Electron Counts for Face-Bridged Octahedral Transition Metal Clusters

Fan, Peidong; Deglmann, Peter; Ahlrichs, Reinhart

doi:10.1002/1521-3765(20020301)8:5<1059::aid-chem1059>3.0.co;2-v

Cited by 21 publications

(16 citation statements)

References 25 publications

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“…Important orbitals involved in the bonding are the occupied 3p 2 lone‐pair donor orbitals of S 8 (= HOMO) and the empty 5s 0 and 5p 0 acceptor orbitals of the silver ion (= LUMO and LUMO+1). At this point we note that there is ongoing controversy about the importance of (n+1)p‐orbital contributions to the bonding of nd metals 40a–40d…”

Section: Resultsmentioning

confidence: 93%

Approaching the Gas-Phase Structures of [AgS8]+ and [AgS16]+ in the Solid State

et al. 2002

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Upon treating elemental sulfur with [AgSbF6], [AgAl(hfip)4], [AgAl(pftb)4] (hfip=OCH(CF3)2, pftb =OC(CF3)3) the compounds [Ag(S8)2][SbF6] (1), [AgS8][Al(hfip)4] (2), and [Ag(S8)2]+[[Al(pftb)4]− (3) formed in SO2 (1), CS2 (2), or CH2Cl2 (3). Compounds 1–3 were characterized by single‐crystal X‐ray structure determinations: 1 by Raman spectroscopy, 2 and 3 by solution NMR spectroscopy and elemental analyses. Single crystals of [Ag(S8)2]+[Sb(OTeF5)6]− 4 were obtained from a disproportionation reaction and only characterized by X‐ray crystal structure analysis. The Ag+ ion in 1 coordinates two monodentate SbF6− anions and two bidentate S8 rings in the 1,3‐position. Compound 2 contains an almost C4v‐symmetric {AgS8}+ moiety; this is the first example of an η4‐coordinated S8 ring (d(AgS)=2.84–3.00 Å). Compounds 3 and 4, with the least basic anions, contain undistorted, approximately centrosymmetric Ag(η4‐S8)2+ cations with less symmetric η4‐coordinated S8 rings (d(AgS)=2.68–3.35 Å). The thermochemical radius and volume of the undistorted Ag(S8)2+ cation was deduced as rtherm(Ag(S8)2+)=3.378+ 0.076/−0.120 Å and Vtherm(Ag(S8)2+)=417+4/−6 Å3. AgS8+ and several isomers of the Ag(S8)2+ cation were optimized at the BP86, B3LYP, and MP2 levels by using the SVP and TZVPP basis sets. An analysis of the calculated geometries showed the MP2/TZVPP level to give geometries closest to the experimental data. Neither BP86 nor B3LYP reproduced the longer weak dispersive AgS interactions in Ag(η4‐S8)2+ but led to Ag(η3‐S8)2+ geometries. With the most accurate MP2/TZVPP level, the enthalpies of formation of the gaseous [AgS8]+ and [Ag(S8)2]+ cations were established as ΔfH298([Ag(S8)2]+, g)=856 kJ mol−1 and ΔfH298([AgS8]+, g)=902 kJ mol−1. It is shown that the {AgS8}+ moiety in 2 and the {AgS8}2+ cations in 3 and 4 are the best approximation of these ions, which were earlier observed by MS methods. Both cations reside in shallow potential‐energy wells where larger structural changes only lead to small increases in the overall energy. It is shown that the covalent AgS bonding contributions in both cations may be described by two components: i) the interaction of the spherical empty Ag 5s0 acceptor orbital with the filled S 3p2 lone‐pair donor orbitals and ii) the interaction of the empty Ag 5p0 acceptor orbitals with the filled S 3p2 lone‐pair donor orbitals. This latter contribution is responsible for the observedlow symmetry of the centrosymmetric Ag(η4‐S8)2+ cation. The positive charge transferred from the Ag+ ion in 1–4 to the coordinated sulfur atoms is delocalized over all the atoms in the S8 ring by multiple 3p2→3σ* interactions that result in a small long‐short‐long‐short SS bond‐length alternation starting from S1 with the shortest AgS length. The driving force for all these weak bonding interactions is positive charge delocalization from the formally fully localized charge of the Ag+ ion.

