18-electron rule

Summary

The 18-electron rule is a chemical rule of thumb used primarily for predicting and rationalizing formulas for stable transition metal complexes, especially organometallic compounds. The rule is based on the fact that the valence orbitals in the electron configuration of transition metals consist of five (n−1)d orbitals, one ns orbital, and three np orbitals, where n is the principal quantum number. These orbitals can collectively accommodate 18 electrons as either bonding or non-bonding electron pairs. This means that the combination of these nine atomic orbitals with ligand orbitals creates nine molecular orbitals that are either metal-ligand bonding or non-bonding. When a metal complex has 18 valence electrons, it is said to have achieved the same electron configuration as the noble gas in the period, lending stability to the complex. Transition metal complexes that deviate from the rule are often interesting or useful because they tend to be more reactive. The rule is not helpful f

Official source

https://en.wikipedia.org/wiki/18-electron_rule

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The gas phase (1 atm, 453 K) hydrogenation of p-chloronitrobenzene (p-CNB) over a series of laboratory-synthesized and commercial Pd (1-10% wt) supported on activated carbon (AC) and non-reducible (SiO2 and Al2O3) and reducible (ZnO) oxides has been examined. Reaction over these catalysts generated the target p-chloroaniline (p-CAN) (via selective hydrogenation) and nitrobenzene (NB)/aniline (AN) as a result of a combined hydrodechlorination/hydrogenation. A range of Pd nanoparticles with mean sizes 2.4-12.6 nm (from HRTEM and H-2/CO chemisorption) were generated. Both the p-CNB transformation rate and H-2 chemisorption increased with decreasing Pd size. Residual Mo (from the stabilizer used in the synthesis of Pd colloids) suppressed activity, but this was circumvented by the use of poly(N-vinyl-2-pyrrolidone) (PVP). Pd/AC generated p-CAN and AN as principal products, Pd on SiO2 and Al2O3 exhibited hydrodechlorination character generating AN and NB, and Pd/ZnO promoted the sole formation of p-CAN at all levels of conversion. Reaction selectivity is linked to Pd electron density with the formation of Pd delta+ on AC and the occurrence of Pd delta+ on SiO2 and Al2O3. Reaction exclusivity to p-CAN over Pd/ZnO is attributed to the formation of PdZn alloy (demonstrated by XPS), which selectively activates the -NO2 group. This is the first report that demonstrates 100% selectivity for p-CNB -> p-CAN over supported Pd.

American Chemical Society2013

Microwave-Assisted Organometallic Syntheses: Formation of Dinuclear [(Arene)Ru(-Cl)3RuCl(L-L')] Complexes (L-L'= Chelate Ligands with P/N/S-Donor Atoms) by Displacement of Arene pi-Ligands

Sébastien Gauthier, Rosario Scopelliti, Kay Severin

Microwave heating was employed to promote arene displacement in reactions of [{(p-cymene)RuCl2}(2)] or [{(1,3,5-C-6,H(3)iPr(3))RuCl2}(2)] with neutral chelate ligands L-L' [L-L': 1,1'-bis(diphenylphosphanyl)methane, 1,1'- bis(diphenylphosphanyl)ferrocene, (S)-BINAP, (S,S)-DIOP, N,N'-bis(2,4,6- trimethylphenyl)-1,2-ethanediylidenediamine], (R)-Ph-PHOX, and 3-(phenylsulfanylpropyl)diphenylphosphane. The reactions gave complexes of the general formula [(arene)Ru(mu-Cl)(3)- RuCl(L-L')] in good yield. The synthesis of [(p-cymene)Ru(mu-Cl)(3)RuCl{PPh2(CH2)(3)NH2} (22) was accomplished in two steps via the intermediate [{(p-cymene)RuCl2}(2){mu- PPh2(CH2)(3)-NH2}] (21). The structures of [(1,3,5-C(6)H(3)iPr(3))Ru(mu-Cl)(3)RuCl- (dppf)] (16), [(1,3,5-C(6)H(3)iPr(3))Ru(mu-Cl)(3)RuCl{(S)-BINAP]} (17), and [(p- cymene)Ru(mu-Cl)(3)RuCl(MesNCHCHNMes)] (18) were determined by single- crystal X-ray diffraction. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

2009

Carbon dioxide reduction by lanthanide(iii) complexes supported by redox-active Schiff base ligands

Nadir Jori, Marinella Mazzanti, Rosario Scopelliti, Davide Toniolo

Here we have explored the ability of Schiff bases to act as electron reservoirs and to enable the multi-electron reduction of small molecules by lanthanide complexes. We report the reductive chemistry of the Ln(iii) complexes of the tripodal heptadentate Schiff base H(3)trensal (2,2 ',2 ''-tris(salicylideneimino)triethylamine), [Ln(III)(trensal)],1-Ln(Ln = Sm, Nd, Eu). We show that the reduction of the [Eu-III(trensal)] complex leads to the first example of a Eu(ii) Schiff base complex [{K(mu-THF)(THF)(2)}(2){Eu-II(trensal)}(2)],3-Eu. In contrast the one- and two-electron reduction of the [Nd-III(trensal)] and [Sm-III(trensal)] leads to the intermolecular reductive coupling of the imino groups of the trensal ligand and to the formation of one and two C-C bonds leaving the metal center in the +3 oxidation state. The resulting one- and two electron reduced complexes [{K(THF)(3)}(2)Ln(2)(bis-trensal)],2-Ln, and [{K(THF)(3)}(2){K(THF)}(2)Ln(2)(cyclo-trensal)],4-Ln(Ln = Sm, Nd) are able to effect the reductive disproportionation of carbon dioxide by transferring the electrons stored in the C-C bonds to CO(2)to selectively afford carbonate and CO. The selectivity of the reaction contrasts with the formation of multiple CO(2)reduction products previously reported for a U(iv)-trensal system.

ROYAL SOC CHEMISTRY2020