HOMOs doubly degenerate

TPP (45, 46), as well as data for octaalkyl and octathioalkyl porphyrazines. As with the phthalocyanines and porphyrins, the electronic spectra of porphyrazines can be rationalized using Gouterman s four-orbital model, shown in Fig. 5 (47, 48). All of these macrocycles, when symmetrically substituted and with a metal ion incorporated in the central hole, for example, the M[pz(A4)] or B4, have D4h symmetry, with a doubly degenerate lowest unoccupied molecular orbital (LUMO) (eg) and two highest lying highest occupied molecular orbitals (HOMOs) that complete the four Gouterman orbitals with alM and a2u symmetry. 

In n symmetry, there is a stabilizing donor-acceptor interaction involving four electrons between the doubly degenerate nHOMO and itLUMo causing two partial n bonds (8). They are opposed by the Pauli repulsive iHomo - homo two-center four-electron (2c-4e) interaction. Note the difference in nature between c and the n bonds the former is an electron pair bond between singly occupied orbitals, whereas the latter evolves from a donor-acceptor or charge transfer interaction between occupied and unoccupied orbitals. 

Phosphaalkynes RC=P display a rich coordination chemistry, and five different modes of ligation (M-Q) may be discerned in phosphaalkyne transition-metal complexes (Scheme 25).3,51 According to theoretical calculations and to photoelectron spectroscopic investigations, the doubly degenerated jr-orbitals of the triple bond are the HOMOs in phosphaalkynes. In keeping with this, the rj2-coordination modes N-Q are usually realized with transition metals. Complexes with rjMigated phosphaalkynes M are only possible when the respective ensemble of metal atom and ancillary ligands form an appropriate pocket, which only allows rj1-coordination by a linear molecule.3,sl... 

The first examples are doubly-degenerate excitations. The simplest ones are doubly-degenerate n-jt excitations in diatomic molecules such as N2(Dooh) and CO (Coov)-The doubly-degenerate highest-occupied MOs (HOMOs), as denoted by n, belong to the irreducible representations of jtu in Dooh and it in Coov, while the doubly-degenerate lowest-unoccupied MOs (LUMOs), as denoted by jt, belong to those of Kg in Dooh and k in Coov, respectively. The direct products between Ku and Kg and between k and k can be decomposed as follows ... 

Figure 30 Qualitative molecular orbital diagram illustrating the bonding in a linear metal nitrosyl species focusing on interactions with the d orbitals. The HOMO of NO is a singly occupied doubly degenerate orbital and its involvement in the bonding requires the metal to contribute three electrons to the derived molecular orbitals. Thus, a d metal center becomes d upon coordination of NO, as illustrated for the case in which n = 3. The interaction may, therefore, be represented via the triply bonded resonance structure M=N-0.

It predicts that a two-electron system II3 should be triangular wdiilc a four-electron system II3 should be linear. In a structure, the HOMO ofH is doubly degenerate and half-filled as depicted in 7.23. The degeneracy is lifted by the distortion since it stabilizes the level but destabilizes lev Throughout the... 

With 14 electrons, the HOMO of staggered ethane, is doubly degenerate and completely filled. The energy of eclipsed ethane differs from that of the staggered conformcr, since the magnitudesof tlic orbital interactions between two CH3 groups... 

Furthermore, we deal with a stfuctural distortion of the trimethylenemethane radical cation (TMM+) [63], The neutfal TMM has a high symmetry of and is in a triplet state with a doubly degenerate c" HOMO in the ground electronic state [64], The Z)3h structure of TMM is expected to be distorted to a Civ structure with a lower symmetry in TMM+ due to the J-T effect, so as to split the original degenerate e" orbitals into ai and b orbitals. 

---