Chemistry 332 Basic Inorganic Chemistry II

Chemistry 332 Basic Inorganic Chemistry II

18- Electron Rule. Recall that for MAIN GROUP elements the octet rule is used to predict the formulae of covalent compounds. This rule assumes that the central atom in a compound will make bonds such that the total number of electrons around the central atom is 8. THIS IS THE MAXIMUM CAPACITY OF THE s and p orbitals. This rule is only valid for Period 2 nonmetallic elements. The 18-electron Rule is based on a similar concept. The central TM can accommodate electrons in the s, p, and d orbitals. s (2) , p (6) , and d (10) = maximum of 18 This means that a TM can add electrons from Lewis Bases (or ligands) addition to its valence electrons to a total of 18. This is also known Effective Atomic Number (EAN) Rule Note that it only applies to metals with low oxidation states. in 18 Electron Rule contd Example 1. [Co(NH3)6] +3 Example 2. [Fe(CO)5] Oxidation state of Co? Electron configuration of Co? Electrons from Ligands?

Electrons from Co? Total electrons? Oxidation state of Fe? Electron configuration of Fe? Electrons from Ligands? Electrons from Fe? Total electrons? What can the EAN rule tell us about [Fe(CO)5]? It cant occur 20-electron complex. EAN Summary 1. Works well only for d-block metals. metals. It does not apply to f-block 2. Works best for compounds with TMs of low ox. state. 3. Ligands which are good -donors and -acceptors utilize all the valence orbitals and thus such compounds obey this rule. 4.

Complexes which contain a combination of -donors and -acceptors conform to this rule. (e.g. Cr(NH3)3(CO)3 , Cr(6-C6H6)(CO)3). 5. Compounds which obey this rule are kinetically inert to substitution reactions. 6. Exceptions to the rule occur at the two ends of the transition series where nd, (n+1)s, and (n+1)p valence orbitals are less well matched in energy. Lets talk about electron counting briefly. Sandwich Compounds Obeying EAN Lets draw some structures and see some new ligands. Each of these ligands is -bonded above and below the metal center. Ferrocene is an interesting example. Half-Sandwich Compounds Obeying EAN Lets draw some more structures. CO, NO, H, and PR3 can be brought together in combination to give 18 electrons. Some other cool ligands. These cyclic ligands need not be planar. Here are some examples of compounds of cyclooctatetraene. Can a reaction involve only compounds which obey the 18 electron rule?

YES. Compounds and the EAN Rule We can divide compounds into three groups. 1. Electronic configurations are completely unrelated to the EAN rule. The central metal may have >, <, = 18 electrons. 2. Electron configurations follow the EAN rule and never have >18 electrons, but may have less. 3. A group that follows EAN rule rigorously. How can we understand this? Chemistry and Magic Numbers The Octet Rule: Period 2 nonmetallic elements tend to form compounds resulting in eight electrons around the central atom. You have been told this is because elements desire a pseudo-noble gas configuration. This is a VAST simplification. Stable Fullerenes: The allotrope of Carbon known as fullerenes (C60 or Bucky-ball is the most famous) take on a cage structure and it has been observed that particular numbers of C atoms yield more stable compounds. C60, C70, C76, C84, C90, C94 Nanoparticles: Metal Nanoparticle are really COOL! It has been observed that magic numbers of atoms preferentially come together to form stable structures. Bonding in TM Complexes: Many TM complexes will form with 18 electrons around the central metal atom. It was first observed by

Sedgwick in 1927. 18- Electron Rule. Recall that for MAIN GROUP elements the octet rule is used to predict the formulae of covalent compounds. Think about Na+ and ClThis rule assumes that the central atom in a compound will make bonds such that the total number of electrons around the central atom is 8. THIS IS THE MAXIMUM CAPACITY OF THE s and p orbitals. This rule is only valid for Period 2 nonmetallic elements. The 18-electron Rule is based on a similar concept. The central TM can accommodate electrons in the s, p, and d orbitals. s (2) , p (6) , and d (10) = maximum of 18 This means that a TM can add electrons from Lewis Bases (or ligands) addition to its valence electrons to a total of 18. in This is also known Effective Atomic Number (EAN) Rule Simple Examples of the 18 Electron Rule Example 1. [Co(NH3)6] +3 Example 2. [Fe(CO)5] Oxidation state of Co?

