Triplet states valence bonds

Hall, G. G., Proc. Roy. Soc. [London) A213, 113, The molecular orbital theory of chemical valency. XI. Bond energies, resonance energies and triplet state energies. ... 

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... 

As expected from the valence-bond descriptions of Fig. 3, the computed geometries of the lowest A singlet and triplet states of 16 have quinonoid geometries displaying bond localization (Fig. 5). On the other hand, the quintet A -16... 

Finally, we shall briefly discuss the compounds of the silicon subgroup elements in their divalent state (germylenes, stannylenes and plumbylenes) R2M (R = alkyl, aryl). The chemical bonds in R2M are formed by the px and py orbitals of M. The pz orbital is unoccupied and there is an unshared electron pair in an s-orbital of M. Therefore, the R2M compounds have both electrophilic and nucleophilic properties. The valence angle R—M—R diminishes as the atomic number of M increases. The distances of M—R bonds in R2M are less than in the corresponding tetracoordinate R4M derivatives87. The spatial structure of R2M compounds, the shielding of the reaction centre (M ) as well as the possibility for R2M molecules to transit from a singlet state into a triplet state depend on the steric bulk of the R substituents. If the R substituents are bulky, some R2M ... 

Figure 1 Representative valence bond diagrams for benzene, their bit representation using two bits per site, and the integer Ik that encodes these correlated states with six 7r-electrons. The Kekule diagrams 1 > and 2 > are covalent singlets, as is the Dewar diagram 3 >, with one electron at each site. Diagram 4 > is an ionic singlet, while 15 > is a covalent triplet.

Several structures may be envisioned for compounds MLx 2( r-C ), which are broadly distinguished by whether or not they describe closed-shell singlet states or open-shell triplet states, and the number and combination of bonds between the metal and carbon as well as adjacent carbon centers. There have been several reports rationalising the valence bond descriptions of complexes containing even387 389 or odd values of / 390,391 and related polymeric materials and Sponsler has offered a comparison of the... 

---