Порядок связи хром-хром 5 !?

korendovych · 29.11.2005 18:24:44

Совсем недавно (4 November 2005) опубликован первый пример соединения с порядком связи хром-хром - 5. (T. Nguyen et al., Science 310, 844 (2005)) Краткий синопсис (Science 310, 796 - 797) www.sciencemag.org/cgi/content/full/310/5749/796
Я могу прислать pdf, если нет доступа, хорошо бы завести на форуме файлообменник.

Что думает по поводу этой статьи широкая общественность?

korendovych · 01.12.2005 20:43:47

выложил абстракт и структуру

Although in principle transition metals can form bonds with six shared electron pairs, only quadruply bonded compounds can be isolated as stable species at room temperature. Here we show that the reduction of {Cr(µ-Cl)Ar'}2 [where Ar' indicates C6H3-2,6(C6H3-2,6-Pri2)2 and Pr indicates isopropyl] with a slight excess of potassium graphite has produced a stable compound with fivefold chromium-chromium (Cr–Cr) bonding. The very air- and moisture-sensitive dark red crystals of Ar'CrCrAr' were isolated with greater than 40% yield. X-ray diffraction revealed a Cr–Cr bond length of 1.8351(4) angstroms (where the number in parentheses indicates the standard deviation) and a planar transbent core geometry. These data, the structure's temperature-independent paramagnetism, and computational studies support the sharing of five electron pairs in five bonding molecular orbitals between two 3d5 chromium(I) ions.

Відредаговано korendovych (01.12.2005 21:10:59)

a_volkov · 04.12.2005 03:56:57

Отвечю, воспользуясь словом из лексикона Эллочки-Людоедочки из "12 стульев":

Жуть!

korendovych · 04.12.2005 04:30:40

Если считать электроны, как учит Анатолий Иванович, то получается Cr-----Cr 10e, Ar - 2e, а еще 6 электронов откуда? Естественно предположить, что с эта-6-ароматической системы, как раз 6. Но они об этом как пленные партизаны. Зато пишут "There is also a weaker interaction between each Cr ion Cr(1)–C(7A) 2.2943(9) А and the ipso carbon C(7) or C(7A) of a flanking ring of the terphenyl group attached to the other Cr." И еще из их обьяснений непонятно, почему угол хром-хром-углерод не 180 градусов. Если учитывать ета-6, то тогда еще куда ни шло.

Но в общем и правда интересно, по Эллочкиной терминологии "шикарно, парниша". smile

vincent · 12.01.2006 11:25:50

Очень интересно. Иван, если сможешь перешли статью. Обьяснения по поводу 5-кратной связи, на сколько я понял, довольно туманны
P.S. Давно хотел тебе написать, но свободного времени было мало.Сейчас полегче. Как насчет партии в шахматы?

Відредаговано vincent (12.01.2006 12:09:01)

korendovych · 12.01.2006 22:10:03

Статью я выложил сюда, через пару недель удалю:

ase.tufts.edu/chemistry/rybak/articles/844.pdf

Кратность связи - понятие вообще довольно сложное и неоднозначное, как впрочем и сама хим. связь.

korendovych · 14.05.2006 05:10:03

А вот и продолжение: обоснование почему пятерная связь должна быть изогнутой.

J. Am. Chem. Soc. 2006, 128, 7335

Origin of Trans-Bent Geometries in Maximally Bonded Transition Metal and Main Group Molecules

Clark R. Landis* and Frank Weinhold

Recent crystallographic data unambiguously demonstrate that neither Ar'GeGeAr' nor Ar'CrCrAr' molecules adopt the expected linear (VSEPR-like) geometries. Does the adoption of trans-bent geometries indicate that Ar'MMAr' molecules are not "maximally bonded" (i.e., bond order of three for M = Ge and five for M = Cr)? We employ theoretical hybrid density functional (B3LYP/6-311++G**) computations and natural bond orbital-based analysis to quantify molecular bond orders and to elucidate the electronic origin of such unintuitive structures. Resonance structures based on quintuple M-M bonding dominate for the transition metal compounds, especially for molybdenum and tungsten. For the main group, M-M bonding consists of three shared electron pairs, except for M = Pb. For both d- and p-block compounds, the M-M bond orders are reflected in torsional barriers, bond-antibond splittings, and heats of hydrogenation in a qualitatively intuitive way. Trans-bent structures arise primarily from hybridization tendencies that yield the strongest -bonds. For transition metals, the strong tendency toward sd-hybridization in making covalent bonds naturally results in bent ligand arrangements about the metal. In the p-block, hybridization tendencies favor high p-character, with increasing avidity as one moves down the Group 14 column, and nonlinear structures result. In both the p-block and the d-block, bonding schemes have easily identifiable Lewis-like character but adopt somewhat unconventional orbital interactions. For more common metal-metal multiply bonded compounds such as [Re2Cl8]2-, the core Lewis-like fragment [Re2Cl4]2+ is modified by four hypervalent three-center/four-electron additions.

