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School of Advanced Sciences Department of Chemistry Module:2: Metal complexes and Organometallics (6 hours) Inorganic complexes - structure, bonding and application; Organometallics – introduction, stability, structure and applic...

School of Advanced Sciences Department of Chemistry Module:2: Metal complexes and Organometallics (6 hours) Inorganic complexes - structure, bonding and application; Organometallics – introduction, stability, structure and applications of metal carbonyls, ferrocene; Metals in biology (haemoglobin, chlorophyll - structure and property). What are Organometallics? An area which bridges organic and inorganic chemistry. A branch of coordination chemistry where the complex has one or more metal-carbon bonds. C always is more electronegative compared to M. "organometallic" compounds are interesting compounds in which there is a bonding interaction (ionic or covalent, localized or delocalized) between one or more carbon atoms of an organic group or molecule and a main group, transition, lanthanide, or actinide metal atom (or atoms) Zeise’s Salt- The first transition metal organometallic compound: Discovery 1827 - Structure ~ 150 years later First σ-bonded Organometallic Compound- Diethyl Zinc: 2 3 C2H5I + 3 Zn → (C2H5)2Zn + C2H5ZnI + ZnI2 Why are organometallics important? ❖ Are we dealing with ‘a’ special element? ❑ What is special about carbon? Forms bonds with other carbon atoms (C-C) readily and they are strong (catenation) Forms strong multiple bonds (C=C) Forms very strong bonds with another special element H!! Cyclic “C=C-C” fragments would be extra stable - AROMATIC ❑ C and its electronic configuration! 1s2 2s2 2p2 ❑ Why is this special ? To form a full shell, it would require 4 covalent bonds Gap between the 2s and 2p is just right! 1s2 2s1 2px12py12pz1 When 4 equivalent covalent bonds are formed, no extra electrons / no vacant 3 orbitals 4 Organic vs Organometallic reactivity Organic chemistry Organometallic chemistry ▪ C is the negative end of the M-C bond ("umpolung") ▪ C-C/C-H bonds are covalent ▪ Reactivity dominated by nucleophilic ▪ Cδ+-Xδ- : polar (partly ionic) attack on metal atom and electrophilic ▪ Reactivity dominated by nucleophilic attack on carbon Atom attack at carbon atom ▪ Associative and dissociative ▪ SN2 and SN1 like reactivity substitution at M 5  Ferrocene, bis(η5-cyclopentadienyl)iron Benzene is a neutral ligand and, so the oxidation state of Cr is ZERO (0) In Cr(C6H5)2, dibenzene chromium 6  Ionic model Cr (0),4s13d5: 6e For benzene: Three pi bond, 3x2= 6e+6e; Total EAN, 6+6+6 =18e Cr(η6-C6H6)2 In Cr(C6H5)2, dibenzene chromium, Benzene is a neutral ligand and, so the oxidation state of Cr is ZERO (0); Not like cyclopentadienyl anion; 2pi bond+ one anionic bond, 2e+2e+2e = 6e; Cr(η6-C6H6)2 7 Some Important Ligand Nomenclature Oxidation state/ionic “eta-x” was originally developed to indicate how 2 ηx many carbons of a π-system were coordinated to a metal center. 4 Hapticity is another word used to describe the bonding mode of a ligand to a metal center. An η5-cyclopentadienyl ligand, for example, has all five carbons of the ring bonding to the transition metal center. ηx values for carbon ligands where the x value is odd 4 usually indicate anionic carbon ligands (e.g., η5-Cp, η1- CH3, η1-allyl or η3-allyl, η1-CH=CH2) 2 The # of electrons donated (ionic method of electron counting) by the ligand is usually equal to x + 1 Even ηx values usually indicate neutral carbon π-system 6 ligands (e.g., η6-C6H6, η2-CH2=CH2, η4-butadiene, η4- cyclooctadiene) Number of electrons donated by the ligand in the even (neutral) case is usually just equal to x. 6 6 η5-Cp η3-Cp η3-allyl η1-allyl 9  Bridging ligand µx “mu-x” is the nomenclature used to indicate the presence of a bridging ligand between two or more metal centers. The x refers to the number of metal centers being bridged by the ligand. Usually, most authors omit x = 2 and just use μ to indicate that the ligand is bridging the simplest case of two metals. μ3η2 ❖ bridging ligands are usually placed next to the metals in question, then followed by the other ligands (note that rules 1 & 2 take precedence): Co2(μ-CO)2(CO)6, Rh2(μ-Cl)2(CO)4. 