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Reactivity of Transition Metal Organometallics L. J. Farrugia Msc

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L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics Reactivity of Transition Metal Organometallics L. J. Farrugia MSc Core Course C5 Text books : Inorganic Chemistry - Housecroft & Sharpe Ch 23 - the very basics Inorganic Chemistry - Shriver & Atkins Ch 16 - the very basics This course assumes familiarity with the Level-2 and Level-3 courses on Organometallic Chemistry, and this is covered in the above texts Organometallics - Elschenbroich & Salzer (library) - much more useful Topic 1 - Introduction to Cyclopentadienyl Compounds First part of course covers the role of the ligand cyclopentadienyl (Cp) and its derivatives. Cp is one of the most important ligands in organometallics after CO. A considerable percentage of organometallic compounds contain this ligand - it is also a good ligand for main group metals and the f-block metals (lanthanides & actinides). H H H Fe CO H OC CO Cyclopentadiene Cyclopentadiene complex (4 e donor ligand) very rare as a ligand Na / -H2 H - Na+ H acidic H atom planar aromatic (6-pi electron) dienyl anion - The anion C 5H5 is a very useful synthetic reagent. It is usually treated as equivalent to occupying THREE coordination sites, so that C 5H5 ≡ 3(CO). In electron counting terms, it can be treated as either as a 6-e donor ANION or a 5- e donor NEUTRAL molecule. The latter is the recommended approach because it is simpler (do not need to worry about oxidation levels). 1.1 Bonding in cyclopentadienyl compounds Cp has 5 π electrons in the 5 out-of-plane p-orbitals on the C atoms. These 5 orbitals combine as a 1 + e 1 + e 2 under five-fold symmetry. With a single Cp ring, 1 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics inspection shows the possible combinations with the metal d-orbitals are shown below Cp orbitals Metal orbitals Symmetry of bond a1 pz dz2 , s σ e1 dxz dyz px py π e2 dx2-y2 dxy δ For two Cp rings in a metallocene M(C 5H5)2 the rings may be either staggered or eclipsed - Fe(C 5H5)2 is eclipsed but Co(C 5H5)2 and Ni(C 5H5)2 are staggered. The bonds can also be divided into sigma, pi and delta symmetry as shown in OHP #1 OHP # 2 shows the formal MO interaction diagram for ferrocene - Fe(C 5H5)2 The main points to remember (i) the 9 filled orbitals have 18 electrons - hence the rule ! (ii) the LUMO is a doubly degenerate π* orbital, so Ni(C 5H5)2 is paramagnetic 2 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics This shows how the metal and ring π-orbitals match by symmetry. The resultant MO scheme for ferrocene shows the way that the basis orbitals having the same symmetry can combine to give new orbitals (NOT necessary to remember the MO scheme) The filling of the nine bonding orbitals in ferrocene explains the high stability of this compound. The mixing of metal and Cp orbitals indicates a strong covalent character to the transition metal - cyclopentadienyl bond. In Co(C 5H5)2 ONE electron fills the e* 1g while in Ni(C 5H5)2 there are TWO unpaired electrons - hence both compounds are paramagnetic 3 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics + Co(C 5H5)2 readily loses an electron to give the cobalticinium cation [Co(C 5H5)2] an 18e cation. Similarly Ni(C 5H5)2 loses one electron to give a 19e cation, but further oxidation results in decomposition. 1.2 Structural types of Cp compounds (i) Metallocenes MCp 2 These are known for the metals Ti, V, Cr , Mn, Co and Ni. Staggered or eclipsed rings leads to D5h or D5d symmetry with virtually NO barrier to rotation of the Cp ring about the metal-Cp axis. So all Cp protons appear as equivalent in the 1H NMR spectrum V, Cr, Fe, Co and Ni give the "classic" sandwich compounds illustrated above. The exceptions are 4 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics (a) Titanocene "TiCp 2" This is really a fulvalene complex made by reducing Cp 2TiCl 2. It contains bridging * hydrides and a Ti-Ti bond - gives 16 e Ti atoms. Real "titanocene" TiCp 2 has recently been made but is extremely reactive. (b) Manganocene MnCp 2 This is ionic at low temperature, with a polymeric chain like structure Above 159 oC it becomes isomorphous with ferrocene, so must adopt a sandwich structure. All the sandwich metallocenes apart from