Agostic distortion

The leading NBO interaction associated with agostic distortions is... 

In particular, we wiU discuss here the results on Ir(H)2(PCy2Ph)3+. In this complex, X-ray data (20) indicate the existence of an agostic distortion, with one of the Ir-P-C angles being 100.9°, significantly different from the standard tetrahedral angle of ca. 109°. In the IMOMM(MP2 MM3) calculation of this species, two different partitioning schemes between the QM and MM domains were applied (Fig. 1). In model I, aU atoms not directly bound to the metal were... 

The pivoting of the methyl group (4-24b) moves its local C3 axis, so that the ncHj orbital is no longer oriented along the z-axis (4-25). The overlap between ruzH, and z therefore decreases, as does the electronic stabilization associated with this interaction. If this were the only factor to consider, this motion would therefore be energetically unfavourable, and no agostic distortion would be expected. 

At this point in our analysis, we must note that the interpretation of the agostic distortion we have suggested does not really match the notion that it is due to an additional interaction between a hydrogen atom, or a C—H bond, and the metal centre. Notice, however, that a weak M... H bonding interaction does appear in the MOs represented in 4-26 for the agostic structure (right-hand side). Moreover, one of the bonding MO of CH3 (often referred to as ttchj), that mainly characterizes the C—Hi bond, does have an increased interaction with the empty yz orbital, due to the substantial overlap between Ishi tid yz (4-27). This interaction... 

Although complex 1 presented an important agostic distortion, the oxidative addition from this species is disfavoured (Scheme 1, pathway a). The introduction of HCO3 in the system results in the coordination of the carbonate to the ruthenium and the formation of adduct 4 which evolves to product 5. The transformation from 3 to 5 is exothermic by 13.7 kcai moi making the proton abstraction mechanism more plausible (Scheme 1, pathway b). 

N(SiMe3)2 ] (M = Ca or Sr, Fi2-Tp ° = fluorinated 3-phenyl-hydrotris(indazolyl)borate) (Scheme 8) have been used as precatalysts for the intramolecular hydroamination of 2,2-dimethylpent-4-en-l-amine. The Sr complex exhibits a Sr- MeSi agostic distortion. The complex... 

The Sm-N distances are consistent with single a bonds suggesting the presence of a (C H NNC.H dianion. However, the 1.25(1) a NN distance is the same" as tire N=N double bond distances in a variety of azobenzene structures. The molecule also contains distorted N-C (phenyl) distances, which are stretched from a normal 1.42 8 distance to 1.56-1.61 8. In addition, the samarium atoms are displaced asymmetrically such that both come within 2.29-2.34 8 of the ortho hydrogens of a single phenyl ring in a bonafide agostic (6<0 Sm-H interaction. The ability of Sm(II) to structurally distort azobenzene in this way is remarkable. 

As an illustration of the agostic effect, let us consider the simple example of YH2(C2H5), the ethyl derivative of the parent YH3 species (Fig. 4.54(a)). The optimized structure of YH2(C2H5) is shown in Fig. 4.55, exhibiting the distortive... 

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. 

Mononuclear amidometal dihalides (obtained from the appropriate LnX3) were the distorted pentagonal bipyramidal [CeBr2 N(SiMe3)2 (thf)3] having agostic CH,- -Ce... 

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