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Section: Resultsmentioning

confidence: 93%

Approaching the Gas-Phase Structures of [AgS8]+ and [AgS16]+ in the Solid State

et al. 2002

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“…The effects on M−M distances of S substitution by either Se or Te in the [MoFe 3 S 3 ] 2+ 13 or [M 6 E 8 (CO) 6 ] clusters have been examined. The lack of significant changes of the short M−M distances in these clusters has been interpreted as indication of (relatively weak) intermetallic bonding.…”

Section: Discussionmentioning

confidence: 99%

Density Functional Theory Calculations and Exploration of a Possible Mechanism of N₂ Reduction by Nitrogenase

Huniar¹,

Ahlrichs

Coucouvanis

2004

J. Am. Chem. Soc.

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Density functional theory (DFT) calculations have been performed on the nitrogenase cofactor, FeMoco. Issues that have been addressed concern the nature of M-M interactions and the identity and origin of the central light atom, revealed in a recent crystallographic study of the FeMo protein of nitrogenase (Einsle, O.; et al. Science 2002, 297, 871). Introduction of Se in place of the S atoms in the cofactor and energy minimization results in an optimized structure very similar to that in the native enzyme. The nearly identical, short, lengths of the Fe-Fe distances in the Se and S analogues are interpreted in terms of M-M weak bonding interactions. DFT calculations with O or N as the central atoms in the FeMoco marginally support the assignment of the central atom as N rather than O. The assumption was made that the central atom is the N atom, and steps of a catalytic cycle were calculated starting with either of two possible states for the cofactor and maintaining the same charge throughout (by addition of equal numbers of H(+) and e(-)) between steps. The states were [(Cl)Fe(II)(6)Fe(III)Mo(IV)S(9)(H(+))(3)N(3-)(Gl)(Im)](2-), [I-N-3H](2-), and [(Cl)Fe(II)(4)Fe(III)(3)Mo(IV)S(9)(H(+))(3)N(3-)(Gl)(Im)], [I-N-3H](0) (Gl = deprotonated glycol; Im = imidazole). These are the triply protonated ENDOR/ESEEM [I-N](5-) and Mössbauer [I-N](3-) models, respectively. The proposed mechanism explores the possibilities that (a) redox-induced distortions facilitate insertion of N(2) and derivative substrates into the Fe(6) central unit of the cofactor, (b) the central atom in the cofactor is an exchangeable nitrogen, and (c) the individual steps are related by H(+)/e(-) additions (and reduction of substrate) or aquation/dehydration (and distortion of the Fe(6) center). The Delta E's associated with the individual steps of the proposed mechanism are small and either positive or negative. The largest positive Delta E is +121 kJ/mol. The largest negative Delta E of -333 kJ/mol is for the FeMoco with a N(3-) in the center (the isolated form) and an intermediate in the proposed mechanism.

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“…Ausnahmen davon finden sich bei späten Übergangsmetallen selten und beschränken sich auf Cluster mit ungewöhnlichen Ligandenanordnungen und/oder Metallen der Gruppen 10 und 11; ein Beispiel ist [Ni 6 (η 5 ‐C 5 H 5 ) 6 ] mit 90 Elektronen 10. Cluster des Typs [M 6 (μ 3 ‐E) 8 (PR 3 ) 6 ] (M=Metall der Gruppe 6–9; E=S, Se oder Te) früher und später Übergangsmetalle sind vielfach bekannt11 und haben Elektronenzahlen12 zwischen 79 für [W 6 (μ 3 ‐S) 8 (PEt 3 ) 6 ] + 13 und 98 für [Co 6 (μ 3 ‐S) 8 (PEt 3 ) 6 ] 14. Demgegenüber besitzt [Rh 6 (P i Pr 3 ) 6 (μ‐H) 12 ] 2+ nur 76 Elektronen, eine Zahl, die bei späten Übergangsmetallen ohne Beispiel ist und bei frühen Übergangsmetallen nur von Halogenidclustern wie [M 6 (μ‐Cl) 12 Cl 6 ] 4− (M=Nb, Ta; ebenfalls 76 Elektronen) erreicht wird.…”

Section: Methodsunclassified

Ein oktaedrischer Rhodium‐Cluster mit sechs Phosphan‐ und zwölf Hydridliganden, dem zehn Elektronen fehlen

Dyson

McIndoe

2004

Angewandte Chemie

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Electron Counts for Face-Bridged Octahedral Transition Metal Clusters

Cited by 21 publications

References 25 publications

Approaching the Gas-Phase Structures of [AgS8]+ and [AgS16]+ in the Solid State

Approaching the Gas-Phase Structures of [AgS8]+ and [AgS16]+ in the Solid State

Density Functional Theory Calculations and Exploration of a Possible Mechanism of N₂ Reduction by Nitrogenase

Ein oktaedrischer Rhodium‐Cluster mit sechs Phosphan‐ und zwölf Hydridliganden, dem zehn Elektronen fehlen

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Electron Counts for Face-Bridged Octahedral Transition Metal Clusters

Cited by 21 publications

References 25 publications

Approaching the Gas-Phase Structures of [AgS8]+ and [AgS16]+ in the Solid State

Approaching the Gas-Phase Structures of [AgS8]+ and [AgS16]+ in the Solid State

Density Functional Theory Calculations and Exploration of a Possible Mechanism of N2 Reduction by Nitrogenase

Ein oktaedrischer Rhodium‐Cluster mit sechs Phosphan‐ und zwölf Hydridliganden, dem zehn Elektronen fehlen

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Density Functional Theory Calculations and Exploration of a Possible Mechanism of N₂ Reduction by Nitrogenase