The seesaw geometry occurs when a molecule has a steric number of 5, with the central atom being bonded to 4 other atoms and 1 lone pair (AX4E1 in AXE notation). An atom bonded to 5 other atoms (and no lone pairs) forms a trigonal bipyramid with two axial and three equatorial positions, but in the seesaw geometry one of the atoms is replaced by a lone pair of electrons, which is always in an equatorial position. This is true because the lone pair occupies more space near the central atom (A) than does a bonding pair of electrons. An equatorial lone pair is repelled by only two bonding pairs at 90°, whereas a hypothetical axial lone pair would be repelled by three bonding pairs at 90° which would make it stable. Repulsion by bonding pairs at 120° is much smaller and less important.[2][1]
Compounds with disphenoidal geometry (See-Saw Geometry) have two types of ligands: axial and equatorial. The axial pair lie along a common bond axis so that are related by a bond angle of 180°. The equatorial pair of ligands is situated in a plane orthogonal to the axis of the axial pair. Typically the bond distance to the axial ligands is longer than to the equatorial ligands. The ideal angle between the axial ligands and the equatorial ligands is 90°; whereas the ideal angle between the two equatorial ligands themselves is 120°.
Disphenoidal molecules, like trigonal bipyramidal ones, are subject to Berry pseudorotation in which the axial ligands move to equatorial positions and vice versa. This exchange of positions results in similar time-averaged environments for the two types of ligands. Thus, the 19F NMR spectrum of SF4 (like that of PF5) consists of single resonance near room temperature.[3] The four atoms in motion act as a lever about the central atom; for example, the four fluorine atoms of sulfur tetrafluoride rotate around the sulfur atom.[4]
Examples
Sulfur tetrafluoride is the premier example of a molecule with the disphenoidal molecular geometry (see image at upper right). The following compounds and ions have disphenoidal geometry:[5]
^ abShields, Shawn P. "The Trigonal Bipyramid Geometry (SN = 5) (VSEPR Part 4)" (PDF). Chemistry. Retrieved 3 April 2022. SN = 5 with One Lone Pair. Molecular Shape: Sawhorse or Seesaw
^Miessler, Gary L.; Tarr, Donald A. (1999). Inorganic Chemistry (2nd ed.). Prentice-Hall. pp. 58–59. ISBN0-13-841891-8.
^Klemperer, Walter G.; Krieger, Jeanne K.; McCreary, Michael D.; Muetterties, E. L.; Traficante, Daniel D.; Whitesides, George M. (1975). "Dynamic nuclear magnetic resonance study of fluorine exchange in liquid sulfur tetrafluoride". J. Am. Chem. Soc. 97 (24): 7023–7030. doi:10.1021/ja00857a012.
^Cass, Marion E.; Hii, King Kuok (Mimi); Rzepa, Henry S. (1 February 2006). "Mechanisms that Interchange Axial and Equatorial Atoms in Fluxional processes: Illustration of the Berry Pseudorotation, the Turnstile and the Lever Mechanisms via animation of transition state normal vibrational modes". Journal of Chemical Education. 83 (2): 336. Bibcode:2006JChEd..83..336C. doi:10.1021/ed083p336.2. Manuscript submitted to the Journal of Chemical Education on April 21, 2005
^Seesaw Molecular Geometry, AX4E1 25 Dec 2007 November 3, 2014, at the Wayback Machine
seesaw, molecular, geometry, disphenoidal, seesaw, also, known, sawhorse, type, molecular, geometry, where, there, four, bonds, central, atom, with, overall, molecular, symmetry, name, seesaw, comes, from, observation, that, looks, like, playground, seesaw, mo. Disphenoidal or seesaw also known as sawhorse 1 is a type of molecular geometry where there are four bonds to a central atom with overall C2v molecular symmetry The name seesaw comes from the observation that it looks like a playground seesaw Most commonly four bonds to a central atom result in tetrahedral or less commonly square planar