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CH307 Organometallic Compounds of d-block elements Organometallic compounds contain at least one metal-carbon bond. Ligands Organometallic complexes have a wide range of ligand types from simple M-R to the -cyclopentadienyl rings of ferrocene. Hapticity of a ligand The hapticity of a ligand indicates the atoms that are directly bonded to the metal. The Greek letter (eta) is used, (5-C5H5) M indicates that all five carbons are bonded to M. P. McArdle 2009 5-C5H5 ligands are usually drawn as in (23,1b) - bonded or 1-alkyls of the M-R type are well known. Examples are WMe6, TiMe4 and MeMn(CO)5. Which Contain simple (2c-2e) M-R bonds. The compound (5C5H5)(1C6H5)Fe(CO)2 also contains one (2c-2e) bond. P. McArdle 2009 The Carbonyl Ligand, CO CO is an unlikely ligand as it does not alter the pH of H2O (i.e. will not complex a H+) It also has a very small dipole moment and the strongest known chemical bond (1100 kJmol-1). Mopac calculation of CO HOMO and LUMO orbitals HOMO LUMO P. McArdle 2009 The accepted bonding scheme has two steps: 2. M to CO back –donation 1. -donation from a filled C based orbital to an empty from a filled metal d-orbital metal orbital. to an empty CO (*) orbital. Step 2 enhances step 1 and thus the scheme is often said to be synergic. P. McArdle 2009 In this scheme CO is acting as a -acceptor or -acid ligand and step 2 enchances step 1 (a synergic effect). CO ligands may be terminal, doubly bridging (2) or triply bridging (3). MC-O IR spectra are very useful as the bands are very intense and narrow. Free CO is at 2143cm-1 and neutral M(CO)x are at ~ 2000cm-1 ~ 100cm-1 lower than free CO. This indicates a reduction in CO bond order on complexation and shows the importance of step 2 in the bonding scheme. P. McArdle 2009 C-O bond lengths from electron diffraction and X-ray diffraction studies show that free CO is 1.13Å MC-O is 1.17 and 2M2C-O is 1.20Å. IR spectra show this even more clearly CO 2143 2000 1800cm-1 Within an isoelectronic series (same no. of valence electrons) The following is generally true a +ve charge increases MC-O ~ 100cm-1 and a –ve charge decreases MC-O ~ 100cm-1. P. McArdle 2009 Hydride Ligands Many molecular hydrides are known which involve transition metal organometallic systems. M-H systems vary from hydridic (basic) types which react with H+ to M-H systems which are strong acids. 1H NMR is the best way to characterise M-H systems. The chemical shift range is from -8 to –30ppm. This is for the most part outside the normal chemical shift range of 0 to 10 ppm and it is easy to detect M-H by NMR. The M-H stretching vibration is observed close to 2000cm-1 However it is often weak and hard to see if CO is present. P. McArdle 2009 Phosphine ligands Substituted PH3, PR3, derivatives are good ligands PF3 ~ CO due to extensive back bonding enhanced by F. PR3 in general are weaker -acceptors than CO. In terms of the bonding scheme used for CO PR3 uses a P d-orbital as an acceptor. The R groups can be used to alter the importance of -donation and -acceptor behaviour. This is an electronic effect. Along the series Me, Ph, OMe and OPh the R group is increasing in electron withdrawing power. P. McArdle 2009 Steric effects The ability of different R groups in PR3 to produce different steric effects has become very important. C.A. Tolman was the first to study this extensively.The cone-angle construction can be used to quantify this property. This is very useful for fine tuning catalysts P. McArdle 2009 The phosphine cone angles cover a very wide range with some taking up more than 180 of the metal’s coordination sphere. 