Organic compounds comprising one or more R1C2R11 groups wherein R1 and R11 are the same or different and each represents hydrogen, halogen or an alkyl, aryl, cycloaliphatic, alkoxy or carboalkoxy group or a silyl group (see Division C3) and C2 represents a carbon-to-carbon bonding are prepared by reacting an organometallic complex, as defined below, with a hydrogenating agent, a halogenating agent, an oxidising agent, an alkylating agent, an alkyne, an element of Group VIa of the Periodic Table (as quoted on pages 392 and 393 of The Handbook of Chemistry and Physics, 37th edition, published by the Chemical Rubber Publishing Co.) or a reactive compound as defined below. The term "organo-metallic complex" is defined as a complex having the general formula Mx(CO)y (R1C2R11)z wherein M represents iron, nickel, ruthenium, rhodium, palladium, osmium, iridium, platinum, manganese, rhenium, chromium, molybdenum, or tungsten, CO represents a carbonyl group bonded to the said metal, to the R1C2R11 group, or to both, R1C2R11 is a group as defined above, x is from 1 to 4, y is from 1 to 10, z is from 1 to 6, prepared by the method described and claimed in Specification 885,514. The expression "reactive compound" is defined as a compound selected from nitrosyl chloride, nitrosobenzene, N,N-dichloro-p-toluenesulphonamide, phenyldichlorophosphine, benzyldichlorophosphine, benzyldibromophosphine, phenyldichloroarsine, polysulphides, mono- and di-halogeno-phosphines, -arsines or -stibines, polyselenides, sulphur dioxide, potassium selenocyanide (KSeCN), sulphur dichloride and triphenylphosphine selenide. Complexes in which the carbonyl group bonded to the metal is partially replaced by a nitronyl, a substituted stibine, a substituted arsine, or a substituted phosphine may also be used. Numerous suitable complexes and reactants of the above types are specified, some of the reactants falling into more than one of the specified classes. Many types of compound which may be obtained by the process of the invention are specified including ketones, phenols, esters, ethers, sulphonamides, halogen compounds and heterocyclic compounds containing nitrogen, phosphorus, arsenic, oxygen, sulphur or selenium as the hetero-atom. Some of the compounds produced are stated to be novel, in particular, 2,5-dimethyl-3,4-diphenylthiophene, tetraphenylselenophene, pentaphenylphosphole, P-benzyl-tetraphenylphosphole, pentaphenylarsole, tetraethylselenophene, tetraphenyl-g -pyrone tetraphenyl - 1 - thia - 4 - pyrone, tetraphenyl - 1 - selena-4-pyrone, 1,4-bis-(p-toluene-sulphonamido) 1, 2, 3, 4-tetraphenylbutadiene, 1,4-bis-(p-toluene-sulphonamido) 1, 4-dimethoxybutane, 1, 2, 3, 4-tetraphenyl-5, 6-bis-(p-chlorophenyl)-benzene and compounds of the formulae: <;FORM:0966131/C1/1>; and <;FORM:0966131/C1/2>; wherein the groups R may be identical or different and may take any of the values given above for R1 and R11, and wherein X represents phosphorus or arsenic. Examples are given. The examples also disclose the following reactions: (6) the isomerisation of cis-dibenzoyl-stilbene to the trans-isomer; (17) conversion of pentaphenylphosphole to the corresponding P-oxide, -sulphide or -selenide; (19) reaction of benzyldichlorophosphine with 1,4-dilithio-1,2,3,4-tetraphenylbutadiene to give P-benzyl-tetraphenyl-phosphole; (20) reaction of phenyldichloroarsine with 1,4-dilithio-1,2,3,4-tetraphenylbutadiene to give pentaphenylarsole; (23) heating hexyne-3 with sulphur to give tetraethylthiophene; (25) heating hexyne-3 with potassium polyselenide to give tetraethylselenophene. The compounds of formula (1) above may be converted to the corresponding phosphole or arsole