geometry Seesaw molecular geometryExamplesSF4Point groupC2vCoordination number4Bond angle s Ideal ax ax 180 eq eq 120 ax eq 90 SF4 ax ax 173 1 eq eq 101 6 m Polarity gt 0The seesaw geometry occurs when a molecule has a steric number of 5 with the central atom being bonded to 4 other atoms and 1 lone pair AX4E1 in AXE notation An atom bonded to 5 other atoms and no lone pairs forms a trigonal bipyramid with two axial and three equatorial positions but in the seesaw geometry one of the atoms is replaced by a lone pair of electrons which is always in an equatorial position This is true because the lone pair occupies more space near the central atom A than does a bonding pair of electrons An equatorial lone pair is repelled by only two bonding pairs at 90 whereas a hypothetical axial lone pair would be repelled by three bonding pairs at 90 which would make it stable Repulsion by bonding pairs at 120 is much smaller and less important 2 1 Contents 1 Structure 2 Examples 3 See also 4 References 5 External linksStructure EditCompounds with disphenoidal geometry See Saw Geometry have two types of ligands axial and equatorial The axial pair lie along a common bond axis so that are related by a bond angle of 180 The equatorial pair of ligands is situated in a plane orthogonal to the axis of the axial pair Typically the bond distance to the axial ligands is longer than to the equatorial ligands The ideal angle between the axial ligands and the equatorial ligands is 90 whereas the ideal angle between the two equatorial ligands themselves is 120 Disphenoidal molecules like trigonal bipyramidal ones are subject to Berry pseudorotation in which the axial ligands move to equatorial positions and vice versa This exchange of positions results in similar time averaged environments for the two types of ligands Thus the 19F NMR spectrum of SF4 like that of PF5 consists of single resonance near room temperature 3 The four atoms in motion act as a lever about the central atom for example the four fluorine atoms of sulfur tetrafluoride rotate around the sulfur atom 4 Examples EditSulfur tetrafluoride is the premier example of a molecule with the disphenoidal molecular geometry see image at upper right The following compounds and ions have disphenoidal geometry 5 SF4 SeF4 IF2 O 2 IOF3 ClF 4 TeF4 XeO2F2 SCl4 AsF 4See also EditMolecular geometry AXE methodReferences Edit a b Shields Shawn P The Trigonal Bipyramid Geometry SN 5 VSEPR Part 4 PDF Chemistry Retrieved 3 April 2022 SN 5 with One Lone Pair Molecular Shape Sawhorse or Seesaw Miessler Gary L Tarr Donald A 1999 Inorganic Chemistry 2nd ed Prentice Hall pp 58 59 ISBN 0 13 841891 8 Klemperer Walter G Krieger Jeanne K McCreary Michael D Muetterties E L Traficante Daniel D Whitesides George M 1975 Dynamic nuclear magnetic resonance study of fluorine exchange in liquid sulfur tetrafluoride J Am Chem Soc 97 24 7023 7030 doi 10 1021 ja00857a012 Cass Marion E Hii King Kuok Mimi Rzepa Henry S 1 February 2006 Mechanisms that Interchange Axial and Equatorial Atoms in Fluxional processes Illustration of the Berry Pseudorotation the Turnstile and the Lever Mechanisms via animation of transition state normal vibrational modes Journal of Chemical Education 83 2 336 Bibcode 2006JChEd 83 336C doi 10 1021 ed083p336 2 Manuscript submitted to the Journal of Chemical Education on April 21 2005 Seesaw Molecular Geometry AX4E1 25 Dec 2007 Archived November 3 2014 at the Wayback MachineExternal links EditChem Chemistry Structures and 3D Molecules permanent dead link Indiana University Molecular Structure Center Interactive molecular examples for point groups Molecular Modeling Animated Trigonal Planar Visual VSEPR Retrieved from https en wikipedia org w index php title Seesaw molecular geometry amp oldid 1101895019, wikipedia, wiki, book, books, library,
