Chemistry 533 Electron Counting Transition metal and organometallic chemistry Electron counting organometallic compounds Organic structure and bonding The nature of organic chemistry self-evidently confines it to the chemistry of the Main Group. In a general view of structure and bonding, structure and bonding in organic chemistry is determined entirely by s and p orbitals. Classical hybridization 1 is the most basic approach that offers any qualitative utility for organic structure and bonding but is not necessarily an accurate picture of the system in question. Hybridization fails when applied to apparently hypervalent main group compounds and is useless when discussing transition metal structure and bonding. Applying group theory to organic systems results in physically reasonable orbitals of mixed atomic parentage that can be stocked with the necessary eight electrons, providing a useful description of the bonding in the system which is in accord with observed measurements of the system. There are several features of organic molecules that differ strongly from those containing transition metals. These include a distinct lack of high order symmetry and an almost invariable eight electron count at any atom in the system that is not hydrogen with concomitant coordinative saturation. Moreover, the number of relatively heavy atoms (C, N, O) is usually high and this generates a relatively high density of electronic states with similar energies. These features of organic molecules dictate that a simple structure and bonding scheme is usually sufficient for synthetic applications but transfer of such a scheme to organometallics or other transition metal system usually results in error. For this reason, electron counting transition metal compounds is a basic and important feature of the description of their chemistry. Appendix I gives some details of the assembly of valence bond hybrids, derived from the work of Linus Pauling. Inorganic structure and bonding There is a natural distinction between inorganic structure and bonding problems that involve transition metals and those that involve the p-block metals. In the latter case, the most important fundamental concepts are the widening ns -np gap and the alternation effect, which involves the 3 d/4 f contractions and the so-called 'inert pair' effect. The transition metals are very different, due to the presence of nd- orbitals and ( n+1) s and ( n+1) p orbitals. Depending on the row, the electron count and the position in the transition series, a very wide variety of reactivities, formal oxidation states and so on are available for stoichiometric and catalytic reactivity. In order to be able to account for the profusion of organometallic and coordination complexes of the transition metals, a most basic step is the accurate determination of the formal oxidation state and the assessment of the coordination sphere and electron count. The most satisfactory method of counting electrons in this respect is the MLX method, which invariably gives the correct answer, without an a priori assumption about oxidation state. For a compound such as WF 6 or WMe 6 , the formal oxidation state is straightforward to assess. For a compound such as C7 H7 Ti C5 H5 , this is less clear, with formal oxidation states of 0 and IV being available. 1 We define 'hybridization' as the use of constructs such as sp 3 sp 2 and sp hybrids with unweighted orbital contributions. 1 Chemistry 533 Electron Counting Transition metal and organometallic chemistry MLX classification The MLX classification assigns the metal, M, an oxidation state of 0 and then homolytically breaks the bonds between ligands so that all the fragments are neutral. This ensures that the ligand leaves as a neutral ligand and the metal retains its full d count. 1 Simple ligands Ligands such as phosphines, alkenes and CO leave as the neutral ligand and therefore act as two electron donors and are designated as L. Ligands such as CF 3 SO 3 , Cl, OMe, H and Me leave as radicals i.e. they are open shell and are therefore one electron donors; these are designated as X. More complex ligands are constructed from these two simple classifications. Examples of these simple ligand types are given in Table 1. X ligands L ligands F, Cl, Br, I, OR, R, Ar, H, CF 3 SO 3 , CN, N 3, NO 2, CO, R 3P, R 3N, N 2, R 2C=CR 2, RCN, RNC, H 2, Table 1: A non-exhaustive set of examples of X and L ligands Examples 3×CO =L 3 2×Ph 2 PCH 2 CH 2 PPh 2=2L2=L4 5×Cl =X 5 3×H2 O=L3 2×N =L MoCl =MX 2 2 + 5 5 Ph PCH CH PPh Mo N =ML [Tc CO 3 H2 O 3 ] =ML 6 2 2 2 22 22 6 Faegri, K.; Martinsen, G.; Aebischer, N.; Schibli, R.; Alberto, Hidai, M.; Tominari, K.; Uchida, Y.; Strand, T. G.; Volden, H. V. R.; Merbach, A. E. Angew. Chemie, Misono, A. J . Chem. Soc. D: Chem. Acta Chem. Scand. (1993), Int. Ed . (2000), 39 (1), 254-256. Comm . (1969), (23), 392 47 (6), 547-53. 