11 Organometallic compounds are classified into three classes. (i) Sigma (σ) bonded organometallic compounds: In these complexes, the metal atom and carbon atom of the ligand are joined together with a sigma bond, (a) Grignard reagents, R–Mg–X where R is an alkyl or aryl group, and X is a halogen. (b) Zinc compounds of the formula R2Zn such as (C2H5)2Zn Other similar compounds are (CH3)4Sn, (C2H5)4Pb, Al2(CH3)6, Al2(C2H5)6, Pb(CH3)4 etc. Al2(CH3)6 is a dimeric compound and has a structure similar to diborane, (B2H6). It is an electron deficient compound, and two methyl groups act as bridges between two aluminium atoms. (ii) Pi (π) bonded organometallic compounds: These are the compounds of metals with alkenes, alkynes, benzene and other ring compounds. In these complexes, the metal and ligand form a bond that involves the π-electrons of the ligand. Three common examples are Zeise’s salt, ferrocene and dibenzene chromium. These are shown below. (iii) Sigma and π-bonded organometallic compounds Metal carbonyl compounds formed between metal and carbon monoxide, belong to this class. These compounds possess both σ-and π-bonding. Generally, oxidation state of metal atoms in these compounds is zero. Carbonyls may be mononuclear, bridged or polynuclear. Stability of Organometallic Compounds ❖ In general terms, the stability of an organometallic compound may refer to either its thermal stability, or resistance to chemical attack (by air and moisture). Obviously, these different types of stabilities would depend both on thermodynamic as well as kinetic factors. The organometallic compounds are generally hydrolysed via nucleophilic attack by water, which is facilitated by: (1) the presence of empty low-lying orbitals on the metal (2) the polarity of metal-carbon bonds. Rate of hydrolysis is dependent on M-C bond polarity – greater the polarity, faster will be the rate 14  The 18-electron Rule or Effective atomic number (EAN) ▪ The 18e rule is a way to help us decide whether a given d-block transition metal organometallic complex is likely to be stable or not. Not all the organic formulas we can write down correspond to stable species. Recall: -the octet rule. ▪ For example, CH5 requires a 5-valent carbon and is therefore not stable. Stable compounds, such as CH4, have the noble gas octet, and so carbon can be thought of as following an 8e rule. ▪ The 18e rule, which applies to many low-valent transition metal complexes, follows a similar line of reasoning. The metal now has one s, and three p orbitals, and also five d orbitals. We need 18e to fill all nine orbitals; some come from the metal, the rest from the ligands. Therefore, we can expect that the low-lying MOs can accommodate up to 18 valence electrons--The 18-Electron Rule. ▪ The rule states that “thermodynamically stable transition metal organometallic compounds are formed when the sum of the metal d electrons and the electrons supplied by the surrounding ligands equals 18” 15 16 Counting electrons for metal complex To count the electrons of a metal complex, one must: a) note any overall charge on the metal complex b) know the charges of the ligands bound to the metal center (ionic ligand method) c) know the number of electrons being donated to the metal center from each ligand (ionic ligand method) Similarly for a transition metal complex, the electron count is the sum of the metal valence electrons + the ligand centered electrons. Covalent Model: # e = # metal electrons (zero valent) + # ligand electrons - complex charge; Metal: The number of metal electrons equals its column number (i.e., Ti = 4e, Cr = 6e, Ni = 10e); Ligands: In general L donates 2 electrons, X donates 1 electron. Ionic Model: # e = # metal electrons (dn) + # ligand electrons Metal: Determined based on the number of valence electrons for a metal at the oxidation state present in the complex Ligands: In general and L and X are both 2 e donors. Covalent Model (Neutral atom method): Metal is taken as in zero oxidation state for counting purpose Oxidation state method: oxidation state of the metal by considering the number of anionic ligands present and overall charge of the complex Complexes with 18 e- counts are referred to as saturated. Complexes with counts lower than 18e- are called unsaturated. 