Page 1 : 40 CHEMISTRY FOR DEGREE STUDENTS — ELECTIVE, , , , , , , , , , , , , , , , , , , , Transition metals can also form metal-carbon o bonds with organic groups althou, unstable. They can easily decompose even at room temperature. For example, the alkyl one, of transition metals such as Ti (CH,),, Nb CH,);, W (CH), are well known but they are ative, and explode even below room temperature. The stability of the y-bonded covalent co eae, increases if some additional x-bonding ligands are also present. For example, the Presence Of |i, such as PR,, CO, amines, etc, increases the stability of these compounds, For example, tetr: "Bands, titanium undergoes spontaneous decomposition violently at —23° C but the addition Of dinys:, , or diamine increases its stability and it does not decompose up to room temperature, Tide, Factors responsible for kinetic instability of transition metal, , sigma bonded organo, compounds, , The following factors contribute to kinetic instability of organometallic ComPounds containing, metal-alkyl bond:, , (4) Formation of reactive species due to cleavage. The cleavage of metal-carbon bond in, transition metal alkyls give reactive alkyl radicals or ions. The reactive species formed react with, each other or solvent to give chain reactions. This behaviour is different from that Of the other, inorganic metal-ligand bonds such as M OH,, M-NR,, M — X (X = halogen), etc. where a cle aaa, leads to relatively less reactive species like H,O, R,N~ or X~ respectively. ‘, , (ii) Cleavage products form strong bonds. In case the cleavage of metal-carhor, occurs, they combine among themselves to form strong bonds such as C—C, C—O, a, (X = halogen) etc. depending upon the nature of organometallic compound. During the, , of new bonds, the cleavage reaction is highly exothermic and is thermodynamically fay, example, metal-alkyl bond undergoes cleavage as, , M—R —> M + R, Let, Reactive, Intermediates, The formation of alkyl radical leads to subsequent reactions., , (iii) Hydrogen transfer from alkyl to metal. The higher alkyl organomet:, particularly those which carry -hydrogen are unstable. B-hydride elimination reacti, , opportunity for the decomposition of metal alkyls into a metal hydride and olefin. |, reaction may be represented as, , , , , , , , bn. In higher alkyl, rogen present on carbon, i!, , compound into, 3). This is called — Fig, 2.3.

Page 2 : ORGANOMETALLIC COMPOUNDS 41, , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , ages) DRC limination i i i, ‘ a, ine for the instability of alkyl organometallic compounds., Ts contain ee reaction can bring about stability in metal alkyls. Alkyls, ie hydrogen atom on f-carbon are therefore st ne, r . a e stable. For le,, a M-CH, CH;, M— CH, - Ar, M CH, — NR, etc. ae, ; (ii) Chelation. The stability of a m i, f etal-carbon sigma bond can be enh, @ : ' i enhanced by chelz, through a terminal group can bring about chelation. ao, , i Increase in coordinatio r. TI ili f, (iii) ii nm number. The stability of metal-alkyl compound can be raised, ing the negative ch ( sei ) T m, th large on carbon \M C}. Thus aromatic compounds having metal, rbon linkage are relative! : ‘, a ble than M—CH,. y more stable. For example M—C,H, is, , .., aa bon Multiple Bonding, , is increased. For example, carbenes, = CR, and carbynes, = CR, and triple bonds with the metal atom. It has been observed, compounds, the metal-carbon bond is somewhat shorter, al metal-alkyl o bond but larger than metal-carbon double, is suggests that the bond between metal-carbon in carbenes is, , , , Emil Fischer, , en single and double bond, Emil Fischer (1852-1919) was, , » first carbene complex (CO), W[: C(R) OMe)] was # German Chemist and 1902, recipient of the Nobel Prize in, , d by Emil. Fisher in 1964. It has been found to have :, Chemistry. He development, , arrangement with one of the positions occupied by the Fischer projection, a symbolic, way of Fischer's esterification, , CR (OMe)., . d synthesis of Hic, carbynes have triple bonds. For example, (CO), M (= CR)X pa |, , = Cr, Mo or W, R= Me, Et or Ph, X = Cl, Br, 1). The crystal, CR group occupies position trans to halo ligand, , © of these types of complexes shows that =, geometry (Fig. 2.4). CR, groups form complexes with metals of groups OG----[ff-----755 co, compounds considered here are those of lithium ff a Ltd, , magnesium (group 2) and aluminium (group 13). a z. i, : Oc: pene co, , LITHIUM COMPOUNDS, , e reagents Xx, ganic and pip. 9.4, structure of (CO), M (-CRIX, °, , anolithium compounds are versatil, ynthesis of a wide variety of inor,, , 4s. These compounds are particularly used ., These compounds are generally polymeric in nature and are soluble in, , um compounds can be prepared as follows:, , direct reaction of Li with alkyl or arylchloride in benzene or petroleum., ba (R= alkyl or ary!), , 2Li+ RCI Solve» LiR + LiCl, , aLi+C,H Cl > CO) Hs Lit bi, , 1 eg

Page 5 : a TIVE, 44 CHEMISTRY FOR DEGREE STUDENTS ELEC, , Structure. This compound offers an example of multicentred bonding. This happens, when carbon is linked to two or more metal atoms. Lithium alkyls are polymeric and generally, exist as tetrameric units such as (Li CH,), (Fig., 2.5). In lithium methyl there is a tetrahedral, , set of four lithium atoms with a methyl group, placed symmetrically above each Li, face. The, Structure clearly involves a methyl group bridging three lithium atoms, The C-atoms have coordination no, 7 being bonded directly to three, H and four Li atoms, The Li-C distance is 231, pm. The Li-Li distance in the structure is 268, pm which is identical with the value of 267.3 a HH, Pm for a gaseous Li, molecule and smaller than Fig. 2.5. Structure of (LiMe), Methy) lithium, the value of 304 pm in Li metal, (Multicentred bonding), , , , , , , , , , , , , , , , Applications of Organolithium Compounds, The important applications of organolithium compounds are given below:, , "9 diy could combine with, Mth an ong,