Syllabus. Definition. Classification based on the nature of Metal-Carbon bond. Metal carbonyls

Transcription

1 Organometallic Chemistry

2 Syllabus Definition Classification based on the nature of Metal-Carbon bond 18 electron rule Metal carbonyls Mononuclear and polynuclear carbonyls (give examples of Fe, Co, Ni)

3 Introduction Organometallic chemistry, the chemistry of compounds containing metal-carbon bonds, it encompasses a wide variety of chemical compounds and their reactions, including compounds containing both σ and π bonds between metal atoms and carbon; many cluster compounds, containing one or more metal-metal bonds Aside from their intrinsically interesting nature, many organometallic compounds form useful catalysts and consequently are of significant industrial interest

4 [Cr(H 2 O) 6 ] is a coordination complex Cr(CO) 6 is an organometallic compound which involve metal-carbon bond Both octahedral, CO and H 2 O are σ donor ligands; in addition, CO is a strong π acceptor Cyclic organic ligands containing delocalized π systems can team up with metal atoms to form sandwich compounds which also involve metal-carbon bond

5 The first organometallic compound to be reported was synthesized in 1827 by Zeise, it is an ionic compound (Zeise's salt) of formula K[Pt(C 2 H 4 )Cl 3 ]H 2 O In 1890, Mond reported the preparation of Ni(CO) 4, Reactions between magnesium and alkyl halides, carried by Grignard in 1899 led to the synthesis of alkyl magnesium complexes now known as Grignard reagents Kealy and Pauson reacted the Grignard reagent cyclo-c S H 5 MgBr with FeC 3, this reaction yield an orange solid of formula (C 5 H 5 ) 2 Fe, ferrocene

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7 Classification based on the nature of Metal-Carbon bond

8 The following five types of organometallic compounds can be distinguished depending upon the nature of metal-carbon bond; [1] Ionic organometallic compounds [2] Organometallic compounds containing metal-carbon sigma bond [3] Ylides [4] Organometallic compounds with multicentre bonds [5] Organometallic compounds with pi bonded ligands

9 [1] Ionic organometallic compounds Most of the organometallic compounds of alkali metals fall in this category They have short life because of their high reactivity. Examples are Na + (CH 2 =CH-CH 2 ) -, Na + (CH 2 -C 6 H 5 ) -, Na + (C 6 H 5 ) -, Na + (C 5 H 5 ) -, etc

10 [2] Organometallic compounds containing metal-carbon sigma bond: Metallic elements of Group II, III, IV and V as well as transition metals form organometallic compounds in which the metal atoms are bonded to carbon atoms by sigma bond

11 Organometallic compounds of transition metals: Very few examples of alkyl compounds of transition metals are known because of their greater reactivity But organic ligand does not contain any β hydrogen will form stable complex, e.g., [CH 3 -CH 2 -Rh(NH 3 ) 5 ] The alkynyl compounds [M-C C-R] are more stable than alkyl or aryl complexes, the reason is that alkynyl group acts as σ donor as well as π acceptor Similar situation occurs in alkenyl compounds [M-CH=CR 2 ], some other compounds are σ cyclopentadienyl complexes containing (η 1 -C 5 H 5 )M linkage

12 [3] Ylides These are the compounds in which the metal is doubly bonded with the carbon atom of the ligand Such compounds are formed by the main group elements as well as by the transition elements The example is Wittig reagent, Ph 3 P=CH 2

13 [4] Organometallic compounds with multicentre bonds Electron-deficient organometallic compounds occur in polymeric forms fall under this category Examples are (Li-CH 3 ) 4, [Be(CH 3 ) 2 ] n, [Al(CH 3 ) 3 ] 2, etc These compounds are considered as intermediate between ionic organometallic compounds of alkali metals and σ bonded organometallic compounds of Si, Sn, Pb, etc Elements which have highest tendency to form this complex are Li, Be, Mg, B and Al

14 [5] Organometallic compounds with pi bonded ligands This category includes organometallic compounds of alkenes, alkynes and some other carbon-containing compounds having electrons in their π molecular orbitals Overlapping of these π orbitals with the vacant orbitals of the metal atom gives rise to an arrangement in which the metal atom gets bound to all the carbon atoms over which the π molecular orbital of the organic ligand is spread The most important compound of this category is ferrocene or (bis-cyclopentadienyl)iron, represented as (η 5 -C 5 H 5 ) 2 Fe It is known to have a sandwitch structure

15 Nomenclature of Organometallic Compounds The number of atoms through which a ligand bonds is indicated by the Greek letter η (eta) followed by a superscript indicating the number of ligand atoms attached to the metal For example, because the cyclopentadienyl ligands in ferrocene bond through all five atoms, they are designated η 5 -C 5 H 5 The formula of ferrocene may therefore be written (η 5 -C 5 H 5 ) 2 Fe Bridging ligands which are very common in organometallic chemistry are designated by the prefix µ, followed by a subscript indicating the number of metal atoms bridged