17 Electron Counting Step 2: Determine the d electron count. Recall: subtract the metal's oxidation state from its group #. Step 1: Determine the oxidation state of the metal. To do this, balance the ligand charges with an equal opposite charge on the metal. This is the metal's formal oxidation state. Step 3: Determine the To determine ligand charges, create an ionic electron count of the model by assigning each M-L electron pair to the complex by adding the # of more electronegative atom (L). This should result electrons donated by each in stable ligand species or ones known as ligand to the metal's d electron reaction intermediates in solution. count. 18  Ionic Covalent Now we can do our electron counting: Fe2+ = 6 Fe = 8 Re(+1) d7-1e 6e- 2Cp = 12 2Cp = 10 2 PR3 4e- 1s22s22p63s23p64s23d6 18 18 2 CO 4e- CH3− 2e- CH2=CH2 2e- Ionic Covalent W(0) = 6 W = 6 Re = 6S25d5 Total: 18e- 6CO = 12 6CO = 12 18 18 Please note that we are using the Ionic Method of electron-counting. 95% of inorganic/organometallic chemists use the ionic method. The ionic method assigns formal charges to the metal and ligands in order to keep the ligands with an even # of electrons and (usually) a filled valence shell. Synthetically, the ionic method generally makes more sense and the one that we will use in this course. 1) There is no overall charge on the complex 2) There is one anionic ligand (CH3−, methyl group) 3) Since there is no overall charge on the complex (it is neutral), and since we have one anionic ligand present, the Re metal atom must have a +1 charge to compensate for the one negatively charged ligand. The +1 charge on the metal is also its oxidation state. So the Re is the in the +1 oxidation state. We denote this in two different ways: Re(+1), Re(I), or ReI. 23 Other examples: Home Work! 16 e- 16 e- 18 e- Pt = 6S15d9 16 e- 16 e- 24 25  Metal-Carbonyls As one goes from a terminal CO- bonding mode to μ2-bridging and finally μ3-bridging, there is a relatively dramatic drop in the CO stretching frequency seen in the IR. Infrared spectroscopy (IR spectroscopy) is the ❖ Standard Bonding Modes spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. 2e- neutral donor 2e- neutral donor 3e- neutral donor IR region: 1mm to 700 nM 26 27  Types of CO-Metal bonding interactions ❖Formation of σ-bond: ▪ The overlapping of empty hybrid orbital on metal atom with the filled hybrid orbital on carbon atom of carbon monoxide molecule through lone pair electrons results into the formation of a M←CO σ-bond. ❖ Formation of π-bond by back donation: ▪ This bond is formed because of overlapping of filled dπ orbitals or hybrid dpπ orbitals of metal 28 atom with antibonding pi orbitals on CO molecule. Structure of Ni(CO)4 29 Applications 1. Metal carbonyls are used in several industrial processes. Perhaps the earliest application was the extraction and purification of nickel via nickel tetracarbonyl by the Mond process. 2. Fe(CO)5 is used for the preparation of inductors, pigments, as dietary supplements in the production of radar-absorbing materials in the stealth technology, and in thermal spraying. 