MO and resonance

The chemical reactivity of these two substituted ethylenes is in agreement with the ideas encompassed by both the MO and resonance descriptions. Enamines, as amino-substituted alkenes are called, are vety reactive toward electrophilic species, and it is the p carbon that is the site of attack. For example, enamines are protonated on the carbon. Acrolein is an electrophilic alkene, as predicted, and the nucleophile attacks the P carbon. 

The biradical resonance structure for ozone requires two singly occupied MOs, and it is clear that an RHF type wave function, which requires all orbitals to be doubly occupied, cannot describe this. A UHF type wave function, however, allows the a and /3 orbitals to be spatially different, and can to a certain extent incorporate both resonance structures. Systems with biradical character will often have a (singlet) UHF wave function different from an RHF. 

Since the time of the quantum-mechanical calculations by Longuet-Higgins, many attempts have been made to calculate tt-electron densities, resonance energies, dipole moments, and optical transitions both by the LCAO-MO and the valence bond method.However, no agreement has been reached on the importance of pd-hybridization of the sulfur atom. This is considered by some workers an essential... 

The VB and MO theories are both procedures for constructing approximations to the wavefunctions of electrons, but they construct these approximations in different ways. The language of valence-bond theory, in which the focus is on bonds between pairs of atoms, pervades the whole of organic chemistry, where chemists speak of o- and TT-bonds between particular pairs of atoms, hybridization, and resonance. However, molecular orbital theory, in which the focus is on electrons that spread throughout the nuclear framework and bind the entire collection of atoms together, has been developed far more extensively than valence-bond... 

The possibility that only some of the outer electrons enter into bond formation needs to be considered. There is a close relation between interatomic distance and bond type, and the values of the interatomic distances in the sequence6 K, A2, 4.62A Ca, Al,A3, 3.93-3.95A Sc, not yet investigated Ti, A3, 2.92-2.95A V, A2, 2.63A Cr, A2, 2.49A, as well as those in the similar sequences from Rb to Mo and Cs to W, indicate that the number of bonds resonating among the available positions increases from one to nearly six. 

This approach explains satisfactorily the decrease in bond length with increasing valency in the sequences of elements K —> Cr, Rb — Mo, and Co — W. The number of electrons available for bond formation in these metals increases with valency, and therefore, according to the resonating valence bond theory, the number of bonds resonating about the available positions increases, resulting in smaller bond lengths. 

The fundamental vibrations have been assigned for the M-H-M backbone of HM COho, M = Cr, Mo, and W. When it is observable, the asymmetric M-H-M stretch occurs around 1700 cm-1 in low temperature ir spectra. One or possibly two deformation modes occur around 850 cm l in conjunction with overtones that are enhanced in intensity by Fermi resonance. The symmetric stretch, which involves predominantly metal motion, is expected below 150 cm l. For the molybdenum and tungsten compounds, this band is obscured by other low frequency features. Vibrational spectroscopic evidence is presented for a bent Cr-H-Cr array in [PPN][(OC)5Cr-H-Cr(CO)5], This structural inference is a good example of the way in which vibrational data can supplement diffraction data in the structural analysis of disordered systems. Implications of the bent Cr-H-Cr array are discussed in terms of a simple bonding model which involves a balance between nuclear repulsion, M-M overlap, and M-H overlap. The literature on M-H -M frequencies is summarized. 

AsPh4]2[Mo S2C2(CN)2 3] was fitted by an axially symmetric spin Hamiltonian with g, = 2.000, g l = 1.974, Att = 10 G and AL = -100 G, interpreted in terms of a 1A1 ground state in D3 symmetry.49 The unpaired electron has predominantly metal dzi character. A molecular orbital scheme (Figure 4) was proposed for the dithiolene complexes.35 This scheme has been used in the discussion of ESR results,49 and electronic and resonance-Raman spectra.34... 

Fig. 4.24 The SHM charges on the atoms of a molecule can be calculated from the number of electrons in each occupied MO and the coefficients of these MOs. The predicted dipolar nature of methylenecyclopropene has been ascribed to a cyclopropenyl-cation-like resonance contributor...

The stability of a molecule based on its filled and empty MOs, and its delocalization energy or resonance energy, based on a comparison of its total n-energy with that of a reference system. The pattern of filled and empty MOs led to Hiickel s rule (the An I 2 rule) which says that planar molecules with completely conjugated p orbitals containing An I 2 electrons should be aromatic. 

Note The MOs of an ML complex can be found by interacting the metal orbitals with the symmetry orbitals of the ligands.21 The foregoing method, where we calculate the Hiickel MOs and then zero the resonance integral, permits symmetry orbitals to be obtained without recourse to group theory. 

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