Hydrogen agostic

Ito et al. (74) obtained a room-temperature spectrum of ethane adsorbed on an evaporated film of palladium. Absorptions were observed at 2850 and 2700 cm 1 characteristic of an alkyl species. The soft vCH 2700-cm 1 band implies the presence of agostic hydrogen-bond-type interactions of C-H bonds with surface metal atoms. 

In a number of cases—viz. Ni(111), Ni(100), Pd(lll) (Table VI), Fe(lll) 234) and Fe( 100) 235)—broad and low-wavenumber vCH absorptions occur in low-temperature spectra. These are attributed to agostic (hydrogen-bond-like) interactions with surface metal atoms 99), leading to weakened and more... 

Complexes of this type are relative few in number indeed, there remain no examples of discrete palladium hydrides, though an agostic hydrogen interaction was reported for Pd (pz)2BBN (96)46 between Pd and the bridgehead... 

An interesting compound is (N-phenyloctaethylporphyrinato) phenyl-ruthenium(II) (entry 10) in which an agostic hydrogen of the N-phenyl group completes the heavily distored octahedral coordination sphere of the Ru(II) ion. The Ru ion protrudes by 14 pm towards the axial phenyl anion. 

The mechanism that is commonly considered to operate in the polymerisation of ethylene and a-olefins in the presence of group 4 metallocene-based catalysts is that devised by Cossee [268, 276, 277] for propylene polymerisation with heterogeneous Ziegler-Natta catalysts, though modifications invoking effects such as a-agostic hydrogen interactions with the metal centre have been proposed [343,344]. 

Density functional molecular orbital calculations suggest that a-agostic hydrogen interaction with the metal atom in a Zr-CH3 unit helps to stabilise it and align the methyl group of this unit for interaction with the ethylene n orbital [351,352]. The a-agostic transition state has one less rotational degree of freedom, which stabilises it and reinforces for the C C bond formation. 

The carbon-carbon bond angle of the Zr-CH2-CH2-Zr structure (compound 4) is unusually narrow—only 76° (64). Compound 6 shows a similar carbon-carbon bond angle of 75° for the Zr-CH2-CH-Al structure. (i-Agostic hydrogen bonds can explain these angles and have been suggested to be a reason for the stabilization of the active site during the polymerization when methylaluminoxane is used as a cocatalyst. 

Treatment of Os3(CO)i2 with KOH in MeOH gives the [HOs3(CO)n] ion, which can be converted via reactions such as those shown in Fig. 16-10 into a methyl derivative, Os3(CO)i0(H)CH3. This methyl compound is electronically unsaturated (46 electrons) and responds to this by developing a strong interaction between one C—H bond and an adjacent osmium atom (see Section 2-16 for agostic hydrogen atoms). This peculiar methyl compound... 

An important mechanism in organometallic chemistry is the activation of C-H bonds. This agostic hydrogen can be detected by IR spectroscopy. Characteristic low-lying CH bands, for instance, are observed in the compound... 

Topological analysis of the experimental and theoretical electron densities in TiCCEtCdmpe) (dmpe = Me2PCH2CH2PMe2) suggests the presence of a bond critical point between titanium and the / -agostic hydrogen atom.236... 

Once complexation of the alkenes occurs, 1,2-insertion is facile and reversible. Insertion gives initially a 16-electron complex, 6, which shows an agostic hydrogen. The high concentration of CO quickly drives step c to completion, resulting in 18-electron intermediate 7. Structures 6 and 7 show the result of anti-Markovnikov insertion from which the linear aldehyde, rather than the branched isomer, will result. 

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