Orbital Participation Versus Democracy

The basic methodology that we have used to study the chemical bonding to hypercoordinate main-group elements is much the same as that described in an earlier Chapter The spin-coupled description of aromatic, antiaromatic and nonaromatic systems . 

The spin-coupled wavefunctions for the systems considered here take the form 

The basic strategy adopted for normal octet and hypercoordinate molecules XYn was first to carry out a standard closed-shell RHF calculation and then to localize the orbitals according to the population or overlap criterion introduced by Pipek and Mezey [12]. In all cases, it was straightforward to identify localized molecular orbitals (LMOs) associated with particular X—Y bonds. 

Visual inspection of the bond LMOs for various phosphorus and sulfur halides revealed no evidence for the active participation of d orbitals. For example, not 

Calculations of this type have also been performed for PXn (n=3,5) and SXn (n=2,4) fluorides and chlorides [9]. Much the same basic picture emerges for all of these systems, whether normal octet or hypercoordinate, with the variations in the amount of central-atom character in the two-centre spin-coupled orbital reflecting the polarity of the particular bond. Analogous descriptions were found to apply for XeFn (n=2,4) and for SiX ions (X=H,F). 

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