107º 118 122 145 P(OMe)3 PMe3 PMe2Ph PPh3 P(4-MeC6H4)3 P(3-MeC6H4)3 PtBu3 P(2-MeC6H4)3 145º 165 182 194 Bi and tri-dentate phosphines are also very important two well known examples are bis(diphenylphosphino)methane ,dppm, and bis(diphenylphosphino)ethane, dppe. The latter is also called diphos. (Ph)2P dppm P(Ph)2 (Ph)2P P(Ph)2 dppe (diphos) P. McArdle 2009 -bonded organic ligands 2 alkenes bond to the metal “side on” and are two electron donors. A scheme similar to that used for CO can also be invoked here. The alkene to M donation is from the filled C=C orbital and the back bond from M is into the * C=C orbital P. McArdle 2009 If the substituents on the olefin are strongly electron withdrawing (e.g. CN) back-bonding is enhanced to such an extent that C2(CN)4 complexes are often considered to be metallacyclopropane complexes. H NC H CN M M H H NC CN Zeise’s salt was reported in 1827 (structure in1960s) K2PtCl4 + ethene or ethanol = K+[(2-ethene)PtCl3]_ Cl Cl Cl H H Pt H H P. McArdle 2009 Dienes and trienes may also bond. (4C4H6)Fe(CO)3 (6-cycloheptatriene)Cr(CO)3 C4H6 = buta-1,3-diene O Fe C C C O Cr O O C C C O O (3-allyl)(4-C4H6)(5-Cp)Mo P. McArdle 2009 Molecular orbitals of ethene P. McArdle 2009 Molecular Orbitals of Buta-1,3-diene P. McArdle 2009 Benzene Molecular Orbitals P. McArdle 2009 Odd numbered -systems have a non-bonding orbital in the middle of the energy range with a node in the middle. P. McArdle 2009 18-electron rule Metal complexes in low oxidation states with high field ligands tend to have 18-valence electrons on the metal. [The 18 electron rule does not work for high oxidation states (>2) and weak field ligands, e.g. [Cr(H2O)6]3+]. To simplify electron counting the metal is always in oxidation state 0 sharing 1e in covalent bonds and accepting 2e in donor bonds. Some 16e compounds are stable at the RHS of the transition series Rh, Pd, Pt, Ir. M .. Cl M .. PR3 In M-Cl the metal gains one electron from the Cl And in MPR3 the metal gains 2e from PR3 P. McArdle 2009 Listing ligands by number of electrons they provide 1e M-H, M-Cl, M-Br, M-I, M-R, bent M-NO and M-M 2e CO, PR3, R2C=CR2, R2C (carbene) 3e 3-allyl, NO (linear), RC (carbyne), -X (Cl, Br, I) 4e 4-diene 5e 5 -dienyl 6e 6 -C6H6 P. McArdle 2009 Example (6-C6H6)Cr(CO)3 Cr(0) group 6 6e 6-C6H6 6e 3CO 3x2 6e 18e This complex would be expected to be stable. P. McArdle 2009 ((2 ethene)2RhCl)2 has a -Cl structure Cl Rh Rh Cl Rh(0) group 9 Cl –Rh ClRh 2 x 2-ethene 9e 1e 2e 4e 16e 16e compounds are often stable for Rh as it is at the RHS of the d-block. P. McArdle 2009 Formulae of the binary metal carbonyls M(CO)x Using the 18 electron rule the formulae of the simplest binary carbonyls can be predicted. Remember the 18 electron rule is a principle of maximum orbital use. It is not easy to increase a metals valence electron count beyond 18 as all s, p and d-orbitals are filled by 18e. The simplest possible formulae are; M(CO)x when M has an even number of valence electrons and M2(CO)x when M has an odd number of valence electrons P. McArdle 2009 Metal valence electron count The number of valence electrons = Group number for d-block No. 3 4 Sc Ti 5 V 6 Cr 7 8 9 10 Mn Fe Co Ni 11 12 Cu Zn Example with even number of electrons Cr Cr(CO)x X = no. of CO per M Cr 6e + (xCO) 2x = 18 x = (18 –6) / 2 = 6 Predicted formula Cr(CO)6 P. McArdle 2009 Example with odd number of electrons Co Co2(CO)2x Cobalt is in group 9 These metals get 18e by forming an M-M bond which gives each metal 1 extra electron. The formula is then for each metal M(e) + 1(from M-M) + 2x = 18 x = (18 –1 – M(e)) / 2 Which for Co is x = (18 – 1 – 9) / 2 = 4 Thus the formula is Co2(CO)8 P. McArdle 2009 Simple Stable Binary Metal Carbonyls M(e) 6 7 Cr(CO)6 Mn2(CO)10 Mo W