Sigma -bonding interactions

Sigma bonding interaction between two ligand orbitals and metal d r orbital... 

TABLE 16.12 Sigma bonding interactions between the ligands in the positions defined by Figure 16.29 and each of the d-orbitals (in units of... 

Figure 13.51 Energy level diagram for an octahedral metal complex showing only sigma bond interactions... 

Figure C3.2.7. A series of electron transfer model compounds with the donor and acceptor moieties linked by (from top to bottom) (a) a hydrogen bond bridge (b) all sigma-bond bridge (c) partially unsaturated bridge. Studies with these compounds showed that hydrogen bonds can provide efficient donor-acceptor interactions. From Piotrowiak P 1999 Photoinduced electron transfer in molecular systems recent developments Chem. Soc. Rev. 28 143-50.

DFTB has been combined with various force field programs, including CHARMM [49], Amber [50, 51], SIGMA [52], TINKER [34] and GROMACS (to be published). All these combined methods have used the same strategy for the interface with respect to the treatment of bonding interactions at the interface and non-bonded Coulomb and van der Waals (vdW) terms. The concepts will not be reviewed in detail here, for this purpose we would like to refer to an excellent and extensive recent review [4],... 

F.E. Inscore, R. McNaughton, B.L. Westcott, M.E. Helton, R. Jones, I.K. Dhawan, J.H. Enemark, and M.L. Kirk, Spectroscopic evidence for a unique bonding interaction in oxo-molybdenum dithiolate complexes implications for sigma electron transfer pathways in the pyranopterin dithiolate centers of enzymes. Inorg. Chem. 38, 1401-1410 (1999). 

The following question remains Why are the sigma-type interactions weaker than the pi-type interactions in this case This is apparently a geometrical effect, enforced by the relative sizes of M and X valence and core orbitals in the ionic bonding environment, and illustrated in the NAO diagrams of Fig. 2.18. We may... 

As shown in (4.72a) and (4.72b), such interactions give rise to formal sigma bonds (omb or oma) that are expected to be oppositely polarized, with atomic charges of opposite sign on the metal atom depending on whether donation is to (4.72a) or... 

From this example one can judge that insertions of metal atoms into sigma bonds may be precluded by unfavorable spin or steric/promofion factors, despite the formal possibility of attractive interactions such as depicted in Fig. 4.62. Long-range s2 steric/promofion barriers may be expected to block direct insertion of metal atoms... 

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