Agostic-bond complexes

The X-H (X = B, C, Si) bonds in some ligand molecules, like the H-H bond in H2, provide their a bonding electron pairs to bind the ligands to metal centers. They form intermolecular coordination compounds (i.e., sigma-bond complexes) or intramolecular coordination compounds (i.e., agostic-bond complexes). All such compounds involve non-classical 3c-2e bonding and are collectively termed a complexes. 

The /3-agostic ethyl complex is similar to a transition state intermediate in the step of the ethene insertion into the Pd - H bond. This might be also relevant to the high catalytic activity of this catalyst. 

X-ray structure analysis of 49 revealed that agostic bonding causes a severe distortion of the carbene ligand, so that the W-C-Si bond angle is reduced to 87.8(6)°, compared with 113.1(4)° in the starting complex. The hydride atom was, however, found closer to the silicon atom, the W-H and Si-H distances being 2.1(1) and 1.5(1) A, respectively. 

Si H M agostic interactions in silylamido complexes have been extensively studied to date. The earlier examples were prepared by halide displacement in the coordination sphere of a metal by a silylated amide, which puts severe limitations on the nature of the substituents at silicon (usually, robust methyl groups are used). More recently, a new route to p-agostic silylamides based on the direct coupling of silanes with imido ligands was discovered that allows one to trace the effect of substitution at silicon on the extent of the Si-H bond complexation (vide infra). 

Finally of note is the l,5-cyclooctanediylbis(pyrazolyl)borate complex BBN(pz)2 2Ni (7, BBN = borabicyclononane), which, as previously observed of its cobalt analogue,8 9 exhibits significant agostic bonding between nickel and the bridgehead protons located above and below the N4 plane. While this situation was confirmed crystallographically for the cobalt system, interactions in 7 were implied solely on the basis... 

Figure 11.5.7 shows the molecular structures of some transition metal complexes in which agostic bonding has been characterized by either X-ray or neutron diffraction. Structural data for these compounds are listed in Table 11.5.2. 

Many complexes containing P-, N-, or O-containing ligands can also form agostic bonds ... 

Molecular structures of some complexes with agostic bonds (a) Half of the dimeric molecule... 

This decomposition pathway leads to equimolar amounts of alkenes and alkanes. Complexes with /3-agostically bonded alkyl groups are likely intermediates structures of this kind may sometimes be the ground state, as in [(P—P)Pt(r72-C2H5)]+., 5... 

While in manganese and other electronically saturated complexes an -bonded acyl (21-XXIXb) is observed as an unstable intermediate, there are numerous examples of stable V-acyls of Group 4-6 metals.153 For molybdenum a third, j8-agostic bonding mode was observed which is in equilibrium with the alkyl-carbonyl isomer 154... 

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