oxides, sulphides or selenides or may undergo Diels-Alder reactions, for example with maleic anhydride. The compounds of formula (2) above, or the corresponding compounds in which the hetero-atom is a Group VIa (as defined above) other than selenium, may be reacted with ammonia to give g -pyridones. Some of the products of the process of the invention are stated to be useful as insecticides, fluorescent materials, chelating agents and polar solvents, and in the production of polymers. The Provisional Specification refers also to the use of complexes of the above type in which the metal is cobalt.ALSO:Organo-silicon compounds are prepared by reacting an organo-metallic complex, as defined below, with a silicon compound such as phenyltrimethylsilyl acetylene and bis-(trimethylsilyl) acetylene. The term "organo-metallic complex" is defined as a complex having the general formula Mx(CO)y(R1C2R11)z wherein M represents iron, nickel, ruthenium, rhodium, palladium, osmium, iridium, platinum, manganese, rhenium, chromium, molybdenum or tungsten, CO represents a carbonyl group bonded to the said metal, to the R1C2R11 group, or to both, R1C2R11 is a hydrocarbon group, x is from 1 to 4, y is from 1 to 10, z is from 1 to 6, prepared by the method described and claimed in Specification 885,514. Products of the formulae: <;FORM:0966131/C3/1>; wherein X represents phosphorus or arsenic and the groups R may be identical or different and may be a silyl group, are stated to be novel. Examples 19 and 20 of the Provisional Specification describe the preparation of 1,3-bis-(trimethylsilyl) - 2, 4, 5, 6 - tetraphenylbenzene, 1, 3, 5 - tris - (trimethylsilyl) - 2, 4, 6 - triphenylbenzene, trimethylsilyl - triphenyl - cyclopentadienones, 2, 5 - bis - (trimethylsilyl) - 3, 4-diphenylcyclopentadienone and 2, 4 - bis - (trimethylsilyl) - 3, 5 - diphenylcyclopentadienone, using complexes of the above type in which the metal is cobalt.ALSO:Hydrocarbons comprising one or more R1C2R11 groups wherein R1 and R11 ar the same or different and each represents hydrogen or an alkyl, aryl or cycloaliphatic group and C2 represents a carbon-to-carbon bonding are prepared by reacting an organo-metallic complex, as defined below, with a hydrogenating agent or an alkyne. The term "organo-metallic complex" comprises complexes having the general formula Mx(CO)y(R1C2R11)z wherein M represents iron, nickel, ruthenium, rhodium, palladium, osmium, iridium, platinum, manganese, rhenium, chromium, molybdenum or tungsten, CO represents a carbonyl group bonded to the said metal, to the R1C2R11 group, or to both, R1C2R11 is a group as defined above, x is from 1 to 4, y is from 1 to 10, z is from 1 to 6, prepared by the method described and claimed in Specification 885,514. Complexes in which the carbonyl group bonded to the metal is partially replaced by a nitrosyl, a substituted stibine, a substituted arsine, or a substituted phosphine may also be used. Numerous suitable complexes and reactants of the above types are specified. Examples describe the preparation of 1, 2, 3, 4-tetraphenyl-butadiene, 1,2,3,4-tetraphenylbut - 1 - ene, 1,2,3,4 - tetraphenylbutane, 1,2,3,4 - tetraphenylbutane, dibenzyl, stilbene, and an unidentified compound of the formula (C6H5.C2.C6H5)3H2, from complexes containing tetraphenylacetylene by reduction with lithium aluminium hydride or sodium and ethanol in liquid ammonia and of 1,2,3,4-tetraphenylbenzene and hexaphenylbenzene by reacting diphenyl-acetylene with Fe(CO)4(C6H5C2H)2. The Provisional Specification also refers to the use of complexes of the above type in which the metal is cobalt.;FORM:0966131/C3/1>;FORM:0966131/C1/2>;FORM:0966131/C1/1>