Electron configuration of Co? Electrons from Ligands? Electrons from Co? Total electrons? Oxidation state of Fe? Electron configuration of Fe? Electrons from Ligands? Electrons from Fe? Total electrons? What can the EAN rule tell us about [Fe(CO)5]? It cant occur 20-electron complex. Approach 1 to counting Oxidation State Electron Count. Ligands are viewed as close-shelled entities. (No radicals). This is what we did in the earlier examples. We dissect the structure When neutral Lewis base ligands (like NH3) are considered they are viewed as neutral molecules with 2 electrons for donation to the metal. Ligands like methyl (CH3 and Cl) are viewed as anions.NOT AS NEUTRAL RADICALS. (By definition H is viewed as H-) After removal of the ligands the metal is assigned a formal charge. [Ni(CO)4] Ni0 10 e-, CO 2 e- each (8) = 18 [PtCl2(PMe3)2] Pt2+ 8 e-, Cl- 2 e- each (4), PMe3 2 e- each (4) = 16

[Ta(Me)5] Ta5+ 0 e-, Me- 2 e- each (10) = 10 Fe(5-C5H5)2 Fe2 6 e-, 5-C5H5 6e- each (12) = 18 Ferrocene Approach 2 to counting Neutral Atom Counting. The general premise to this approach is: REMOVE ALL THE LIGANDS FROM THE METAL AS NEUTRAL SPECIES. This approach results in no difference for neutral ligands like NH3 or CO. BUT For ligands such as methyl we remove the ligand as a radical. It is therefore a single electron donor in this model. Furthermore, in this model both the ligand and the metal must donate an electron to the bond. This method provides NO information about the metal oxidation state. Electron Counting Examples 7 9 Mn

Co Look at CO complexes of Mn You may expect to have the following structure for a CO complex of Mn. CO OC Mn CO OC CO Mn 7 3 CO Terminal 10 Total 17 electrons Prediction of Structure. (metal carbonyls) You may expect to have the following structure for a CO complex of Mn. CO OC Co CO OC What about ? Co 9 3 CO Terminal 6 2 CO Bridging 2 1 Co-Co 1

OC O C OC Co OC CO Co C O CO CO Is this the only possible structure for bis[tetracarbonylcobalt]? The EAN Rule cannot differentiate structures of compounds but it CAN provide possibilities for investigation. Compounds and the EAN Rule We can divide compounds into three groups. 1. Electronic configurations are completely unrelated to the EAN rule. The central metal may have >, <, = 18 electrons. 2. Electron configurations follow the EAN rule and never have >18 electrons, but may have less. 3. A group that follows EAN rule rigorously.

(This is what I have shown you so far) How can we understand this? Group I TiCl4(THF)2 Ti(H2O)63V(urea)63CrCl63CrI2(DMSO)4 Mn(H2O)62+ CoF63CuCl5 3Ni(H20)62+ Cu(H20)62+ ZnCl2Valence (biuret)2 (d-electrons, valence) (O,12) (1 ,13) (2 ,14) (3 ,15) M ML6 6L p s

You figure out. d o filled electrons from 12 to 22. Ligands are weak field, o is small. Weak sigma interaction and NO pi interaction by 6L Little or no pi interaction between metals and ligands. Energy of the t 2g orbitals is the same as the free metal. There are 6 low energy bonding MOs, 5 medium energy MOs and and 4 strongly antibonding MOs (too high energy to be occupied). 12 electrons from the ligands fill the lowest energy orbitals (blue). Up to 6 metal electrons reside in the t2g set (nonbonding) without any destabilization of bonding. o is so small that up to 4 electrons can be put into the eg set with only a small penalty. Group II Zr(CH3)22Ti(en)33-Re(NCS)6Mo(NCS)63Os(SO3)68Ir(NH3)4Cl22+ ReH92- (d-electrons, valence) (O,12) (1 ,13) (2 ,14)