Tc Re 8 9 10 Fe(CO)5 Co2(CO)8 Ni(CO)4 Ru Os Rh Ir While other formulae are known the 2nd and 3rd rows follow the first row for the most part. There are no simple carbonyls known for Pd or Pt What about Ti and V ? Ti Group 4 x = (18 – 4) / 2 = 7 Ti(CO)7 does not exist, there are not enough orbitals to bond 7 CO groups. However salts of [Ti(CO)6]2– have been made P. McArdle 2009 V Group 5 V2(CO)2x M(e) + 1(from M-M) + 2x = 18 x = (18 – 1 – 5) / 2 = 6 This suggests a formula V2(CO)6 This would be 7-coordinate which would exceed 6 and be too sterically hindered. Salts of 18e [V(CO)6]– are known and the very unstable V(CO)6 radical has been isolated. P. McArdle 2009 Another approach No. 3 Sc 4 Ti 5 V 6 Cr 7 8 Mn Fe 9 10 Co Ni 11 12 Cu Zn Remember 1 simple formula e.g. Fe(CO)5 or Ni(CO)4 and work from there. Fe(CO)5 is 18e Mn(CO)5 is 17e + 1 M-M = 18e Formula is (Mn(CO)5)2 Co(CO)5 is 19e Co(CO)4 is 17e + 1 M-M gives (Co(CO)4)2 Many other formulae are known e.g. In the Fe group Fe forms Fe(CO)5 Fe2(CO)9 Fe3(CO)12 Ru and Os also form M(CO)5 but are much more stable as M3(CO)12 P. McArdle 2009 Structures of some binary carbonyls Cr(CO)6 Oh Mn2(CO)10 staggered Co2(CO)8 bridged + non-bridged (soln.) P. McArdle 2009 Fe(CO)5 D3h Ni(CO)4 Td ED Synthesis of M(CO)x Reaction of metal powder (often prepared in situ) with CO under pressures of up to 200 bar is the general method. 390K 70bar CrCl3 + LiAlH4 + CO Cr(CO)6 400K 90bar RuCl3.xH2O +CO +Zn Ru3(CO)12 Physical properties Cr(CO)6, white solid Mn2(CO)10, yellow solid Fe(CO)5, orange liquid b.p.103º (toxic), Co2(CO)8, orange air sensitive solid Ni(CO)4, colourless liquid b.p. 45º (very very toxic) P. McArdle 2009 Reactions of Organometallic Compounds Ligand Substitution Carbonyl substitution h or M(CO)x-1L + CO M(CO)x + L M(CO)x 18e h or slow M(CO)x-1 16e L fast M(CO)x-1L 18e The substitution mechanism for 18e complexes is dissociative. The reaction rate does not depend on the concentration of L. P. McArdle 2009 Oxidative Addition A reaction in which the metal is oxidized by two units and the metal coordination number is increased usually by 2. Addition of a molecule X-Y to a metal centre. N M(L)x + X X-Y N+2 M(L)x Y Oxidation state N Oxidation state N+2 Coordination No. x Coordination No. x+2 P. McArdle 2009 The best know examples are provide by the complexes of Rh(I) Ir(I) Pd(0) Pd(II) Pt(0) and Pt(II). The metal must have 2 stable oxidation states separated by 2 units. e.g. Rh(I) and Rh(III) or Pd(0) and Pd(II) Perhaps the best known is Vaska’s complex [Ir(PPh3)2(CO)Cl] This complex reacts with a very large number of X-Y molecules. [Ir(PPh3)2(CO)Cl] + MeI = [Ir(Me)(PPh3)2(CO)(I)Cl] 16e Ir(I) 4 coord. 18e Ir(III) 6 coord. P. McArdle 2009 Oxidation state of the metal In the [MLaXb]c+ formalism a is the number of L type ligands b is the number of X type ligands c is the charge on the complex Oxidation state of the metal = b + c [ML4X2]+ The metal is in oxidation state III P. McArdle 2009 Conversion of (n-ligand)M complexes to [MLaXb]c+ formalism Ligands which donate an even number of electrons are L type Monodentate neutral ligands e.g. PR3 are L type In general n-ligands with even n have La with a = (no. of electrons / 2) e.g. 6-C6H6 = L3 Ligands which donate an odd number of electrons have an X type interaction and possible L interactions n-ligands with odd n have an X interaction and (n-1) / 2 L interactions Only X type interactions affect oxidation state P. McArdle 2009 (3-allyl)(4-butadiene)(5-cyclopentadienyl)Mo (X L) ( L2 ) ( X L2 )Mo MoL5X2 Mo oxidation state = II P. McArdle 2009 [Mn(5-C5H5)(CO)3] Mn( X L2 ) ( L3 ) [Co(5-C5H5) (CO)(PPh3)2], [Mo(CO)3(PPh3) 2I] CoL5X MoL5X [Co(2-Buta-1,3-diene)(CO)4Br] [(3-allyl)Mo(CO)4I] [Fe(CN)5NO]2nitroprusside MoL5X2 MnL5X Mn(I) Co(I) Mo(I) CoL5X Co(I) Mo(II) [Fe ( X5 ) ( X L )]2- [FeLX6]2- Fe(IV) NO is more easily oxidized than Fe(II) Fe(II)NO+ [FeX5(L+)]2The iron is diamagnetic low spin d6 [NO]+ is isoelectronic with CO P. McArdle 2009 You can buy [NO]+[PF6]- Oxidative addition is reversible the reverse reaction is called reductive elimination Vaska’s complex is an oxygen carrier Ph3P Cl Ir CO PPh3 + PPh3 OC Ir O Cl O PPh3 O O Oxidative addition followed by reductive elimination is the basic mechanism of many catalytic processes. O OC OC Me Co CO H H H Co CO OC CO + O Me H In this example reductive elimination gives a new C-H bond P. McArdle 2009 Mechanism of oxidative addition – There are 3 established reaction mechanisms. 1. Concerted addition M H + H2 MH2 H H H M H H * M H H filled empty This is the most likely mechanism when X=Y in X-Y and the reaction is conducted in a non-polar solvent. P. McArdle 2009 2. Ionic stepwise slow fast + M + RX MR + X M(R)(X) A likely mechanism when X≠Y and the reaction takes place in a polar solvent. 3. Free radical In the presence of O2 peroxides or other free radical sources radical mechanisms have been detected. For example in the case of Vaska’s complex Init + Ir(I) Init-Ir(II) Init-Ir(II) + RX Init-Ir-X + R R + Ir(I) R-Ir(II) R-Ir(II) + RX R-Ir(I)-X + R P. McArdle 2009 Alkyl and hydrogen migrations OC OC Me Mn CO O CO CO + OC CO OC Mn Me CO CO CO This is an example of alkyl migration This has also been called CO insertion into a metal alkyl The reaction mechanism suggests the former The term migratory insertion tries to satisfy everyone Using 13CO the mechanism can be deduced. If 13CO is used none of it ends up in the MeC=O group P. McArdle 2009 Me OC OC Mn C CO Me O O Mn OC CO CO 6-coordinate When this 13CO labelled complex is heated it looses CO and the 13C label position in the products can be used to test the mechanism. OC Mn CO CO Me 25% + OC OC 5-coordinate O Me OC 13 13CO OC CO CO O OC OC OC OC C CO Mn 13 CO CO - CO CO Mn CO Me 13 CO 25% + OC Me P. McArdle 2009 Me C Mn CO O CO 6-coordinate CO is lost trans to another CO (trans effect of CO > COMe) and Me then moves into the vacant position. Thus the result supports the alkyl migration mechanism. If only CO insertion was involved the only products would be those in the box. CO Mn CO CO 13 CO 25% + Me OC CO Mn CO 25% CO 13 CO -Hydrogen elimination -H elimination, giving an unstable [M(alkene)H] complex, is the principle decomposition pathway for metal alkyls which have a -hydrogen. This is why the most stable alkyl complexes are formed by Me, Ph, CH2CMe3, CH2SiMe3 and CH2Ph. Ln M H H H R H H H R Ln M H H H R H H H Ln M After -elimination the olefin may dissociate (L type ligand) leaving a very reactive metal hydride which itself then reacts/decomposes. P. McArdle 2009 “Insertion Reactions” Olefin into metal hydride Metal hydride + olefin metal alkyl Ln M H + H H olefin insertion H H -elimination Ln M H H This is really H migration H CO CO Ln M CO This is really alkyl migration R O Olefin into M-R Ln M R H Metal alkyl + CO metal acyl CO into M-R Ln M R H H H R' H H Ln M R' R P. McArdle 2009 This is another alkyl migration. It is a mechanism for olefin polymerization Reactions involving M-M bonds Oxidative clevage M-M + X2 2M-X (OC)5Mn-Mn(CO)5 + Br2 2 (CO)5Mn-Br Reductive clevage M-M + reducing agent 2 M¯ (OC)5Mn-Mn(CO)5 + Na / Hg 2 (OC)5Mn¯ These anions are good nucleophiles. (OC)5Mn¯ + MeI (CO)5Mn-Me + I¯ P. McArdle 2009 (OC)5Mn-Mn(CO)5 + Br2 2 (OC)5Mn-Br Na / Hg (OC)5Mn¯ MeMgBr MeI (OC)5Mn-Me CO (OC)5Mn-(CO)-Me P. McArdle 2009 Metal carbene complexes M=CR2 There are two types 1. Hetero atom stabilized or Fischer