Відредаговано korendovych (04.04.2007 00:17:46)

korendovych · 04.04.2007 00:19:17

Roos and Gagliardi, along with Antonio C. Borin of the University of São Paulo, in Brazil, now report the results of an even broader computational study to show that the maximum multiplicity of a covalent chemical bond between any two elements is six and that the tungsten dimer (W2) has the distinction of being most likely to have the strongest sextuple bond (Angew. Chem. Int. Ed., DOI: 10.1002/anie.200603600). Some chemists had already recognized that in transition metals the s and d orbitals could combine to support a sextuple bond in a naked diatomic molecule—a compound in which there are no ligands to tie up a bonding orbital. A few sextuple-bonded compounds with a very short bond length, such as Cr2, have been trapped in the gas phase at low temperature and observed by laser-induced fluorescence spectroscopy.

It turns out the number of available valence electrons and available orbitals leads to a maximum bond order of 6.0 for only a few elements—the group 6 transition metals (chromium, molybdenum, and tungsten) and uranium. Among sextuple-bonded dimers made from these elements, Mo2 and W2 have the highest calculated actual bond order of 5.2. The W2 dimer claims the highest bond energy of the calculated sextuple bonds. The researchers attribute the higher energy to decreased electron repulsion in the 5d orbitals of tungsten, which are more diffuse than the 3d and 4d orbitals of chromium and molybdenum.

korendovych · 29.12.2007 00:58:08

Гонка за пикометры продолжается smile Установлен новый рекорд: 1.8028(9) А.

www.rsc.org/chemistryworld/News/2007/No … 110701.asp

Відредаговано korendovych (29.12.2007 01:01:32)

korendovych · 29.08.2008 02:37:19

не прошло и года как...

www.nature.com/nchem/reshigh/2008/0808/ … em.54.html

Nature Chemistry
Published online: 22 August 2008 | doi:10.1038/nchem.54

Metal–metal bonds: Shorter and shorter
Neil Withers

Abstract
Two dichromium compounds with the shortest intermetallic bond distances yet have been made with nitrogen ligands

Bonding is at the very heart of chemistry and understanding the complexities and breadth of bonds is of great interest to a variety of chemists. Extremely high-order bonds have been observed to exist between metal atoms: quadruple- and quintuple-bonded species, for example. Although quintuple bonds have been predicted theoretically for several metals, only chromium atoms have been observed experimentally to share five electron pairs in five bonding orbitals. Now, groups based in Taiwan and Germany have independently prepared dichromium(I) complexes with the shortest reported metal–metal bond distances.

Yi-Chou Tsai and Chia-Wu Hsu at the National Tsing Hua University, in collaboration with three other Taiwanese universities, have made two coordination compounds that boast quintuply bonded chromium atoms. The first contains two chromium atoms only 1.82 Å apart, surrounded by three amidinate ligands. Density functional theory calculations confirmed the fivefold bonding between the metal atoms. This can be reduced by potassium graphite (KC8) to give a second complex with an even shorter Cr–Cr distance: 1.74 Å. This is the first metal–metal bond shorter than 1.8 Å.

Meanwhile, Rhett Kempe and Awal Noor at the University of Bayreuth, working with Frank Wagner at the Max Planck Institute for Chemical Physics of Solids in Dresden, have reduced chromium(II) compounds to create a dichromium(I) complex with an intermetallic distance of 1.75 Å. Interestingly, this contains only two coordinating ligands, in this case amidopyridinato groups. Although this also has a formal bond order of five, calculations involving electron density and the electron localizability indicator suggested a bond order of only 4.2. Kempe and colleagues attribute this to the repulsive nature of the 4s–4s bond at such short distances.

Both groups' complexes contain dichromium stabilized by nitrogen ligands, as was the case for the diazadiene dichromium compound that was previously the record holder at 1.80 Å. Although it is clear that the ligands have a crucial role in creating the conditions for such short bonds to exist, how exactly they do so is less clear. For example, other amidinate dichromium complexes, similar to those made by Tsai and colleagues, have much longer Cr–Cr bond lengths, up to 2.61 Å. Kempe and colleagues have ruled out weak coordination as an explanation for the strong bonding between the chromium atoms, because their ligand is strongly bonded to the metal atoms.

Відредаговано korendovych (29.08.2008 02:39:10)

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Оголошення

#1 29.11.2005 18:24:44

Порядок связи хром-хром 5 !?

#2 01.12.2005 20:43:47

Re: Порядок связи хром-хром 5 !?

#3 04.12.2005 03:56:57

Re: Порядок связи хром-хром 5 !?

#4 04.12.2005 04:30:40

Re: Порядок связи хром-хром 5 !?

#5 12.01.2006 11:25:50

Re: Порядок связи хром-хром 5 !?

#6 12.01.2006 22:10:03

Re: Порядок связи хром-хром 5 !?

#7 14.05.2006 05:10:03

Re: Порядок связи хром-хром 5 !?

#8 04.04.2007 00:19:17

Re: Порядок связи хром-хром 5 !?

#9 29.12.2007 00:58:08

Re: Порядок связи хром-хром 5 !?

#10 29.08.2008 02:37:19

Re: Порядок связи хром-хром 5 !?

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