Orbital hybridization carbenes

The two non-bonding electrons at the sp -hybridized carbene carbon atom can occupy the two empty orbitals (p and a) with a parallel spin orientation leading to a triplet ground state (cr pn, state, Figure 1.1). Alternatively, the... 

The two non-bonding electrons at the sp -hybridized carbene carbon atom can occupy the two empty orbitals (p and o) with a parallel spin orientation, leading to a triplet ground state (o Pn state. Figure 1.1). Alternatively, the two electrons occupy the a orbital with an antiparallel spin orientation (a Pn , % state). An additional, generally less stable, singlet state (a p Aj state) and an excited singlet state with an antiparallel occupation of the p and c orbitals (a p state) are coneeivable but of no relevanee for the present discussion. 

Divalent carbon species called carbenes are capable of fleeting existence. For example, methylene, CH2, is the simplest carbene. The two unshared electrons in methylene can be either spin-paired in a single orbital or unpaired in different orbitals. Predict the type of hybridization you expect carbon to adopt in singlet (spin-paired) methylene and triplet (spin-unpaired) methylene. Draw a picture of each, and identify the valence orbitals on carbon. 

These observations are compatible with the model for the carbene complex presented in Section II,A. Both metal and w-donor substituents compete to donate electron density to unfilled carbenepz orbitals, and with good 7r-donors such as nitrogen, the metal is less effective. In terms of resonance formalism, the resonance hybrid 39 makes a more significant contribution than 40 to the structure of the carbene ligands in these compounds. Similar conclusions are reached when the structures of Group 6, 7, and other Group 8 heteroatom-substituted carbene complexes are considered. 

Spectator substituents, bonded to the carbene s migration terminus (Ci), directly influence the lifetime and philicity of the carbene, but they do not primarily alter the migratory aptitudes of migrants on C2. Oxa spectator substituents stabilize singlet carbenes by electron donation to the vacant carbenic p orbital (LUMO) cf. resonance hybrid 69. 

Phenylcarbene (la). Just as in triplet methylene (CH2), in triplet phenylcarbene (3A"-la) one electron occupies the p-jr atomic orbital on the carbene carbon and one electron occupies the in-plane a hybrid orbital. However, in the lowest singlet state of CH2 and of phenylcarbene ( A -la), both electrons occupy the hybrid a orbital, because this orbital is substantially lower in energy than the p-jt AO. 

After filling of the lbi, 2bi, and la2 bonding it MOs, the last two electrons must be distributed between the ai, 3bi, and 2a2 orbitals. As depicted in Fig. 8, the carbene hybrid orbital (ai) is lower than but close in energy to the 3bi and 2a2 it MOs. Five electronic states are thus worthy of consideration 1 Ai, 2, Bi, 3A2, and 3Bi. The electron configurations of these states and the CASSCF optimized geometry of each are shown in Fig. 9.57... 

The two substituents and a pair of electrons occupy the three sp -hybrid orbitals, while a formally vacant p orbital remains at the carbene carbon atom. Because of the similar chemical behavior of these nucleophihc carbenes compared to phosphines, organometallic chemists have in many cases successfully replaced phosphines by stronger donating N-heterocychc carbenes. 

Phenylnitrene is intrinsically more stable than the isomeric pyridylcarbene. Kamey and Borden explained that the reason why the N—H BDEs in RNH radicals are smaller than the C—H BDEs of RCH radicals, that is, rehybridization, is also responsible for the fact that nitrenes are thermodynamically more stable than carbenes.The lone pair of electrons of a nitrene reside in a low-energy sp hybrid orbital. This effect dramatically stabilizes nitrenes relative to carbenes in which the nonbonding electrons reside in either pure p or pseudo sp orbitals. 

A general cheletropic reaction is shown in Figure 12.2. This reaction involves the addition to, or extrusion from, a conjugated system of a group bound through a single atom. The reaction usually involves the elimination of simple stable molecules such as SO2, CO, or N2. The atom to which there were two a bonds carries away a pair of electrons, usually in a spn hybrid orbital. The addition of a carbene to a simple olefin to form a cyclopropane is also a cheletropic reaction which, as discussed in Chapter 14, is not predicted to be concerted. Cheletropic reactions incorporate features of both cycloaddition and electrocyclic reactions. 

Answer to 4(d). We can consider the orbitals of cycloheptatrienylidene to arise from the interaction of the orbitals of hexatriene and the valence orbitals of a di-coordinated carbon atom (a 2p orbital and an sp" hybrid orbital). The orbitals of hexatriene may be obtained from an SHMO calculation. The interaction diagram is shown in Figure B7.2. The p orbital of the carbene site is raised as a result of the dominant interaction with 713 of hexatriene. The orbital 714, which is closest in energy to the carbene s p orbital, does not interact because of symmetry, and 5 interacts less strongly because the coefficients at the terminal positions of the hexatriene are... 

Another anomalous cycloaddition is the insertion of a carbene into an alkene. 6-Electron cheletropic reactions (p. 28) are straightforward allowed pericyclic reactions, which we can now classify with the drawings 3.47 for the suprafacial addition of sulfur dioxide to the diene 2.179 and its reverse. Similarly, we can draw 3.48 for the antarafacial addition of sulfur dioxide to the triene 2.180 and its reverse. The new feature here is that one of the orbitals is a lone pair, which is given the letter co to distinguish it from o- and n-bonds, with suprafacial and antarafacial defined by the drawings 3.45 and 3.46, which apply to all sp3 hybrids and p orbitals, filled or unfilled. 

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