type carbenes OMe (OC)5Cr R The O atom is the hetero atom in this case. 2. Unstabilized or Schrock type carbenes R' M R There is no hetero atom attached to the carbene carbon. P. McArdle 2009 Synthesis of metal carbene complexes The Fischer type are obtained by reaction of Li alkyls with coordinated CO. Works best for Cr, Mo and W. (OC)5Cr + C O Me (OC)5Cr Li C O Li+ Me The anion is then alkylated using Me3O+ BF4¯ OMe (OC)5Cr C R The hetero atom stabilizes the molecule and polarizes the Cr=C bond + OMe The carbene carbon is (OC)5Cr C attacked by nucleophiles R P. McArdle 2009 The Schrock type are synthesised by -H abstraction Ta(CH2tBu)3Cl2 LiCH2tBu -LiCl -CMe4 H (tBuCH2)3Ta tBu Schrock type carbenes involve early t-metals in high oxidation states. - (OC)5Cr + OMe C + - R' M R R Fischer type are polarized M¯C+ and are not olefin metathesis catalysts, C attacked by nucleophiles Schrock type are polarized M+C¯ and are good olefin metathesis catalysts, C attacked by electrophiles. P. McArdle 2009 Metal carbyne complexes OMe (OC)5 M Ph + BX3 X(OC)4M C Ph + BX2OMe Fischer carbenes react with BX3 (loss of OMe¯) The cation formed is susceptible to X¯ attack and CO loss + (OC)5 M C Ph + BX3OMe X(OC)4M C Ph + M = Cr, Mo, W and X = Cl, Br, I -H abstraction from a Schrock carbene can also yield carbyne complexes. Ta C Cl Cl tBu H PMe3 Ph3P=CH2 - Ph3MePCl P. McArdle 2009 Ta C tBu Cl PMe3 CO + CO Bond length and bond order An M-C single bond should be close to the sum of the covalent radii. M-C double and triple bonds should be shorter than M-C This is the case M C >M M = Cr ~2.10 C 2.04 >M C 1.69 Å Metal carbonyls M-C bond lengths are shorter than M=C in carbenes M C O M C O P. McArdle 2009 A resonance form with some MC multiple character is involved Metallocenes di-cyclopentadienyl sandwich complexes Ferrocene is by far the best known and the most stable [Fe(5-cyclopentadienyl)2] FeL4X2 Fe 8 L4 8 X2 2 18 Fe( X L2 )2 FeL4X2 Only Ru and Os can also do this All non-iron group metallocenes are less stable than ferrocene Cobaltcene has 19e and is unstable in air The cobaltcinium cation is 18e and air stable [Co(5-cyclopentadienyl)2]+ [PF6]¯ P. McArdle 2009 Bonding in 5-cyclopentadienyl complexes cyclopentadienyl molecular orbitals Chapt 18 box 18.2 Each C is sp2 e2 hybridized and after the -frame 5 pz orbitals remain e1 dxz, dyz a1 dz2 D5d Metal orbitals P. McArdle 2009 + - + + - + + - + + Chemistry of ferrocene O Fe Ac2O + H3PO4 Fe mono acetylation mild conditions nBuLi RCOCl AlCl3 Fe Me Me Fe O Me O di-acetylation more vigorous conditions P. McArdle 2009 Li Zirconocene derivatives Zirconium is stable in oxidation state IV and forms the Zirconocene derivative (5-C5H5)2ZrCl2 Cl Zr Cl This compound Is 16e Zr Group 4 ( X L2 )2 Zr X2 ZrL4X4 = 16e The analogous W compound is 18e as W is in Group 6 Cl A related Zr system has led to important new chiral Metallocene Ziegler catalysts W Zr Cl Cl P. McArdle 2009 Cl This type of compound is sometimes called a bent metallocene Questions What is meant by the term -acid ligand ? Explain the term "ligand cone angle". Which of the following have the largest and smallest ligand cone angles, P(Ph)3, P(o-tolyl)3 and PH3. Give metal valence electron counts for the following systems and indicate those which are likely to be stable and those which are not; [Mn(5-C5H5)(CO)3], [Co(5-C5H5)(CO)(PPh3)2], [Mo(CO)3(PPh3) 2I], [Co(2-Buta-1,3-diene) (CO)4Br] and [Mo(3-allyl)(CO)4I]. Give metal valence electron counts for the following systems and indicate those which are likely to be stable and those which are not; [Cr(5-C5H5)(CO)2], [Mn(5-C5H5)(CO)4],[Mo(CO)3(PPh3)I2], [Co(2-Butene)(CO)3Br] and [Mo(3-allyl)(5-C5H5)(CO)2]. P. McArdle 2009