Framework electrons

Closo Clusters 2n + 2 Systems). The assignment of valence electrons and the factoring out of those electrons involved in exopolyhedral bonds provides 2n framework electrons for a B H molecule, two electrons short of the 2n + 2 closo count. In fact, stable neutral B H molecules are not... 

For main group elements the number of framework electrons contributed is equal to (t + a — 2) where v is the number of valence shell electrons of that element, and x is the number of electrons from ligands, eg, for Ff, x = and for Lewis bases, x = 2. Examples of 2n + 2 electron count boranes and heteroboranes, and the number of framework electrons contributed by their skeletal atoms, ate given in Table 1. 

Table 2. Framework Electron Contributions for Metal Moieties ...

The general contribution for a metal with v valence electrons and exopolyhedral ligands donating x electrons is t + a — 12 framework electrons. See text. 

The closo, nido, arachno classification is given on the basis of framework electron count and not stmcture. 

Characterization of these clusters indicate an unusual 2n framework electron count having geometries reminiscent of stricdy metallic clusters (11,164). 

Further checks, uhich can readily be verified from the equations of balance, are (a) the number of atoms in a neutral borane molecule = 2(s + I + y + x), and (b) there are as many framework electrons as (here are atoms in a neutral borane B H , since each BH group supplies 2 electrons and each of the m — n) "extra H atoms supplies I electron, making n + m in all. 

The electrostatically favored cation (Li) and anion (RE) arrangement implies the presence of two different E-, Si- and Li sorts, which has been established by solution and solid-state NMR spectroscopy. The electronic structures of the mixed-valent pnictides 10 and 11 have been simply described as electron-deficient clusters with delocalized framework electrons. Formally the latter consist of two low-valent anediyl moieties RE and eight andiides (RE)2- (E = P, As). The relatively large E-E distances of >4 A exclude the occurrence of localized E-E bonds. However, delocalization of the cluster valence electrons is achieved without Li-Li bonds via Li-mediated multiple bonding. Evidence for this has been seen in the NMR spectra (31P, 7Li, 29Si), which are in accordance with the electron delocalization model (see later discussion). 

A number of novel products have been isolated from the reaction of [B Hg]- [31426-87-6] and CoCl and [C HJ in THF (162,163). The predominant product is /< -2-(CpCo)-B4H8 [43061-99-0]. Also obtained are isomeric clusters containing up to four cobalt atoms, eg, (t]5-C5H5Co)4B4H8 [59370-82-0]. Characterization of these clusters indicate an unusual 2n framework electron count having geometries reminiscent of stricdy metallic clusters (11,164). 

The stability of azole carbenes can be attributed to electronic factors which operate in both the Tran d CT-frameworks (92JA5530). In the TT-framework, electron donation into the carbene out-of-plane p-orbital by the electron-rich system moderates the typical electrophilic reactivity of carbenes. In the o-framework, additional stability for the carbene electron pair may be gained from the o-electron-withdrawal effects on the carbene center by the more electronegative nitrogens, which moderates the carbene nucleophilic reactivity. The combination of these a- and TT-effects serves to increase the singlet-triplet gap and stabilize the singlet carbene over the more reactive triplet state. For carbenes with bulky substituents (tert-butyl, 1-adamantyl, etc.) steric effects provide additional stabilization. 

In Chapter 15 we observed that I he l electron rule was adequate For predicting stabilities of small organometallic clusters. In this chapter we have seen that Wade s rules allow us Lo make predictions about borune structures based on the number of framework electrons. These rules ulso are adequate for most curboranes. metallacar-boranes, and other heteroboranes. i1" Furthermore, organometallic clusters that are not derived from boranes can be dealt with in a similar fashion. More sophisticated extensions are required for complex larger clusters.139... 

One of our goals here is to be able to predict the structure of a cage or cluster from its molecular formula. We do this by first finding the number of framework electrons. The structure will then be predicted to be closo, nkJo, or arachito if the number of framework electrons is 2n + 2,2n + 4, or In + 6, respectively. As an example let us consider B3H7(Fe(CO)3] , for which n equals five. The three BH units and the two FefCO>3 units contribute two electrons each and the four extra hydrogen atoms contribute one electron each to give a total of 14 framework electrons ... 

Since n = 5. we see that there are 2/1 + 4 framework electrons and we predict a nido structure which is found experimentally. The square pyramidal structure (Fig. 16.54) can be thought of as resulting from substitution of two BH units with two FefCO units in B5H9 (Fig. I6.49a). 

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