1 The d count on the metal includes the s2 electrons for the neutral metal atom 2 Chemistry 533 Electron Counting Transition metal and organometallic chemistry 3×Ph P=L 3 3 6×H =X 6 × = 1×N2=L i 3 H2 CCH 2 L3 2× Pr 2 PhP =L2 1×H=X H2 CCH 2 Pt =ML 3 iPr PhP Os H =ML X 2 = 2 2 6 2 6 Ph 3 P 3 Co N2 H ML 4 X Howard, J. A. K.; Johnson, O.; Pu, L. S.; Yamamoto, A.; Ikeda, S. Howard, J. A. K.; Spencer, J. L.; Koetzle, T. F.; Spencer, J. L. J. Am. Chem. Soc. (1968), Mason, S. A. Proc. Roy. Soc. Lon. A Inorg. Chem. (1987), 26 (18), 90 (14), 3896 (1983), 386 (1790), 145 2930 Complex ligands Unsaturated hydrocarbons These are ligands that donate more than one or two electrons to the metal center. They can be similarly divided into 'compound LX' classifications as shown below in Table 2. The 'eta' notation, n denotes the number of atoms bound to the central metal atom in question by the superscript employed ( n). The absence of a superscript implies that the maximum number of available atoms is bonded. Under this notation, ferrocene is written Cp 2 Fe , bis (allyl)nickel is written as C3 H52 Ni and so on. Taking the [(allyl)Ni] fragment as an example, there are two possible resonance extrema that can exist as shown in figure 1. Figure 1: Hypothetical resonance extrema for the (allyl)Ni fragment The arrow represents the L interaction, whereas the single bond to Ni represents the X interaction. The accurate picture for the structure of bis (allyl)Ni shows no contribution from either resonance extremum , but for 'bookkeeping' purposes, it is a useful tool. The allyl fragment in [(allyl)Ni] is therefore acting as an LX ligand and C3 H5 is always counted as a 3 electron, LX ligand. Similarly, Cp acts as an L2X ligand when it displays Cp coordination. The coordination of an unsaturated carbocycle to a transition metal can take a variety of possible values, usually varying in steps of 2 as individual L interactions change due to the electronics and sterics at 3 Chemistry 533 Electron Counting Transition metal and organometallic chemistry the metal center. Some examples of differing coordination of the same ligand are given below. 1Cp M Cp Nugent, K. W.; Beattie, J. K.; Hambley, T. W.; Snow, M. R. Aust. J. Chem. (1984), 37 (8), 1601-6 Almenningen, A.; Haaland, A.; Lusztyk, J. J. Organomet. Chem . (1979), 170 (3), 271-84 1Cp Be Cp Bennett, M. J., Jr.; Cotton, F. A.; Davison, A.; Faller, J. W.; Lippard, S. J.; Morehouse, S. M. J. Am. Chem. Soc. (1966), 88 (19), 4371-6. 1 Cp Cp Fe CO 2 Green, M. L. H.; Konidaris, P. C.; Michaelidou, D. M.; Mountford, P. J. Chem. Soc. Dalton Trans. (1995), (2), 155-62 Green, J. C.; Green, M. L. H.; James, J. T.; Konidaris, P. C.; Maunder, G. H.; Mountford, P. J. Chem. Soc. Chem. Commun. (1992), (18), 1361-4 t Cp 2 Mo N Bu Green, M. L. H.; Konidaris, P. C.; Michaelidou, D. M.; Mountford, P. J. Chem. Soc. Dalton Trans. (1995), (2), 155-62 Green, J. C.; Green, M. L. H.; James, J. T.; Konidaris, P. C.; Maunder, G. H.; Mountford, P. J. Chem. Soc. Chem. Commun. (1992), (18), 1361-4 Cp 3Ind Mo Nt Bu 4 Chemistry 533 Electron Counting Transition metal and organometallic chemistry ncht M Menconi, G.; Kaltsoyannis, N. Organometallics (2005), 24 (6), 1189-1197. Zeinstra, J. D.; De Boer, J. L. J. Organomet. Chem . (1973), 54 207-11 Cp Ti cht Muhandiram, D. R.; Kiel, G.-Y.; Aarts, G. H. M.; Saez, I. M.; Reuvers, J. G. A.; Heinekey, D. M.; Graham, W. A. G.; Takats, J.; McClung, R. E. D. Department of Chemistry, Organometallics (2002), 21 (13), 2687-2704. 5 cht Fe CO 2 SnPH 3 Muhandiram, D. R.; Kiel, G.-Y.; Aarts, G. H. M.; Saez, I. M.; Reuvers, J. G. A.; Heinekey, D. M.; Graham, W. A. G.; Takats, J.; McClung, R. E. D. Department of Chemistry, Organometallics (2002), 21 (13), 2687-2704. 3 cht Re CO 4 Whereas ligands with higher potential hapticities, such as cht (cycloheptatrienyl), have the electronic flexibility to display a number of hapticities, ligands derived from Cp tend not to do so and examples of 3Cp M are rare; an exception to this is the indenyl ligand, which can undergo a facile 5 3 3 'ring-slippage' or can, in certain cases, form an Ind M ground state.