3. Metal carbonyls are used in a number of industrially important carbonylation reactions. In the oxo process, an alkene, hydrogen gas, and carbon monoxide react together with a catalyst (such as HCo(CO)4) to give aldehydes (hydroformylation). H2 + CO + CH3CH=CH2 → CH3CH2CH2CHO 4. Several other Metal-Carbonyl complexes have been employed in the hydrocarboxylation and hydrogenation reactions. Dicobalt octacarbonyl [Co2(CO)8] can be used for hydrosilylation of olefins also. 5. Many organometallic complexes are the sources for the pure metal particles/ metal coatings using Chemical Vapour Deposition (CVD) process. 30 Structure and Bonding Ferrocene In its most common form, Mössbauer Mössbauer spectroscopy indicates that the iron center in absorption spectroscopy, a solid ferrocene should be assigned the +2 oxidation state. sample is exposed to a beam of Each cyclopentadienyl (Cp) ring should then be gamma radiation, and a detector allocated a single negative charge. Thus ferrocene measures the intensity of the beam could be described as iron(II) bis(cyclopentadienide) transmitted through the sample. The Fe2+[C5H5- ]2. atoms in the source emitting the The number of π-electrons on each ring is then six, gamma rays must be of the same which makes it aromatic according to Hückel's rule. isotope as the atoms in the sample These twelve π-electrons are then shared with the metal absorbing them. via covalent bonding. Since Fe2+ has six d-electrons, the complex attains an 18-electron configuration, which accounts for its stability. In modern notation, this sandwich structural model of the ferrocene molecule is denoted as Fe(η5-C5H5)2. Crystallography reveals that the cyclopentadienide rings are in staggered conformation. Hybridization: d2sp3 Magnetic Nature: Diamagnetic 31 Applications of Ferrocene Fuel additives: Ferrocene and its derivatives could be used as antiknock agents in the fuel for petrol engines. They are safer than previously TEL. Pharmaceutical: Ferrocene derivatives have been investigated as drugs e.g. one drug has entered clinic trials, Ferroquine (7-chloro-N-(2-((dimethylamino)methyl)ferrocenyl)quinolin-4-amine), an antimalarial. Ferrocene-containing polymer-based drug delivery systems have been investigated. Solid rocket propellant: Ferrocene and related derivatives are used as powerful burn rate catalysts in ammonium perchlorate composite propellant. As a ligand scaffold: Chiral ferrocenyl phosphines are employed as ligands for transition-metal catalyzed reactions. Some of them have found industrial applications in the synthesis of pharmaceuticals and agrochemicals. 32 Fuel additive, smoke suppressant and chiral catalyst precursor Ferrocene powder Ferrocene crystals Ferox Gas & Diesel Fuel Additive is a catalyst that is an eco-friendly fuel additive and horsepower booster. It allegedly increases mileage from between 10 and 20% while also significantly reducing harmful emissions. 33  Metals in biology Contents……Metals in biology (haemoglobin, chlorophyll- structure and property) 34 35 36 Chlorophyll - Structure and Property ❖ Structure of Chlorophyll Chlorophylls are green pigments with polycyclic, planar structures resembling the protoporphyrin system present in haemoglobin In chlorophyll, Mg2+ is the metal centre The four inward-oriented nitrogen atoms of the porphyrin ring in chlorophyll are coordinated with the Mg2+ II III All chlorophylls have a long phytol side chain, esterified to a carboxyl-group substituent in ring IV I IV Chlorophylls also have a fifth five membered ring not present in heme The heterocyclic five-membered ring system that surrounds the Mg2+ has an extended polyene structure, with alternating single and double bonds Such polyenes characteristically show strong absorption in the visible region of the electromagnetic spectrum Chlorophylls have unusually high molar extinction coefficients (higher light absorbance) and are therefore particularly well- suited for absorbing visible light during photosynthesis 37 ❖ Chloroplasts always contain both chlorophyll a and chlorophyll b ❖ Both are green, their absorption spectra are sufficiently different that they complement each other’s range of light absorption in the visible region ❖ Both chlorophyll a & b absorb in the blue and red region so that the remaining green region is transmitted – hence chlorophylls are green in colour ❖ Most plants contain about twice as much chlorophyll a as chlorophyll b ❖ Chlorophyll is always associated with specific binding proteins, forming light-harvesting complexes (LHCs) in which chlorophyll molecules are fixed in relation to each other, to other protein complexes, and to the membrane. 