Coordinatively unsaturated 16-electron

The crucial experiment suggesting that the H2 molecule might act as a dihapto ligand to transition metals was the dramatic observation that toluene solutions of the deep purple coordinatively unsaturated 16-electron complexes [Mo(CO)3(PCy3)2] and [W(CO)3-(PCy3)2l (where Cy = cyclohexyl) react readily and cleanly with Ha (I atm) at low temperatures to precipitate yellow crystals of [M(CO)3H2(PCy3)2] in 85-95% yield. The... 

We synthesized cationic y2-acetyl compounds 28,25 by combining iron acetyl complexes CpFe(C0)L(C0CH3) (g7) [L=C0,PPh3] with a coordinatively unsaturated (16-electron) metal carbonyl salt CpM(CO)n+[M=Fe,n=2 M=Mo,n=3], as indicated in Scheme 5. Thus... 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

Major achievements in cyclometallation processes involving NHC ligands have been achieved by Nolan and coworkers. It was observed that the solvent could play a crucial role in the formation of rhodium- and iridium-based complexes. As shown in Scheme 39, the reaction performed in pentane between [M(COE)2Cl]2 (M = Rh or Ir) and four equivalents of FBu led only to COE substitution by the NHC ligand affording rhodium-based product (243) (the iridium complexing proving difficult to isolate). On the other hand, the same reaction carried out in hexanes gave, via C H activation, the hydride complexes (244) and (245). Finally, in benzene, a unique double cyclometallation process occurred to yield the coordinatively unsaturated 16-electron... 

It is particularly hazardous to the eyes because of its ready reaction with organic matter, a property exploited in electron microscopy in order to stain and fix biological tissues. Although osmium is a d8 transition metal, its tetroxide is a coordinatively unsaturated 16-electron species and has a tetrahedral structure3. It is highly soluble in carbon tetrachloride (375 g/ 100 g), less so in benzene, and moderately soluble in water (7.24 g/100 g). 

The compound Mo(CO)2 [S2P( -C3H7)2] 2 is monomeric (cryoscopy) and thus is a coordinatively unsaturated, 16-electron species that is potentially very reactive. The compound reacts reversibly with CO and triphenylphosphine to... 

Coordinatively-unsaturated 16-electron complexes typically undergo associative substitution. Here the mechanism involves a slow bimolecular step where the incoming ligand and 16-electron complex combine to form a coordinatively saturated 18-electron intermediate. The intermediate rapidly expels the leaving group to give the new substituted 16-electron product. This is outlined in equation 7.4. 

A good number of mononuclear metal complexes are known to activate C-S bonds, and it is clear that a high electron density around the metal atom generally favors the oxidative addition reaction. One commonly encountered case is the interaction of electron rich, coordinatively unsaturated (16-electron) metal fragments with thiophenes which results in the rupture of one of the C-S bonds with the consequent formation of the corresponding saturated 18-electron thiametallacycles. 

Throughout this section, the role of coordinatively unsaturated 16-electron species (see Section 23.7) and the ability of the metal centre to change coordination number (essential requirements of an active catalyst) should be noted. 

The CM-oxidative addition of H2 to RhCl(PPh3)3 yields an octahedral complex which dissociates giving a coordinatively unsaturated 16-electron species (equation 26.9). The solvated complex RhCl(PPh3)2(solv) (formed from RhCl(PPh3)2 in reaction 26.8) is also involved in the catalytic cycle (but at... 

In the Heck-Breslow mechanism, formal reductive cleavage of the acyl-Co complex 8 with molecular hydrogen or HCo(CO)4 (2) is proposed. However, it is more than likely that the actual acyl-Co complex that reacts with molecular hydrogen is the coordinatively unsaturated (16-electron) acyl-Co(CO)3 7, and the oxidative adduct 10 is formed from 7, which then reductively eliminates to give aldehyde and HCo(CO)3 (3) (Scheme... 

Scheme 5. Plausible explanations for the formation of coordinatively unsaturated 16-electron ruthenium species...

Pd catalysts have two important dimensions (1) as a late transition metal catalyst and (2) as a Lewis acid catalyst. There is no need to describe the former catalysis, which is usually based on the Pd(0)-Pd(II) redox catalytic system. The latter Lewis acid catalysis, despite the soft metal, is based on the empty orbital of the coordinatively unsaturated 16-electron Pd(II) species. 

The olefin conversion in the reaction with HCo(CO)4 and the acylcobalt tetracarbonyl conversion in the reaction with HCo(CO)4 or with dihydrogen are faster if performed in the presence of dinitrogen or argon instead of in the presence of carbon monoxide. The negative effect of carbon monoxide was explained by the assumption that in these reactions in small equilibrium concentrations highly reactive coordinatively unsaturated 16-electron cobalt complexes are involved according to the dissociation equilibria below ... 

Oxidative addition reactions usually involve a coordinatively unsaturated 16-electron metal complex or a five-coordinate 18-electron species, and take the following general form ... 

The two generally recognized routes by which an organometallic complex can catalyze the hydrogenation of alkenes are referred to as the olefin or unsaturated route and the hydride route, as shown in Scheme 5.26. Both pathways start from a coordinatively unsaturated (16-electron) metal complex, M(L)4, which might be formed by ligand dissociation, as shown. The two routes differ in the first step which is olefin complexation or oxidative addition of Hj. Both routes lead to the key dihydride-olefin species in the center of the Scheme. 

While 1,3-butadiene itself is a gas at room temperature, substituted dienes are liquids or solids. In general, therefore, pressure vessels are not required in the preparation of the ij -diene complexes. Starting materials include Fe(CO)5, Fe lCO) or Fe3(CO)j2. The crucial initial step in each case is the formation of the coordinatively unsaturated 16-electron intermediate FelCO). This can be induced thermally or, in the case of FelCO), by irradiation. 

It was first discovered by Noyori that an N-H functionality has a remarkable enhancing effect on the catalyst activity (the NH effect ) by facilitating the formation of a coordinatively unsaturated 16-electron Ru(II) intermediate B under basic conditions from the catalyst precursor, which is then transformed into the catalytically active Ru-hydride species [45] (see Fig. 5). Indeed, the Ru(II) complex 73 containing a chiral l//-pyrazolyl-pyridyl-oxazoline NNN ligand exhibited higher... 

Thereafter, coordinatively saturated, cationic Ru-vinylidene complexes have been obtained from terminal alkynes and Ru complexes, most of them containing Cp and various ancillary ligands. Coordinatively unsaturated, 16-electron Ru-vinylidene complexes, RuX2("C=CHR)L2 (X = Cl, Br, R = tBu, L = PPh3) (2), first reported by Wakatsuki et al., have been prepared from RuX2(PPh3)3 and iBuCM H (Scheme 3). 

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