Three-center bonding interactions fragments

For nickelocene in the triplet state, a very different bonding picture can be expected (Table 4.52). The two unpaired electrons now require two nonbonding d orbitals (beyond the three filled d orbitals for the remaining six electrons), so that only a valence s orbital remains for 2c/2e bonding. This leads to formal M—Cp+ and nonbonded Cp- fragments at the lc/2e Lewis-like level, which naturally interact with strong three-center tu-bonding resonance of the form... 

So far, we have looked at different modes of bonding and how Pauli repulsive orbital interactions may either influence them (2c-2e bond) or be an essential part (2c-3e bond). In this section, we examine a different role of Pauli repulsion, namely, the one it plays in the absence of bonding interactions between groups. Here, it is responsible for the fact that such groups, A and B say, repel each other. Or, to put it in another way, steric repulsion between A and B is a pure quantum effect, caused by the Pauli repulsion between same-spin electrons of the different fragments, such as the well-known two-center, four-electron (2c-4e) repulsion (19) or the two-center, two-same-spin-electron (2c-2sse) repulsive component (20) of the three-electron bond. 

A relatively common interaction in molecular coordination or organometallic compounds that nominally are coordinately unsaturated is the formation of a three-center two-electron bond between a metal center in the compound and a C-H bond of a hydrocarbon, a hydrocarbon fragment, or a hydrocarbon derivative that is a ligand in the complex. This interaction can be the prelude, the intermediate or transition state, to a subsequent reaction in which the CH hydrogen atom is transferred to the metal center and a direct a bond is formed between the carbon atom and the metal atom especially if the C-H bond is an activated bond. Internal oxidative addition of CH is a term often applied to this subsequent reaction step. The overall sequence is schematically outlined in 1. Factors that materially... 

Hydrocarbon Fragments - Modeling by Molecular Orbital and Cluster Chemistry. A basic guideline for metal surface coordination chemistry with respect to hydrocarbon or hydrocarbon derivatives may be formulated as follows If the stereochemistry of the chemisorption state allows C-H hydrogen atoms to closely approach surface metal atoms then the chemisorption state should be further stabilized by the formation of a three-center two-electron C-H-metal bond. This effect should be more pronounced the more electron deficient the metal surface. There should be an activation of the C-H bond and the hydrogen atom should become more protonic in character. If the C-H bond is sufficiently weakened by this interaction then C-H bond cleavage should result. 

The transannular interaction in germatranes is explained by the model of hypervalency that assumes the formation of a three-center four-electron N Ge—X hypervalent bond . This model successfully explains the fraw -inhuence in such fragments, i.e. the inverse interdependence and the constancy of the sum of Ge—X and Ge—N bond orders. The calculated energies of the transannular bonds in some germatranes are higher than those in the corresponding silatranes . The three-center N Ge—X bond has a predominantly a-nature with a minor contribution from jt-interaction of the 4d-orbitals of the Ge atom. 

The addition of more triple bond fragments that can conjugate with the anionic center allows for even greater stabilization of the anion, leading to the reduction in DPE in the series 5 (377.1 kcal mol ) to 7 (353.7 kcal mol ) to 8 (334.1 kcal mol ). Since the anion of 3cb can also interact with three triple bonds, it is reasonable to expect that the DPE of 3 should be similar to that of 8. 

Fig. 40.7. Fragment molecular orbitals that produce the three-center/two-electron bonds (one of three by symmetry), which constitute the principal metal-to-metal bonding interactions of Ru3(CO)i2[Pd(PR3)].

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