Planar six-coordinate carbon

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Compounds with planar six-coordinate carbon, which on the basis of quantum chemical calculations at the highest level were found to be suitable for experimental verification (CCSD (T) / cc-pVTZ, real energetic minima, activation barriers for rearrangement 96–147  kJ / mol for the B. 4 C 3 isomers).

Planar sixfold coordinated carbon is an extreme form of chemical bond , for which possible implementation possibilities have been identified with the help of complex quantum chemical methods.

prehistory

Carbon is usually found in four bonds , with the coordination numbers (types of coordination) four (tetrahedral; sp 3 hybridization), three (trigonal; sp 2 hybridization) or two (linear; sp hybridization) being encountered according to the concept of hybridization . With their knowledge of the tetrahedral coordination of sp 3 -hybridized carbon, van't Hoff and Joseph LeBel provided an early example of the importance of an understanding of cohesion and coordination numbers and types (see chirality ).

Investigations

There has been no lack of attempts to get beyond the known bonding situations of carbon. This includes studies on planar four-fold coordinated carbon (anti-van't Hoff-LeBel compounds) or six-fold coordinated carbon.

The experimentally verified examples of six-coordinate carbon are distinguished by the fact that the carbon is spatially surrounded by the coordination partners; i.e., is in the center of a trigonal prism or an octahedron. A planar six-fold coordination is therefore particularly challenging and should be possible if

  1. Bonding situations can be found that do not have unusual element-element bond lengths
  2. Attachment situations can be found in which no “unfavorable”, i.e. H. strongly antibonding orbitals must be occupied
  3. a stabilization through aromaticity (= cyclic delocalization) of 4n + 2 electrons is to be expected.

Using these three criteria, K. Exner and Paul von Ragué Schleyer were able to show by quantum chemical calculations that the B 6 C dianion, with a carbon surrounded in a planar manner by six boron atoms, must indeed be an observable species. The doubly protonated form of the dianion (H 2 B 6 C) and the three isoelectronic, neutral compounds B 4 C 3 , which have a planar six-fold coordinated central carbon by a B 4 C 2 ring, are also used by demanding quantum chemical calculations predicted sufficiently stable species for experimental observation.

Individual evidence

  1. ↑ Lai-Sheng Wang, Alexander I. Boldyrev, Xi Li, Jack Simons: Experimental Observation of Pentaatomic Tetracoordinate Planar Carbon-Containing Molecules . In: J. Am. Chem. Soc. 122, No. 32, 2000, pp. 7681-7687, doi : 10.1021 / ja993081b .
  2. ↑ Walter Siebert, Anuradha Gunale: Compounds Containing a planar-tetracoordinate carbon atom as analogues of planar methane . In: Chem. Soc. Rev. 1999, 28 , pp. 367-371, doi : 10.1039 / A801225C .
  3. ↑ Dirk Röttger, Gerhard Erker: Compounds Containing Planar-Tetracoordinate Carbon . In: Angew. Chem. Int. Ed. Engl. 36, No. 8, 1997, pp. 812-827, doi : 10.1002 / anie.199708121 .
  4. ↑ Robert Choukroun, Patrick Cassoux: Planar Tetracoordination of Carbon in Groups 4 and 5 Organometallic Chemistry . In: Acc. Chem. Res. 32, No. 6, 1999, pp. 494-502, doi : 10.1021 / ar970304z .
  5. ↑ Xi Li, Lai-Sheng Wang, Alexander I. Boldyrev, Jack Simons: Tetracoordinated Planar Carbon in the Al 4 C-Anion. A Combined Photoelectron Spectroscopy and ab Initio Study . In: J. Am. Chem. Soc. 121, No. 25, 1999, pp. 6033-6038, doi : 10.1021 / ja9906204 .
  6. Jump up ↑ VG Albano, M. Sansoni, P. Chini, S. Matinego: Synthesis and crystallographic characterization of the carbidopentadecacarbonylhexarhodate dianion in its bis (benzyltrimethylammonium) salt, the first example of a trigonal prismatic cluster of metal atoms . In: J. Chem. Soc., Dalton Trans. 1973, pp. 651-655, doi : 10.1039 / DT9730000651 .
  7. ↑ François P. Gabbaï, Annette Schier, Jürgen Riede, Hubert Schmid Baur: Synthesis of the hexakis [(triphenylphosphanes) gold (I)] methanium (2+) Cation from Trimethylsilyldiazomethane ; Crystal Structure Determination of the Tetrafluoroborate Salt . In: Chem. Ber. Recl. 130, No. 1, 1997, pp. 111-114, doi : 10.1002 / cber.19971300118 .
  8. ↑ K. Exner, P. v. R. Schleyer: Planar Hexacoordinate Carbon: A Viable Possibility . In: Science . 290, 2000, pp. 1937-1940, doi : 10.1126 / science.290.5498.1937 .