Delta-orbital electron, bonding

The important point to remember is that an electron in the delta-bonding orbital of M02(O2CCH3)if has a substantial influence on the strength of the metal-metal interaction. This influence is directly evidenced by the metal-metal vibrational fine structure observed with ionization from the delta orbital, which shows a lowering of the metal-metal stretching frequency and a lengthening of the equilibrium metal-metal bond distance. 

The newly-developed capability to observe metal-metal vibrational fine structure in the valence ionizations of quadruply bonded dimers is illustrated for the delta-bond ionization of Mo2(02CCH3)if. Observation of this structure provides direct information on the bonding influence of an electron in a delta-bonding orbital by showing the significant changes in metal-metal force constant and bond distance that occur when that electron is removed. 

The "classic" molecule is M02(C CCHa), which is an important representative member of di-metal molecules containing a quadruple bond. The occupation of the delta-bonding orbital, which completes formation of the quadruple bond, is a special feature of these molecules. The classic question is the following To what extent does an electron in the delta-bonding orbital contribute to the total bond strength and force constant between the two metal centers ... 

The obvious approach to answering this question is to remove an electron from this orbital and observe the effect on, for example, the metal-metal stretching frequency or metal-metal bond distance. Of course, removal of an electron from the delta bonding orbital creates a positive molecular ion for which determination of these properties may not be possible using normal techniques. In those cases where the ion is sufficiently stable that these properties can be measured, the meaning of the information may be clouded by changes in intermolecular interactions or other internal factors. 

Additional insight is obtained if these results are compared with the related absorption experiments in which an electron from the delta-bonding orbital is excited to the delta-antibonding orbital (2). The pertinent data is summarized in the Table. The state obtained by 6 ionization has a greater formal bond order than the state obtained by 6-h5 excitation, but has a weaker metal-metal force constant and a longer metal-metal bond. It is... 

As in the planar systems, the bonds between neighboring atoms form roads along which the electrons move most readily. For simplicity we assume that the electrons are confined to the center line of each road and that they move at constant potential along the line. The net effect from branching at various junctions, occupied localized orbitals, and attraction to the nuclei is allowed for by adding a positive delta potential (a spike) at each junction (or joint) in the cage. ... 

The delta-density computations showed a heavy loss of the electron density from the n (i.e., Py) orbital of the planar triplet 43T(n-Jt ) as well as from the carbonyl 71-bond. This is then identified as having an n-71 configuration (column 2... 

Again a one-dimensional model, using delta wells to represent the atoms, provides a simple illustration of how the inquiry might proceed. The appendix to this chapter finds six independent wave functions, three bonding and three antibonding, for the problem. The six electrons can occupy, by spin-opposed pairs, the three bonding orbitals. It turns out that the resulting collection... 

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