Chemistry 253 Organometallic Chemistry - Semester 2, 2024 PDF

CHEMISTRY 253 Organometallic Chemistry Dr. Erin Leitao School of Chemical Sciences [email protected] Office 302 1063 Semester 2, 2024 What is organometallic chemistry?  Review Problem 1. Which...

CHEMISTRY 253 Organometallic Chemistry Dr. Erin Leitao School of Chemical Sciences [email protected] Office 302 1063 Semester 2, 2024 What is organometallic chemistry?  Review Problem 1. Which of the following bonds is the most ionic? Why? NaCl, CsF, LiCl, HCl, PCl CsF due to greater EN  Review Problem 2. Which of the following is a Lewis base?  Review Problem 3. Which of the following is a Lewis acid?  Review Problem 4. Order the following elements from least to most electronegative F, Cs, Ba, Zn, O, B, Mg Cs, Ba, Mg, Zn, B, O, F 6  Review Problem 5. Order the following elements from smallest to largest radius F, Cs, Ba, Zn, O, B, Mg F, O, B, Zn, Mg, Ba, Cs 7 Main-Group? Transition Metals?  Organometallics Compounds containing a bond between M or E and C - main-group non-metals (e.g. B, Si, P, S) - main-group metalloids (e.g. As, Ge, Sb, Se, Te, etc.) Denote main-group elements as E (metal, semi-metal, non-metal) Denote transition metal elements as M   and  bonds - E-C -bonds are important - E=C are more rare - chemistry is determined largely by the specific properties of E and its group - size and electronegativity are important to determine stability, reactivity and bond type  Main-Group Trends ELECTRONEGATIVITY SIZE d-block isn’t quite as straightforward  Size of E? Can use ionic or covalent radii to measure Covalent radius of C (r = 77 pm) determined from half C-C single bond length (d = 154 pm) Covalent radius of each element (r/pm) determined by subtracting covalent radius of C from E-C bond length (d/pm) d(E-C) = r(C) + r(E)  Problems 6. How is d(E-C) measured? X-ray crystallography 7. In-C bond length 223 pm. What is the covalent radius of In? d(In-C) = r(C) + r(In) 223 pm = 77 pm + r(In) r(In) = 146 pm  E-C Bond Lengths (d/pm) and Covalent Radii (r/pm) Group 2/12 Group 13 Group 14 Group 15 E d r E d r E d r E d r Be 179 102 B 156 79 C 154 77 N 147 70 Mg 219 142 Al 197 120 Si 188 111 P 187 110 Zn 196 119 Ga 198 121 Ge 195 118 As 196 119 Cd 211 134 In 223 146 Sn 217 140 Sb 212 135 Hg 210 133 Tl 225 148 Pb 224 147 Bi 226 149  Covalent Radii (r/pm) Covalent radius for an element can vary depending on type of bonding. As amount of s character increases (sp3 to sp2 to sp), effective radius becomes smaller d(C-C) r(C) R3C-CR3 154 pm 77 pm for sp3 hybridised C R2C=CR2 136 pm 68 pm for sp2 hybridised C RC≡CR 124 pm 62 pm for sp hybridised C  Electronegativity 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 H He 2.2 Li Be B C N O F Ne 1.0 1.6 2.0 2.5 3.0 3.4 4.0 tendency to attract electrons Na Mg Al Si P S Cl Ar 0.9 1.3 1.6 1.9 2.2 2.6 3.1 K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 0.8 1.0 1.3 1.5 1.6 1.6 1.6 1.8 1.9 1.9 1.9 1.7 1.8 2.0 2.2 2.6 2.9 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe 0.8 1.0 1.2 1.3 1.6 2.1 1.9 2.2 2.3 2.2 1.9 1.7 1.8 1.8 2.0 2.1 2.6 Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 0.8 0.9 1.1 1.3 1.5 2.3 1.9 2.2 2.2 2.3 2.5 2.0 1.6 1.9 2.0 2.0 2.2  Effect of Electronegativity Electronegativity of C is 2.5 – Only N, O, (S, Se) and halogens are more electronegative than carbon – All metals are more electropositive, as are non metals H, B, Si, P, As, etc.) All organometallics are polarised M+-C- or E+-C- Electronegativity also affected by hybridization Compare electronegativities of C: sp3 2.5 sp2 2.75 sp 3.29 Reflects increase in CH acidity C2H6 < C2H4 300 Sn-Cl 323 P-C 276 P-O 351 P-Cl 331  Example Zn(CH2CH3)2 Sn(CH3)4 ΔH(combustion)/kJ mol-1 -1920 -3590 Thermodynamics unstable unstable Property pyrophoric air stable Kinetics labile inert Main-group organometallic compounds are typically thermodynamically unstable with respect to oxidation Lability (reactivity) towards H2O and O2 creates practical concerns for handling compounds  Problem 8. Given the following information, discuss CCl4 and SiCl4 in terms of reactivity and stability towards water. CCl4 SiCl4 E-Cl bond strength /kJ mol-1 327 381 E-O bond strength /kJ mol-1 358 452 Electronegativity 2.5/3.1 1.9/3.1 Size Cl > C Cl ~ Si Lone Pairs? no no Storage Under air Under N2 C-Cl < C-O; Si-Cl < Si-O in terms of bond enthalpies. Therefore both are, thermodynamically unstable with respect to hydrolysis Si > C size, and Si-Cl > C-Cl polarity. Storage is key as CCl4 is kinetically inert and SiCl4 is labile with respect to hydrolysis  Basicity vs. Nucleophilicity Thermodynamics Basicity A base donates a pair of electrons (to a proton) measured by pKa (equilibrium constants for reversible acid- base reactions) relative stabilities (higher pKa = stronger base) Kinetics Nucleophilicity A nucleophile donates a pair of electrons to an atom (electrophile) measured by the rate of the reaction (faster rate = better nucleophile)  So aren’t they the same thing? No. You can have a strong base which doesn’t react with your substrate. Need to consider steric hindrance and solvation iPr2N- (pKa = 36) Me- (48) < nBu- (50) < sBu- (51) < tBu- (53) Strong nucleophiles: Me- < nBu- < sBu- < tBu- Weak nucleophile iPr2N- can have very different reactivity (iPr2N- is selective for deprotonation) Non-polar Common Solvents Polar protic Polar aprotic