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1、Chapter 2,Complexes with metal-carbon s bonds,Metal alkyls, aryl, hydride and related s-bonded ligands,Outline,1. The stability of transition metal-carbon s bonds 2. Preparation of complexes with metal-carbon s bonds 3. Chemical properties of complexes with metal-carbon s bonds 4. Related s-bonded l
2、igand complexes (other metal s-bonded complexes) M-X (X = -BR2, -SiR3, -NR2, -OR, -F, -Cl, etc.) 5. Metal hydrides,Silyl amino,References and suggested readings: The Organometallic Chemistry of the Transition Metals, Robert H. Crabtree, 3rd Edition, 2001, Chapter 3 Organometallic Chemistry, G. O. Sp
3、essard, G. L. G. L. Miessler, Prentice-Hall: New Jersey, 1997, Chapter 6 Organotransition Metal Chemistry, Akio Yamamoto, 1986. Chapters 3.1, 4.2.,1. The stability of transition metal-carbon s bonds,1). Thermal stability of transition metal-carbon bonds,2). Factors affecting M-C bond strength,3). Or
4、igin of earlier failure in obtaining TM alkyls: Kinetic consideration.,Prior to 1960s: Non-transition metal alkyls are well known, e.g.,AlMe3 ZnEt2,Transition metal alkyls are rare. Only a few transition metal alkyls were prepared. e.g.,Cubic 立方的,Prior to 1960s，transition metal alkyls are rare. Are
5、transition metal-carbon bonds weak and unstable? But today: Transition metal-carbon bonds are known for virtually all transition metals.,Questions:,Are transition metal-carbon s bonds really unstable? What affects M-C bond strength? What causes the earlier failure to obtain transition metal alkyls?,
6、1). Thermal stability of transition metal-carbon bonds,The thermal stability can be related to bond dissociation energy.,M-X (g) M(g) + X(g) DH = BDE (M-X),If BDE (M-X) is large, M-X is thermally more stable.,Range of M-C bond dissociation energy (BDE). (See next two pages),BDE (M-C) values in MRn c
7、omplexes (KJ/mol),Non-transition metals Transition metals M-C bond BDE Compound M-C bond BDE,Li-Et 209 Ti(CH2C(CH3)4 Ti-CH2R 170 Li-Bu 248 Ti(CH2Ph)4 Ti-CH2R 240 Zn-Me 176 Ti(CH2Si(CH3)4 Ti-CH2R 250 Zn-Et 145 Cp2Ti(CH3)2 Ti-CH3 250 Cd-Me 139 Cp2TiPh2 Ti-Ph 350 Hg-Me 122 (CO)5MnCH3 Mn-CH3 150 Hg-Et 1
8、01 (CO)5ReCH3 Re-CH3 220 Hg-i-Pr 89 Zr(CH2C(CH3)4 Zr-CH2R 220 Hg-Ph 136 Zr(CH2Ph)4 Zr-CH2R 380 B-Me 363 Zr(CH2Si(CH3)4 Zr-CH2R 225 B-Et 342 CpPt(CH3)3 Pt-CH3 165 Al-Me 276 (Et3P)2PtPh2 Pt-Ph 250 Al-Et 242 Ta(CH3)5 Ta-CH3 260 Ga-Et 237 W(CH3)6 W-CH3 160,Non-transition metals: BDE(M-C): 89-280 kJ/mol
9、Except B-C Transition metals: BDE (M-alkyl) : 150 - 260 kJ/mol BDE (M-aryl): 250 - 350 kJ/mol Normally, BDE (M-alkyl) BDE (M-aryl) Conclusion: Transition metal-carbon bond energies are not so different from those of main group metals.,2). Factors affecting M-C bond strength,Affecting Factors : M eff
10、ect; Ln effect; R effect; C-(sp3, sp2, sp) effect.,a) Metal effect For main group metals, BDE (M-CH3), kJ/mol B-Me 363 Zn-Me 176 Al-Me 276 Cd-Me 136 Ga-Me 247 Hg-Me 122 BDE(M-C) decreases when going down a group.,Cd: cadmium,Hg: mercury,gallium,For transition metals, BDE (M-CH3), kJ/mol, (CO)5Mn-CH3
11、 150 (PhCH2)3Ti-CH2Ph 240 (CO)5Re-CH3 220 (PhCH2)3Zr-CH2Ph 380 BDE(M-C) increases when going down a group.,Benzyl: -CH2Ph,b) R effect (M-R):,Bond dissociation energies DR-Mn(CO)5,Complex BDER-MnCO)5 (kJ/mol) Mn(CO)5CF3 172 Mn(CO)5C6H5 170 Mn(CO)5 CH2C6H5 87 Mn(CO)5CH3 153 Mn(CO)5COCH3 129 Mn(CO)5COC
12、6H5 89 Mn(CO)5COCF3 147 Mn(CO)5H 213 Mn(CO)5I 195 Mn(CO)5Br 252 Mn(CO)5Cl 294,acetyl,Relative bond strength: (M-Cl M-Br M-H = or M-I) Reason: Electron density: Cl Br I M-H M-CF3 M-Ar M-CH3 M-CO-CH3 M-C-R: stability increases with more electron-withdrawing R.,* M-H is stronger than M-CR3 s orbital is
13、 indirectional and more dense. * M-CF3 is stronger than M-CR3 -CF3 is electron withdrawing. * M-Ar is stronger than M-CR3. More effective back-donation for M-Ar.,Would you expect M-CH3 bond be stronger or weaker than,(answer: M-CH3 M-CH=CH2 M-CCH),Why?,* M-CH3: no dp-pp interaction; M-CHCH2: one dp-
14、pp interactions; M-CCH: two dp-pp interactions * Evidence:,Stability of the following M-C bonds: M-CH3 M-CH2-NO2 Why ?,NO2: nitryl,Summary: 1. The M-C bond in transition metal alkyl complexes is not as weak as was thought previously. There is no inherent instability attached to transition metal alky
15、ls. 2. The BDE(M-R) value increases as the atomic number increases among the congeners in the same group, in an opposite trend to non-transition metal alkyls. 3. The D(M-R) value decrease in the order M(M-H) D(M-CF3) D(M-Ph) D(M-CH3) D(M-CH2CH3) D(M-CH2Ph). The earlier failure in obtaining TM alkyls
16、 is not due to thermodynamic reasons !,Exercises Which one has the strongest stability of the following complexes ?,Answer: a) last one b) last one c) first one,3). Origin of earlier failure in obtaining TM alkyls: (Kinetic consideration),General questions. Q1. Why could many TM alkyl complexes not be isolated? The reason for the instability of TM alky
