Electron Counting in Reactions

We therefore need to be very careful to specify whether a process involves proton, hydrogen atom or hydride transfer, because each has a completely different reactivity. The same holds for many other reactants, for example, different Br-containing reagents can act as Br+, Br, or Br transfer agents methyl transfer is typically either of Me+ or Me . 

By looking at the equations in the pages to come, you will become more familiar with electron counting of stable complexes and with counting the ligands that are gained or lost in reactions. In proposing new structures, be sure that the rules discussed in this chapter are obeyed. 

Even d configurations are much more common than odd ones, particularly for the second and third row. Diamagnetic complexes are easier to study and so are more often reported, and the high A value for the second- and third-row metals favors electron pairing in the d, levels. An exception exists for M-M bonded compounds, where odd 

TABLE 2.6 Relationships between Oxidation States and d Configurations 

It is often useful to refer to the oxidation state and d configuration of a metal, but these only represent a formal classification and do not 

The smaller metal clusters, such as Os3(CO)i2, often obey the I8e rule for each metal, but for clusters of six metals or more, there can be deviations for which special cluster counting rules have been devised (Chapter 13). The rule is not useful for main-group elements, such as ZnMe2, I4e MeHgfbipy), I6e I(py)2] . 20e [SbFs] , 22e and IF7, 24e, where no particular electron count is favored. The lanthanides and actinides have seven / orbitals to fill before they even start on the d orbitals, and so they are essentially never able to bind a sufficient number of ligands to raise the electron count to the or 32e 

Because H is a 2e reagent like PPh3, we would not expect H to attack the metal in ferrocene. Note that this result is the same whether we use the ionic or covalent 

The reaction of Eq. 2.6 turns a le alkyl group into a 2e alkene group. To retain the 18e configuration, the complex must become positively charged, which implies that the H must be lost as H and that an electrophilic reagent (such as PhsC ) must be used. In this way the 18e rule helps us pick the right reagent. 

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With regard to the valence electron count, this number determines whether the transition metal ion is using its full complement of valence shell orbitals— i.e., the five nd s, the (n + l)s, and the three (n + l)p s. If the valence electron count is eighteen, all of the orbitals are fully utilized in bond formation and electron pair storage, the effective atomic number (EAN) rule is fulfilled and the metal ion is said to be saturated. If it is seventeen, the metal ion is covalently unsaturated, and if it is sixteen or less, the metal ion possesses at least one vacant coordination site and is said to be coordinatively unsaturated. The importance of the valence electron count in homogeneously catalyzed reactions has been discussed by Tolman (7). 

Next, multiply the half-reactions by factors that make the electron count in each the same. 

The [2+2] reactions of the zirconium-imido compounds with alkynes and alkenes occurs by a mechanism similar to that for the [2+2] reactions of carbenes with alkynes and alkenes. The alkene or alkyne first binds to an open coordination site at the metal, and this coordination is followed by conversion of the alkyne or alkene complex to the metallacyhc product (e.g. Equation 13.76). Thus, the [2+2] reaction requires a 16-electron intermediate to bind the olefin or alkyne, even though the metallacyHc product and the imido complex have the same overall electron count. In support of the coordination of alkyne or alkene, albeit weakly, to the d° metal center, the rate of the reaction of alkynes with the 18-electron zirconocene-imido compound containing bound pyridine-N-oxide was inhibited by added pyridine-N-oxide (Equation 13.76). ... 

The first step in the Cativa process is the reaction between Mel and c -[Ir(CO)2l2]. However, the catalyst may also react with HI and this step initiates a water gas shift reaction that competes with the main catalytic cycle, (a) What chemical is manufactured in the Cativa process Why is this product of industrial importance (b) Why is HI present in the system (c) Give an equation for the water gas shift reaction, and state conditions typically used in industry, (d) Figure 25.22 shows the competitive catalytic cycle described above. Suggest identities for species A, B, C and D. What type of reaction is the conversion of czj -[Ir(CO)2l2] to A What changes in iridium oxidation state occur on going around the catalytic cycle, and what is the electron count in each iridium complex ... 

The circle-in-a-hexagon symbol was first suggested by the British chemist Sir Robert Robinson to represent what he called the aromatic sextet —the six delocalized TT electrons of the three double bonds. Robinson s symbol is a convenient time-saving shorthand device, but Kekule-type formulas are better for counting and keeping track of electrons, especially in chemical reactions. 

The influence of electron-count on cluster geometry has been very elegantly shown by a crystallographic study of the deep-red compound [K(ctypt)]g [Ge9]- [Ge9] .2.5en, prepared by the reaction of KGe with cryptand in ethylenediamine. [Ge9] has the C4, unicapped square-antiprismatic structure (10.10c) whereas [Ge9]- , with 2 less electrons, adopts a distorted Dit, structure which clearly derives from the tricapped trigonal prism (p. 153).The field is one of... 

John D. Corbett once said There are many wonders still to be discovered [4]. This certainly holds generally for all the different areas and niches of early transition cluster chemistry and especially for the mixed-hahde systems. The results reported above so far cover a very Hmited selection of only chloride/iodide systems and basically boron as the interstitial. Because of the very sensitive dependence of the stable stracture built in the soHd-state reaction type on parameters like optimal bonding electron counts, number of cations present, size and type of cations (bonding requirements for the cations), metal/halide ratio, and type of halide, a much larger mixed-hahde cluster chemistry can be expected. Further developments, also in mixed-hahde systems, can be expected by using solution chemistry of molecular clusters, excised from solid-state precursors. 

Os4Pd6(CO)8(//-CO)8(/x-dppm)2] in low yields, while the reaction of [Os5(/u5-C)(CO)i5] with [Pd2(/u-dppm)2Cl2] afforded [Os5Pd4(/u6-C)(CO)12(/u-CO)3(/u-dppm)2] and [Os5(/X5-C)(CO)13(/r-dppm)] in moderate yields.289 The electron counts found in these osmium-palladium clusters do not always agree with those predicted by skeletal electron counting rules. This may simply be ascribed to the ability of Pd to be satisfied with both 16- and 18-electron counts.289... 

Transition metal centered bond activation reactions for obvious reasons require metal complexes ML, with an electron count below 18 ("electronic unsaturation") and with at least one open coordination site. Reactive 16-electron intermediates are often formed in situ by some form of (thermal, photochemical, electrochemical, etc.) ligand dissociation process, allowing a potential substrate to enter the coordination sphere and to become subject to a metal mediated transformation. The term "bond activation" as often here simply refers to an oxidative addition of a C-X bond to the metal atom as displayed for I and 2 in Scheme 1. 

Our work was initiated on the reduced ternary molybdenum oxides with the thought that the metal cluster electron count (MCE) should be variable for the Mo308 cluster units. Based on Cotton s previous molecular orbital treatment of such clusters (16) it appeared that MCE s from 6 to 8 could be accommodated, but it was not clear whether the seventh and eighth electrons would occupy bonding or antibonding orbitals with respect to the M-M interactions. We thus set about to determine this from structural data on suitable compounds. The attempted replacement of Zn2+ with Sc3+ to secure the compound ZntSc°Mo308 was conducted via the reaction shown in equation 1. 

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