ferrocene are paramagnetic No. unpaired electrons Cp 2V 3e Cp 2Cr 2e 2+ Cp 2Mn 5e high spin Mn Cp 2Co 1e Cp 2Ni 2e (ii) 'bent' metallocenes These have non-parallel rings due to the presence of other ligands. Some examples are H Cl W Ti V CO Re H H Cl 18 e 16 e 17 e 18 e 5 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics (iii) half-sandwich compounds - piano stool compounds These have one Cp ring and a variety of other ligands. Some examples are OC V CO Mn Ru Co PPh3 OC CO Cl CO OC CO PPh3 OC CO 4-legged stool 3-legged stool Chem-3 lab 2-legged stool The 17 electron species CpMo(CO) 3 and CpFe(CO) 2 are not stable as such, but dimerise to give the familiar compounds with a Mo-Mo or Fe-Fe metal-metal bond. This is typical behaviour of odd-electron organometallic species. (iv) other types of bonding mode 5 So far only the so-called eta-5 η -C5H5 bonding mode has been illustrated, where (in principle) all five C atoms are equally bonded to the transition metal. However 3 1 there are other possibilities, most common are η -C5H5 and η -C5H5 H M M 1 3 η -C5H5 η -C5H5 1 e donor - like alkyl group 3e donor - like allyl group H Mo ON 5 One good example is Mo(NO)Cp 3 - with "normal" η -C5H5 bonding modes, electron counting give a 24 electron compound (!!) - simply not possible. In fact it is an example of a compound containing all three types. 1 η -C5H5 ligands are usually fluxional. Consider the compound (C 5H5)4Ti - again 5 cannot be 4 η -C5H5 bonding modes because it would be a 24 e compound. In 5 1 this case it has two η -C5H5 and two η -C5H5 ligands. Ti 6 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics At the very lowest temperature measured, the 1H NMR spectrum shows a singlet 5 for the two (equivalent) η -C5H5 groups and an AA'BB'C multiplet for the two 1 (equivalent) η -C5H5 groups (three different H environments). This compound shows two fluxional processes 1 (i) migration of metal atom around the η -C5H5 group via 1,2 shifts (ring whizzing) 1 5 (ii) exchange of the η -C5H5 and η -C5H5 groups Process (i) is a very low energy process, which is frozen out only at the lowest temperatures. Process (ii) occurs at higher temperatures ~ room temperature. The net result of (i) and (ii) is that all 20 protons appear equivalent at the highest temperatures measured. Process (i) could in principle occur also by 1,3 shifts or random shifts. How can we tell which ? We can if it is possible to assign the protons of the AA'BB' signal M c c M a M a a a b c a b b b b b a 1, 2 shift The resulting exchanges are : a→c a→b i.e. both a type protons exchange b→a b→b i.e. only half of b type protons exchange c→a So... the rate of broadening for the a type protons is twice as fast as for the b type protons 5 1 The actual spectrum of ( η -C5H5)( η -C5H5)Fe(CO) 2 which exhibits a similar exchange is shown below. 7 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics 1.3 Oxidation states in Cp compounds Since Cp is formally charged with a -ve charge, each Cp ligand present in a compound contributes a charge of +1 towards the formal oxidation state of the metal. Hence a metal must be in a +ve oxidation level. The only exception would be with +ve charged ligands - NO + is the only common example. CpNi(NO) has a zerovalent Ni atom. In terms of its ability to stabilize oxidation states, Cp is comfortable with both low and high oxidation states of the metal (unlike many pi-acid ligands like CO which are only found for low oxidation states). In general Cp is a good sigma- and pi- donor, but a poor pi-acceptor. Substituting H for Me on the Cp ring makes it an even better sigma-donor - Cp* is the common symbol for C 5Me 5 . H2O2 CO Re Re O CO O OC O CO Re (+1) Re(+7) - oxo complex 8 L.J. Farrugia : MSc Core2 Course C5 - Reactivity of Transition Metal Organometallics Topic 2 - Reactivity of Cyclopentadienyl-type Ligands Very often Cp is a spectator ligand (i.e. it does not take part in the reaction and it is unchanged at the end). Under some circumstances however the ring will react. Ferrocene Fe(C 5H5)2 has been the most studied (because it is a stable 18 e metallocene) but ruthenocene Ru(C 5H5)2 and osmocene Os(C 5H5)2 will give similar reactions - they are in the same periodic group ! However, most Cp compounds will not survive the reaction conditions described below.
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