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17 The 18-Electron Rule

18 Inert Gas rule or 18-Electron Rule or EAN Rule A complex compound in which the central metal atom appears to have acquired the configuration of an inert gas by sharing of electrons tends to be more stable, this generalization is knownasinertgasrule The total number of electrons which the central metal atom appears to possess in the complex including those gained by it in bonding is called Effective Atomic Number of the central metal atom

19 According to this rule, the effective number of electrons in the (n-1)d, ns and np orbitals of metal (valence shell) in its complex should be equal to = 18 The total number of electrons gained by the metal through bonding plus the number of original electrons already present in (n-1)d, ns and np orbitals of metal should be equal to 18 in any of the stable complexes of the metal There are many exceptions to the 18-electron rule, but the rule nevertheless provides some useful guidelines to the chemistry of many organometallic complexes, especially those containing strong π-acceptor ligands

20 Electrons in the complex countedbydonorpairmethod) This method considers ligands to donate electron pairs to the metal To determine the total electron count, we must take into o Account the charge on each ligand and o Determine the formal oxidation state of the metal

21 Cr(C0) 6 A Cr atom has 6 electrons outside its noble gas core, each CO is considered to act as a donor of 2 electrons Cr 24 1s 2 2s 2 2p 6 3s 2 3p 6 3d 4 4s 2 Noble gas, Kr 36 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6

22 Pentahapto-C 5 H 5 (η 5 - C 5 H 5 )Fe(CO) 2 Cl is considered as C 5 H 5-, a donor of 3 electron pairs; it is a 6-electron donor As in the first example, CO is counted as a 2-electron donor Chloride is consideredcl -, a donor of 2 electrons Therefore, this complex is formally an iron(ii) complex. Iron(II) has 6 electrons beyond its noble gas core Fe 26 1s 2 2s 2 2p 6 3s 2 3p 6 3d 6 4s 2

23 Carbonyl (CO) Complexes Carbon monoxide is the most common ligand in organometallic chemistry It serves as the only ligand in binary carbonyls such as Ni(CO) 4, W(CO) 6, and Fe 2 (CO) 9 or, more commonly, in combination with other ligands, both organic and inorganic CO may bond to a single metal or it may serve as a bridge between two or more metals

24 Two features of the molecular orbitals of CO deserve attention. First, the highest energy occupied orbital (the HOMO) has its largest lobe on carbon It is through this orbital, occupied by an electron pair, that CO exerts its a-donor function, donating electron density directly toward an appropriate metal orbital (such as an unfilled d or hybrid orbital) Carbon monoxide also has two empty π* orbitals (the lowest unoccupied, or LUMO); these also have larger lobes on carbon than on oxygen.

25 A metal atom having electrons in a d orbital of suitable symmetry can donate electron density to these π* orbitals. These σ-donor and π-acceptor interactions are illustrated in Figure The overall effect is synergistic. CO can donate electron density via a σ-orbital to a metal atom; the greater the electron density on the metal, the more effectively it can return electron density to the π* orbitals of CO The net effect can be strong bonding between the metal and CO; however, as will be described later, the strength of this bonding depends on several factors, including the charge on the complex and the ligand environment of the metal.

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27 The metal-carbon bond in metal carbonyls possess both s andpcharacter The M C σ bond is formed by the donation of lone pair of electrons on the carbonyl carbon into a vacant orbital of the metal The M C π bond is formed by the donation of a pair of electrons from a filled d orbital of metal into the vacant antibonding π* orbital of carbon monoxide The metal to ligand bonding creates a synergic effect which strengthens the bond between CO and the metal

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29 Bridging modes of CO

30 A characteristic of metal atoms bonded to organic ligands, especially CO, is that they often exhibit the capability to form covalent bonds to other metal atoms to form cluster compounds These clusters may contain only two or three metal atoms or as many as several dozen; there is no limit to their size or variety They may contain single, double, triple, or quadruple bonds between the metal atoms and may in some cases have ligands that bridge two or more of the metals

31 Although CO is most commonly found as a terminal ligand attached to a single metal atom, many cases are known in which CO forms bridges between two or more metals In cases in which CO bridges two metal atoms, both metals can contribute electron density into π* orbitals of CO to weaken the C - O bond Ordinarily, terminal and bridging carbonyl ligands can be considered 2-electron donors, with the donated electrons shared by the metal atoms in the bridging cases For example, in the complex the bridging CO is a 2-electron donor overall, with a single electron donated to each metal

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