookkeeping device of treating CH and Co(CO)3 or Ni(n -Cd 1) units as similar sources of three AOs and two electrons for cluster forming use followed from analyses of the frontier orbitals [HOMOs and lowest unoccupied molecular orbitals (LUMOs)] of such conical transition metal units by Hoffmann,Mingos, " and others who coined the term isolobal to describe their relationship to a CH unit. Although the transition metal units use pd hybrid AOs where carbon uses p or sp hybrids, the numbers, energies, extensions in space, and lobal characteristics of these orbitals are very similar, justifying the description of these units as isolobal, written as follows (cf. Fig. 1.19 in Chapter 1) ... 

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