Recoupled pair bond dyad

The presence of one or more recoupled pair bond dyads in a compound has imphcations beyond the geometry and energetics of the equihbrium structure. For example, asymmetric vibrational stretching of the atoms in the F-S-F dyad reintroduces antibonding character as each of the S-F bonds lengthens in turn, and this results in an infrared vibrational feature that is very intense. The predicted... 

As SF4 has no unpaired electrons, further bond formation requires recoupling the remaining lone-pair of S, which is derived from the 3s orbital (though this orbital has acquired considerable 3p character as interactions with the bond pairs displace it away from the S nucleus). SF5(X Aj) has two F-S-F dyads and an S-F recoupled pair bond. SFg(X Aj) is formed by completing the third F-S-F dyad. The bond energies Dfj for forming SFj and SFg via addition to their respective SFj j species are 41.1 (recoupled pair bond) and 109.2 kcal mol (recoupled pair bond dyad). 

Our studies have shown that covalent and recoupled pair bonding are competitive alternatives. The results for SF2 show that it is possible to find states of the same compound that involve pure covalent bonding, a recoupled pair bond dyad, or a mixture of covalent and recoupled pair bonding. There is no suggestion in the 3c-4e model that such variety is possible. The 3c-4e model offers only limited insight into the excited states of a compound where the bonding may be very different from that in the ground state. 

Keywords Edge inversion Vertex inversion Transition state Recoupled pair bond Recoupled pair bond dyad Generalized valence bond (GVB) theory... 

In the last section, we posed three questions. In this section, we show that the answers to all of these questions center on the ability of the P atom to form recoupled pair bonds with F and, more specifically, p-recoupled pair bond dyads with two F atoms. In molecules other than F2PH and PF3, the inversion transition states involve formation of s-recoupled pair bond dyads. 

NH3 and PH3 each have three equivalent bonds. Upon bond formation, the orbitals on the two atoms polarize, hybridize, expand or contract, delocalize, etc. in response to the presence of the other atoms. However, it is clear that, for instance, one of the orbitals participating in the NH bond closely resembles the 2s lobe orbital of N shown in Fig. 4, although with somewhat more 2p character than in the N atom, and the other orbital resembles the 1 s orbital of the H atom. The lone pair orbital is essentially the out-of-plane 2px orbital in the N( D) state. This is consistent with the orbital diagram la in Fig. 3. The three equivalent NH bonds result from resonance between the 2s-recoupled pair bond dyad and the normal covalent bond with the 2py orbital. The resulting stmcture has D3h symmetry because this arrangement reduces the PauU repulsion between the three bonds, and so, the GVB orbitals on the N atom in the NH3 transition state are resonance averages (hybrids) of the two 2s lobe orbitals and the 2py orbital. The situation is similar in PH3. The reason for the shorter bond distances in... 

Further comparison of s- and p-recoupled pair bond dyads... 

The explanation for the anomalous behavior of the F2PH and PF3 molecules is simple. In the transition states of both F2PH and PF3, the nearly collinear PF bonds are a result of the formation of p-recoupled pair bond dyads. A hydrogen atom is not sufficiently electronegative to recouple the lone pair in the P( D) excited state, and therefore, these bonds are only found in F2PH and PF3. Formation of the p-re-coupled pair bond dyads in the transition states of these two... 

The concept of recoupled-pair bonding was introduced in 2009, and applied in a number of subsequent publications - see for example Refs. 1-4. A recoupled-pair bond forms when an electron in a singly occupied ligand orbital recouples the pair of electrons in a doubly occupied lone pair orbital on the central atom, leading to a central atom-ligand bond. A recoupled-pair bond dyad occurs when a second ligand forms a bond with the orbital left over from the initial recoupled pair bond ". ... 

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