Organometallic Chemistry PDF

CHM 303: ORGANOMETALLIC CHEMISTRY Lecture content: Free radical detection. Electron rule, bonding. 1 The Effective Atomic Number (EAN) rule or the 18-electron rule and bonding scheme The effective atomic number rule also known as the 18-electron rule s...

CHM 303: ORGANOMETALLIC CHEMISTRY Lecture content: Free radical detection. Electron rule, bonding. 1 The Effective Atomic Number (EAN) rule or the 18-electron rule and bonding scheme The effective atomic number rule also known as the 18-electron rule states that stable 2  organometallic compounds should have 18 electrons in their outermost (valence) shell. This rule was first proposed by Sidgwick and extended later by Bailey. Therefore, it is also referred to as sidgwick-Bailey's rule. Although there are many exceptions to this rule, it still provides useful guides to the chemistry of many organometallic complexes. The 18-electrons in the valence shell of the metal will occupy ns, np and (n-1)d orbitals a total of 9 orbitals, which can accommodate two electrons each giving a total of 18 electrons for a complete shell. This makes an organometalliccomplex to be kinetically stable. If there are less than 18 electrons in the valence shell of the metal, an empty low-lying orbital into which electrons may be promoted will be available. This will lead to decomposition on slight heating. If there are more than 18 electrons, the excess electron will move to the antibonding orbitals which will reduce the stability by decreasing the bond order due to interaction with bonding electrons. Note that a = a single non-degenerate orbital, e = double degenerate orbitals and t = triple degenerate orbitals. 1 indicates symmetrical to the plane of reflection and 2 indicates unsymmetrical to the plane of reflection. Gerade (g) implies symmetrical to center of inversion and ungerade (u) implies unsymmetrical to center of inversion. Application of 18-electron rule The 18-electron rule helps to predict the stability of organometallic complexes and also helps to predict the most probable structure of newly synthesized organometallic complexes. The application of the 18-electron rule involves the determination of electrons in the valence shell of the metal in zero oxidation state. This is added to the electrons contributed by the ligands and if the complex is positively charged the Lost electron must be removed or deducted from 3 the total electrons. However, if the complex is negatively charged the gained electrons must be added to the total number of electrons. If the compound or complex has 18 electrons in the valence shell of the metal, it is considered stable and on heating such complex it will not decompose at relatively low temperature. However, if the number of electrons in the valence shell of a metal is less than or greater than 18, the complex is unstable and may decompose on heating except those that have a special stability associated with 16-electron Square planar complexes. Note that the second row and third row transition metals under these group will have the same number of valence electron: nickel, palladium and platinum will have 10 valence electron is zero station state. Similarly, iron osmium and ruthenium will have eight electrons in valence shell in zero oxidation state. The number of electrons donated by the ligands depend on the nature of the ligand and the natureof the bonding below are some examples: Ligand Contributions Below is a list of common organometallic ligands and their respective electron contributions. Exceptions Generally, the early transition metals (group 3 to 5) could have an electron count of 16 or less. Middle transition metals (group 6 to group 8) commonly have 18 electron count while late transition metals (group 9 to group 11) generally have 16 or lower electron count. When a structure has less than an 18 electron count, it is considered electron-deficient or coordinately unsaturated. This means that the compound has empty valence orbitals, making it electrophilic and extremely reactive. If a structure has "too many electrons," that means that not all of the 4 bonds are covalent bonds, and thus some has to be ionic bonds. These bonds are weaker compared to covalent bonds. However, these organometallic compounds that have an electron count greater than 18 are fairly rare. 5 Electron counting Two methods are commonly employed for electron counting: 1. Neutral atom method: Metal is taken as in zero oxidation state for counting purpose 2. Oxidation state method: We first arrive at the oxidation state of the metal by considering the number of anionic ligands present and overall charge of the complex To count electrons in a transition metal compound: 1. Determine the oxidation state of the transition metal and the resulting d-electron count. o Identify if there are any overall charges on the molecular complex. o Identify the charge of each ligand. 