38 In molecular spectroscopy, a Jablonski diagram is a diagram that illustrates the electronic states and often the vibrational levels of a molecule, and also the transitions between them. The states are arranged vertically by energy and grouped horizontally by spin multiplicity. 39 Role of Mg in chlorophyll ❖ Without Mg2+ the chlorin ring is fluorescent – i.e. the absorbed light energy is emitted back immediately ❖ With Mg2+ chlorophyll becomes phosphorescent. Phosphorescence is emission of light from triplet- excited states, in which the electron in the excited orbital has the same spin orientation as the ground- state electron. ❖ In the case of fluorescence, the absorbed light energy is lost immediately – will not be used for chemical reaction ❖ In the case of phosphorescence, there will be excited state of finite life time and the energy can be used for chemical reactions ❖ The Mg2+ coordination increase the rigidity of the planar chlorin ring: The energy loss as heat due to vibration of the ring during light absorption is prevented 40 Photosynthesis Reaction Two types of photosystems cooperate in the light reactions NADP is Nicotinamide adenine dinucleotide phosphate 41 42 Hemoglobin Hb Hb is not an exact Four units of Hb tetramer of Mb 3 major types of Hb Hb A (Adult) Hb F (Fetal) Hb S (Sickle cell) Hemoglobin is a two-way respiratory carrier, transporting oxygen from the lungs to the tissues and facilitating the return transport of carbon dioxide. In the arterial circulation, hemoglobin has a high affinity for oxygen and a low affinity for carbon dioxide, organic phosphates, and hydrogen and chloride ions. 43 ❖ Each of these subunit polypeptides contains a heme group—an iron atom at the center of a poryphyrin ring—which reversibly binds a single O2 molecule in the ferrous state (Fe2+). ❖ Whereas free heme binds O2 irreversibly and is converted to the ferric state (Fe3+) in the process, Hb can reversibly bind O2 because the valence state of the iron atom is protected by encapsulating the heme in the globin protein fold ❖ Each tetrameric (α2β2) Hb can therefore reversibly bind four O2 molecules. ❖ Oxygenation changes the electronic state of the Fe2+ heme iron, which is why the color of blood changes from the dark, purplish hue characteristic of venous blood to the brilliant scarlet, dark red color of arterial blood. 44 ❖ The organic component of the heme group— the protoporphyrin—is made up of four pyrrole rings (A, B, C & D) linked by methine bridges to form a tetrapyrrole ring. Four methyl groups, two vinyl groups, and two proprionate side chains are attached. ❖ The iron atom at the center of the protoporphyrin is bonded to the four pyrrole atoms. ❖ Under normal conditions the iron is in the ferrous (Fe2+) oxidation state. The iron atom can form two additional bonds, one on each side of the heme plane, called the fifth and sixth coordination sites. ❖ The fifth coordination site is covalently bound by the imidazole side chain of the globin chain (the “proximal histidine,” α87 and β92). ❖ The sixth coordination site of the iron ion can bind O2 or other gaseous ligands (CO, NO, ❖ CN−, and H2S 45 ▪ Role of distal histidine: Makes O2 to bind in a bent fashion and makes it difficult for CO to bind in a linear fashion. ▪ An isolated heme binds CO 25000 times as strongly as O2 in solution. In the living system binding affinity for oxygen is reduced considerably. For CO to bind strongly, it has to bind linearly which is Tense (T) state Relaxed (R) state made difficult by distal histidine 46 47