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Terms to know Tellurium • • • • • • • • • • Symbol: Te Atomic Number: 52 Atomic weight: 127.60 GROUP: 16 the Chalcogens [Kr].4d10.5s2.5p4 Melting point [/K]: 722.66 Boiling point [/K]: 1261 solid at 298 K Colour: silvery lustrous grey Oxidation numbers: 2,4,6 History • Ligand: A Lewis base that bonds to a metal ion to form a complex ion • Lewis Base: A species that can form a covalent bond by donating an electron pair • Lewis Acid: A species that can form a covalent bond by accepting an electron pair • Trigonal Bipyriamidal: Five atoms (or four and a lone electron pair) are bonded to a central atom • Octahedral: The geometry of a molecule in which six vertices or a regular octahedron with the central atom at the centre • Dimer: a molecule formed by the joining together of two identical molecules Discovered in 1783 by Franz Joseph Muller von Reichenstein in Romainia. It was named by M. Klanroth The origin of its name: From the Latin word "tellus" meaning "earth“ Uses • • • • • semiconductors alloying with cast iron, copper and stainless steel addition to lead to prevent corrosion Ceramics Tinting Glass Synthesis and Crystal structure of the monomeric organotellurium(IV) trihalides: trans-2-ethoxy-cyclohexyltellurium(IV), trichloro,(2-chlorobicyclo[2.2.1]hept-7yl)-λ4-tellurane, and mesityltellurium(IV) tribromide Focus of the Article • To FIND the conformation of three different organotellurium trihalides compounds • To COMPARE the conformation with that predicted by VSEPR • To EXPLAIN why and when deviations from VSEPR theory occur and how to avoid them By Dainis Dakternieks, Jenny O’Connell, Edward R.T. Tiekink 1 Organometallic Synthesis Of Tellurium • Organometallic compounds contain carbon-metallic bonds. • Some more familiar Organometallic compounds are the organolithium and organomagnesium compounds (Grignard reagents) • Tellurium is capable of forming organometallic compounds because of its’ ability to act as a Lewis acid • Three different organotellurium compounds have been synthesis and their molecular and crystal structure has been determined through crystallography and compared with the predicted VSEPR structures. Not All Organotellurium compounds have Trigonal bipyramidal structure The most common organometallics are those with the molecular formula R2TeX2 and RTeX3 R=organic group X= Halide (Br, Cl) R2TeX2 has the predicted structure trigonal bipyrimidal RTeX3 have been found to have octahedral geometry due to intermolecular and intramolecular attractions. The Expected Conformation • Tellurium like other atoms in the 16th group has six valence electrons. Organotellurium Trihalides RTeX3 • Tellurium is capable of bonding to four ligands Intermolecular secondary bonding interactions • The tellurium atom has four of it’s six valence electrons participating in bonds, leaving one lone pair. • VSEPR would give the structure as trigonal bipyramidal with the lone pair of electrons in the equatorial position. Oligomeric or polymeric Octahedral geometries Secondary intramolecular interactions monomeric Pseudo octahedral Trigonal bipyramidal Geometry 2 The synthesis • The authors of the article prepared three different organotellurium trihalides compounds • 1) Trans-2-ethoxy-cyclohexyltellurium(IV) trichloride Characterization • All were characterized using H-NMR as well as crystallography • the crystallography was preformed at room temperature and yielded the bond lengths of the compounds as well as the angles. • 2) Trichloro(2-chlorobicyclo[2.2.1]-hept-7-yl)-λ4tellurane • 3) Mesityltellurium(IV) tribromide 1) Refluxing Cl4Te in dried ethanol with cyclohexene for 2hrs, filter and recrystallize with petroleum spirits. 2) Dissolve norbornylene and Cl4Te and CCl4,reflux for 3hrs, recrystallize from hot CCl4. 3) Make a solution of Br , CH2Cl2 and dimesityl ditelluride, when reaction is complete rotovap off excess Br and CH2Cl2. Results It was found that two of the compounds synthesized (1 and 2) had very similar geometries due to intramolecular interactions. The geometry was found to be typical of organotellurium trihalide molecules Compound 3 was to have a structure different from the first two compounds because of a lack of intramolecular interactions. 3 The Compounds Compound 1 C8H15Cl3O Te 361.2 g/mol Molecular Formula Molecular mass Crystal size 0.10x0.16x 0.32 mm Density 1.866 g/cm3 Compound Compound 2 3 C7H10Cl4Te C9H11Br3Te 363.6 g/mol 486.5 g/mol 0.03x0.15x 0.30 mm 2.227 g/cm3 0.03x0.03x 0.08 mm 2.497 g/cm3 • The S,S, isomer is only capable of forming loosely attracted dimers between the tellurium atom and the oxygen atom. • The difference in length is caused by the orientation of the lone pair of electrons out toward one of the chlorides causing the bond length in the S,S isomer to length. • The other two bonds are not effected by the arrangement of the lone pair and thus are shorter then the bond lengths on the R,R, isomer Trans-2-ethoxy-cyclohexyltellurium(IV) trichloride • The geometry of compound 1 was found to be somewhere between Trigonal bipyramidal and octahedral as suggested earlier • Has two isomers an S,S isomer and an R,R isomer • The major difference between these two isomers noted in the article was the length of the Te---Cl bonds. • The explanation for this arises when the ability of the molecule to form a dimer or a chains examined. • It was found that the R,R isomer can form a chain with intermolecular interactions between the Tellurium atom and the chlorides on other molecules. • The formation of a chain lengths two of the Te—Cl bonds as the Cl is pulled toward the Te atom on the other molecule Trichloro(2-chlorobicyclo[2.2.1]-hept-7-yl)-λ4-tellurane Found to have similar geometry to compound 1Somewhere between trigonal bipyramidal and octahedral The major intramolecular interaction is between the Te atom and the chloride atom on the norbonyl group. Which forms a quasi bond Like compound 1 compound two is capable of forming a loose dimer between two molecules 4 Conclusions Mesityltellurium(IV) tribromide • In this final compound there are no significant inter- or intramolecular interactions • The conformation of the molecule matches that predicted by VSEPR theory because of the lack of interactions • This anomaly for organotellurium trihalides arises because of the halide group bonded to the tellurium • As well as the lack of electronegative species on the organic group • The conclusion in the article primarily surround the fact that the it was found that the Te atom prefers a pseudo-octahedral conformation (5 bonds and one lone pair). • This can be achieve through inter or intramolecular interactions • This tendency can be weakened by the use of bulky organic groups like the one used in compound three, however some intermolecular interactions still may occur • The use of a halide that will weaken the Lewis acidity of the tellurium atom will further prevent intermolecular interaction, leading to the formation of molecular species with conformations consistent with the VSEPR theory Why?? • The Br group on the Tellurium reduces the Tellurium’s Lewis acidity making it less willing to interact with Lewis bases • The molecule is capable of forming very loosely associated dimers which cause some deviation in the Te---Br bond lengths. • Organometallic compounds are particularly important to the formation of new bonds. • Finding new compounds that can participate in similar reactions is very important • Especially if we are able to direct the stereochemistry of the new compounds being formed. • This would be useful especially for pharmaceutical research 5