Dative ligands with more than one donor atom s Metal complexes Textbook H: Chapter 2.8 Textbook A: Chapter 8.7.4

Metal dihydrogen complexes Discovered in 1984 by Kubas: M(CO)3(PR3)2(H2) (M = Mo, W; R = Cy, i-Pr) Bonding dH-H = 0.84 Å (0.74 Å in free H2); dW-H = 1.75 Å nHH = 2690 cm-1; nHD = 2360 cm-1 dHH = -4.21 (24 Hz wide), dHD = -4.21 (8 Hz wide, 1:1:1 triplet, JHD = 33 Hz, JHD (HD gas) = 43 Hz) Good p acceptors (CO, NO) are effective in stabilizing the H2 ligand. Reference: Kubas, G. J. et al. J. Am. Chem. Soc. 1984, 106, 451

Agostic interactions Characterization Agostic: Greek for “to hold on to oneself”; term coined by Brookhart. It refers to a C-H/Si-H bond, which interacts with a metal. B: first agostic complex, Cotton, 1974 C is more stable than either the alkyl or the alkene- hydride complexes. Characterization 1H NMR: peak shifted from that of a normal aryl or alkane towards that of a hydride ligand JCH is typically around 70-100 Hz versus the 125 Hz of a normal sp3 C atom Usually, fluxional, exchange between agostic and terminal Hs IR: sometimes, a reduced nCH can be observed Neutron diffraction: dM-H in agostic complexes is from 1.85 to 2.4 Å B: Mo-H, 2.1 Ǻ; IR, 2704 and 2664 cm-1; 1H NMR, -3.8 ppm

Alkane coordination Possible coordination modes: X-ray Reed 1997 X-ray Meyer 2003 h2-H,C

Published in: James A. Calladine; Simon B. Duckett; Michael W Published in: James A. Calladine; Simon B. Duckett; Michael W. George; Steven L. Matthews; Robin N. Perutz; Olga Torres; Khuong Q. Vuong; J. Am. Chem. Soc.  2011, 133 (7), pp 2303–2310 DOI: 10.1021/ja110451k Copyright © 2011 American Chemical Society

Metal Hydrocarbyls and Related Ligands Textbook H: Chapter 3.2.1, 3.8 Textbook A: Chapter 8

Stability and reactivity First metal alkyls: 1848, Frankland, EtZnI and ZnEt2. Alkyl compounds of main group elements (Al, Mg, Si, Sn, Pb) followed the discovery of zinc alkyls. Similar alkyl compounds of TMs were considered unstable until 1960s. Main group alkyl DHf (kcal/mol) BDE (kcal/mol) TM alkyl CMe4 SiMe4 GeMe4 SnMe4 PbMe4 -40 -59 -17 -5 +32 86 75 60 52 36 Ti(CH2tBu)4 Zr(CH2tBu)4 Hf(CH2tBu)4 TaMe5 WMe6 47 64 62 38

Binding modes

Bond strengths for classical s-ligands Classical s-bonding ligands (X = H, CH3, Cl) form strong M-X bonds. Bond dissociation energies (BDEs) of organometallic compounds are more difficult to determine than for organic compounds. Useful in predicting reaction outcome. There is a good correlation between M-X BDE and H-X BDE, except LnM-H is usually stronger than LnM-CH3 for middle to late TM. Explanation: destabilizing orbital interactions. References: Martinho Simöes, J. A.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629 Ziegler, T. Pure & Appl. Chem. 1991, 63, 873 (DFT)

Decomposition temp (°C) General trends M-C bond enthalpy increases within a TM group. In main group alkyls, the M-C bond enthalpy decreases down in a group. M-C: 35-70 kcal/mol Similar to main group alkyl bond strength. Comparable to the strength of a C-I bond. The instability of TM alkyls is kinetic not thermodynamic. One strategy to isolate TM alkyls is to block available orbitals by using coordinating ligands (bipy, phosphines). Decomposition temp (°C) TiMe4 TiEt4 ~ -50 not observed PbMe4 PbEt4 ~ 200 (b.p. 110) ~ 100

Kinetic instability: b-H elimination Modes of blocking b-H elimination No b-hydrogens The alkyl is oriented so that the beta position cannot access the metal center (steric bulk or rigidity). The alkyl would give an unstable alkene as the product.

a (or g)-H elimination a-hydride elimination g-hydride elimination

Other decomposition modes Reductive elimination: isolable alkyl hydrides are rare because the reaction is kinetically facile and thermodynamically favorable. The reverse reaction: C-H activation Homolytic cleavage: rare

Empirical relative stabilities 1-norbornyl > benzyl > trimethylsilyl > neopentyl > Ph ~ Me >> Et (1° R) > 2 °, 3 ° R Fluoroalkyl > alkyl (i.e. -CnF2n+1 > -CnH2n+1 for late TMs) CF bonds are very strong (120-130 kcal/mol vs. 98-104 kcal/mol for alkyl C-H). Chelating (metallacycles) > nonchelating (acyclic) 3rd row > 2nd row > 1st row transition metals

Metal alkyls: bonding and characterization 1H and 13C NMR d for the C and H atoms a to the metal are shifted to high field vs. those in the parent alkane. Coupling of the 13C and 1H nuclei of the alkyl to the metal with spin I = ½: 103Rh (100% abundance); 195Pt (34%); 183W (14%); 199Hg (17%); 187Os (1.6%) or to phosphines (if present). X-ray: M-C bonds are 1.9 – 2.2 Å Reactivity M-R + Br2 gives M-Br M-R + HgCl2 gives R-HgCl

Synthesis: nucleophilic attack on the metal Very useful and rather general method Common reagents are RLi, RMgX (or R2Mg), ZnR2, AlR3, BR3, and PbR4.

Metal hydrides: importance and characterization 1931, Hieber reports H2Fe(CO)4 1955 - 1964, Cp2ReH, PtHCl(PR3)2, K2[ReH9] M-H bonds can undergo insertion reactions with unsaturated substrates Characterization 1H NMR d: +25 to -60 ppm (usual: -5 to -15 ppm) Coupling With the metal (if it has I = 1/2) With cis and trans phosphines: stereochemistry determination With each other (if inequivalent, J = 1 – 10 Hz) IR: n(M-H) 1500 – 2000 cm-1, not very useful (weak intensities) Neutron diffraction (vs. X-ray diffraction): large crystals are needed (1 mm3 vs. 0.01 mm3)

Metal hydrides: synthesis from a ligand From alkyl ligands: a-hydride elimination: Ziegler-Natta polymerization, carbene formation b-hydride elimination: stability of metal-alkyl complexes From other ligands

Silyl complexes: M-SiR3 (R = alkyl, aryl, OR) First complex: CpFe(CO)2(SiMe3), Wilkinson 1956 Trimethylsilyl (TMS) complexes are more numerous than t-Bu complexes (rare) b-elimination inhibited due to instability of the Si=C bonds Sterically less congested because M-Si is longer than M-C M-SiR3 bonds are stronger than M-C bonds due to p-interaction between M and SiR3 fragment Most common synthesis method: oxidative addition of Si-H bonds (in contrast to C-H activation) Rich chemistry http://www.cchem.berkeley.edu/tdtgroup/organometallic.html

Amide, oxides, and halide ligands Extra lone pairs present on the heteroatom Late TM: when 18e, repulsion between lone pairs and filled d orbitals; weakening of the M-heteroatom bond Early TM: when d0, empty orbitals available for p interaction: strengthening of the M-heteroatom bond