2. Determine the number of electrons from each ligand that are donated to the metal center. 3. Add up the electron counts for the metal and for each ligand. Typically for most compounds, the electron count should add up to 18 electrons. However, there are many exceptions to the 18 electron rule, just like there are exceptions to the octet rule. 6  Examples 1 and 2 Example 3 Figure 1: Saturated Re metal complex with 18 electron count. 1. There is no overall charge on the molecule and there is one anionic ligand (CH 3-)  The Re metal must have a positive charge that balances out the anionic ligand charge to equal the 0 overall molecular charge. Since there is a -1 charge contribution from the methyl ligand, the Re metal has a +1 charge.  Because the Re metal is in the +1 oxidation state, it is a d6 electron count. It would have been its regular d7 electron count if it had a neutral (0) oxidation state. 2. The CH - ligand contributes 2 electrons. Each CO ligand contributes 2 electrons. Each PR 3 3 ligand contributes 2 electrons. The H2C=CH2 ligand contributes 2 electrons. 3. Adding up the electrons:  Re(1): 6 electrons  CH -: 2 electrons 3  2 x CO: 2 x 2 electrons = 4 electrons  2 x PR3: 2 x 2 electrons = 4 electrons  H2C=CH2: 2 electrons  Total: 18 electrons In this example, the molecular compound has an 18 electron count, which means that all of its orbitals are filled and the compound is stable. 7 Example 4 The 18-electron rule can also be used to help identify an unknown transition metal in a compound. Take for example [M(CO)7]+. To find what the unknown transition metal M is, simply work backwards: 1. 18 electrons 2. Each (CO) ligand contributes 2 electrons o 7 x 2 electrons = 14 electrons 3. 18 - 14 = 4 electrons 4. d4 5. M(I) oxidation state 6. The unknown metal M must be V, Vanadium Example 5 Similarly, to Example 2, the 18 electron rule can also be applied to determine the overall expected charge of a molecule. Take for example [Co(CO)5]z. To find the unknown charge z: 1. 18 electrons 2. Each CO ligand contributes 2 electrons 5 x 2 electrons = 10 electrons 3. Co is typically d9 4. 9 + 10 = 19 electrons 5. To satisfy the 18 electron rule, the [Co(CO)5]z compound must have a charge of z = +1. Reactivity: The 18-electron rule allows one to predict the reactivity of a certain compound. The associative mechanism means that there is an addition of a ligand while a dissociative mechanism means that there is a loss of a ligand. When the electron count is less than 18, a molecule will most likely undergo an associative reaction. For example: (C 2H4)PdCl2  16 electron count  Would it more likely lose a C2H4 or gain a CO? Losing a C2H4 results in a 14 electron complex while gaining a CO gives an 18 electron complex. From the 18 electron rule, we will expect that the compound will more likely undergo an associative addition of CO. Limitations of the effective atomic number (18-electron) rule There are no known cases in which the meta center in organometallic compounds do not have 18 electrons in their valence shell and they are very stable. Examples include: CH 3TiCl3, (CH3)2NbCl3, W(CH3)2, [Rh(CO)2Cl]2, which are: 8, 10, 12 and 16 electron systems respectively. The geometry adopted by these complexes is square planner. Another example is: 8 [Ni(P(cyclohexyl)3)2(C2H4)]. The bulkiness of bonded ligands imposes steric effect on the approach of incoming ligand. this can also impose limitations with regards to the number of ligands that can be accommodated by the metal and limit the number of electrons in the valence shell. Questions i. Using the above given values predict the stability of the following organometallic compoundsbased on effective atomic number rule. ii. Predict the most probable structures for the complexes given below if the effective atomicnumber rule is obeyed 1. [(C7H7)Co(CO)3] 2. [Ni(η5-C5H5)(NO)] iii. Draw the most probable structure for [IrCl(PPh3(NO)(CO)]Cl if the number of valence electrons is 16 in the metal. 9