(3 ,15) You figure out. M ML6 6L p s o d filled Valence electrons equal to 12 to 18. Strong sigma donation increases eg energy and increases o . Strong sigma interaction and NO pi interaction by 6L Little or no pi interaction between metals and ligands. Energy of the t 2g orbitals is the same as the free metal. Their occupation has no impact on the stability of the complex. There are 6 low energy bonding MOs, 3 medium energy MOs and and 6 strongly

antibonding MOs (too high energy to be occupied). 12 electrons from the ligands fill the lowest energy orbitals (blue). Up to 6 metal electrons reside in the t2g set (nonbonding) without any destabilization of bonding. o is so large that electrons cannot be put into the eg set without large penalty. Group III Ti(cp)2(CO)2 V(CO)5NO Cr(C6H6)2 MnH(CO)5 Fe(NO)2(CO)2 Co(NO)(CO)3 Ni(CO)4 (d-electrons, valence) (4, 18) (5 ,18) (6 ,18) (7 ,18) You figure out. M ML6 6L t2g vacant

p s o filled d Valence electrons always equal to 18. Strong sigma interaction and strong pi acceptor interaction by 6L. Strong sigma donation increases eg energy Pi accepting ligands lower t2g energy. BOTH increase o . There are 9 low energy bonding MOs, 9 strongly antibonding MOs (too high energy to be occupied). 12 electrons from the ligands and 6 metal electrons in the t2g orbitals fill the lowest energy orbitals (blue). Removal of the d electrons from the t2g set would destabilize the bonding. o is so large that electrons cannot be put into the eg set without large penalty. EAN Summary 1. Works well only for d-block metals.

metals. It does not apply to f-block 2. Works best for compounds with TMs of low ox. state. 3. Ligands which are good -donors and -acceptors utilize all the valence orbitals and thus such compounds obey this rule. 4. Complexes which contain a combination of -donors and -acceptors conform to this rule. (e.g. Cr(NH3)3(CO)3 , Cr(6-C6H6)(CO)3). 5. Compounds which obey this rule are kinetically inert to substitution reactions. 6. Exceptions to the rule occur at the two ends of the transition series where nd, (n+1)s, and (n+1)p valence orbitals are less well matched in energy. This Rule allows for prediction of structures, reactivity, and reaction mechanisms.

Bridging or Terminal CO OC OC I Fe CO Terminal CO bonding at 2021.5 cm-1 and 1975.7 cm-1 also, because of very small symmetry differences between carbon monoxides. O C Fe Fe C O CO Terminal CO bond1887 cm-1 Bridging CO bond at 1770 cm-1

Bonding in TM Carbonyls CO bonding-the orbital picture O C Filled M ML6 HOMO Filled 6L t2g vacant p s Filled d

o filled 10 valence electrons C (4), O(6) Strong sigma interaction and strong pi acceptor interaction by A cartoon of M-CO bonding. The HOMO in carbon monoxide is the high energy NB which is primarily derived from a carbon 2p orbital. This means a lone pair of electrons is residing on the C atom. The LUMO on CO is the *2p which are antibonding orbitals with significant 2p character. CO acts as a Lewis Base and a Lewis Acid. The back bond appearing in this systems is known as a synergistic effect. Reactions of Metal Carbonyls. i) Substitution of CO by other L (L is often a -acid or Soft Lewis base; L= PR3, polyolefins, SR2, CH3CN) Recall that TM carbonyls obey the 18 electron rule.

This means two things. They are inert toward substitution. Reactions must proceed via a Dissociative mechanism (via M-CO bond cleavage) This provides a basis for photochemistry: If light of a suitable energy is supplied such that * can occur some interesting things happen. * * hv=E E G.S. E.S. M-CO Photochemistry * * B.O.= 0 This negates the

M-CO bond. hv=E E E.S. G.S. Bond Order = 1/2 (electrons in bonding orbitals - electrons in anti-bonding orbitals) CO is photoejected! LnM-CO 18 electrons hv slow - CO [LnM] L' LnM L' fast 16 electrons 18 electrons High energy

reactive intermediate. E~390nm In theory, by filtering the excitation light it should be possible to remove only 1 CO. This is not simple given the broad nature of the UV-vis bands. M-CO photochemistry Examples (CO)4 Ru (OC)4Ru hv, >370nmnm Ru(CO)4 3 LRu(CO)4 (L= olefin) L Orange, colour arises from *(Ru-Ru)Ru-Ru) ~390nmnm Another example involving Fe and an 18 electron transition state OC OC CO hv

Fe CO - CO 18 electrons alkyne 2e- donor OC OC CO Fe 18 electrons alkyne 4e- donor L This intermediate is not 16e- and is stabilized by a 4e- donor alkyne. It substitutes 1013x faster than Fe(CO)5. Reduction of TM Carbonyls What will happen if electrons are added to 18e- TM carbonyls? High energy 19 or 20 electron systems will result and CO will be ejected. (This can be viewed as the two electrons taking the place of the CO or

breaking M-M bonds) 2Na/Hg Fe(CO)422Na/Hg 2Mn(CO)5- These anions are of significant importance. They are nucleophiles and react further to form M-C and M-H bonds. Formation of M-H and M-C bonds CO CO Mn CO OC R CO A RX Mn(CO)5- RCOX The difference between A and B is the presence of CO between M and R.

CO CO Mn CO OC CO R CO and heat CO CO Mn CO OC CO R CO CO CO Mn CO OC C CO RB O CO CO Mn CO OC CO CO R Empty bonding

site. This is referred to as CO insertion although the mechanism involves migration of R. Mn(CO)5- + H+ H-Mn(CO)5- Collmans Reagent Application of carbonylmetallates in organic synthesis. Na2Fe(CO)4 RD D+ RCOCl RX CO OC OC R Fe R O CO

R'X R'X R CO O2 X2 OC R' OH CO Fe CO CO O2 O R OC

- X2 O HNR2 R X H2O R'OH Disodium tetracarbonylferrate is useful in the functionalization of organic halides. Oxidation of TM Carbonyls Oxidation weakens the M-CO or M-M bonds and results in CO elimination or M-M cleavage with the formation of TM carbonyl halides. + X2 Fe(CO)4X2 + CO 18 electrons OC heat

+ X2 OC 2Mn(CO)5X Mn OC 18 electrons X CO X CO Mn CO CO CO 18 electrons RLi Mn(CO)5X RMgX

Mn(CO)5R Special Case. Oxidative Addition (4-coordinate Vaskas Compound 1961, 16 electron species) H H Ph3P Ir PPh3 CO Cl (activation of H2) H2 Cl Ph3P Ir PPh3 CO Ir(I), d8, 16e- RX R

Cl Ph3P Ir PPh3 CO X Ir(III), d6, 18e- Reactions of Coordinated M-CO The attachment of CO to a TM makes the C electrophillic and may be attacked by a nucleophile) (CO)5Co CO R- (CO)5Co R C O- R'X (CO)5Co

R C O R' This is a carbene complex; E.O. Fischer discovered this type of molecule and shared the Nobel Prize with Wilkinson. The (CO)5Co structural unit acts as an electron withdrawing; It is a pseudo ester. (CO)5Co R C HNR2'' OR' (CO)5Co Trans-esterification R C NR2''

The Mond Process Nickel carbonyl, a gas formed from carbon monoxide and metallic nickel. Scientific Serendipity In 1890 Ludwig Mond, was investigating the rapid corrosion of nickel valves used in apparatus for the Solvay process*, and discovered Ni(CO)4. In contrast to many nickel compounds which are usually green solids, Ni(CO)4 is a colourless, volatile, toxic liquid with a very "organic character". He used it as the basis of a method to purify nickel, called the "Mond process". Ni reacts with CO (leaving the impurities behind), to form Ni(CO) 4. The Ni(CO)4 is passed through a tower filled with nickel pellets at a high velocity and 400 K. Pure Ni plates out on the pellets. * A commercial process for the manufacture of Na2CO3. NH3 and CO2 are passed into a satd NaCl(aq) solution to form soluble (NH4)(HCO3), which reacts with the NaCl to form soluble NH 4Cl and solid NaHCO3 if the reactor temperature is maintained below 15C. The NaHCO3 is filtered off and heated to produce Na2CO3. Hemoglobin and Heme

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