Multiple bonds valence-bond descriptions

Valence-bond descriptions of multiple bonds are also similar to the simple LCAOMO description. A double bond consists of a sigma bond and api bond, and a triple bond consists of a sigma bond and two pi bonds. Each bonding factor replaces one LCAOMO, but nonbonding orbitals are the same in both methods. There are no analogues to antibonding orbitals in the simple valence-bond method, so molecules with unpaired electrons such as O2 also cannot be well described in the simple valence-bond method. The extended valence-bond methods mentioned in Chapter 20 are more versatile. 

The multiple bond to the metal in a rhenium(V) compound is best represented by the MO description of it being a triple bond, consistent with the symmetry of the px and p orbitals on the ligand see 1 (1). In keeping with that, these bonds are found to be relatively short and quite strong. The structure has also been presented as 1 a valence bond formulation that facilitates the counting of oxidation states but does not provide an accurate representation. 

The crystal structure of 864(118207)2 90, 91) has shown 864 to be square planar with an 8e-8e bond distance of 2.283(4) A, significantly less than that of 2.34(2) A found in the 8eg molecule (92), indicating some degree of multiple bonding. 8uch a result is consistent with a valence bond description of the molecule involving four structures of type VII. Alternatively the structure can be understood in terms of molecule orbital theory. The circle in structure VIII denotes a closed-shell (aromatic ) six-w-electron system. Of the four tt molecular orbitals,... 

Recent valence bond studies of multiple bonds in molecules with only s, /7-orbitals indicate that bent bonds are preferred to the usual <7 and tt bonds. This has potentially important implications for the description of multiple metal-metal bonds. However, the description of E, IT and A ion states in photoemission from a ground state of bent bonds is not so obvious as in the <7, tt, -molecular orbital model. We examine these issues in the present contribution. 

The EBO concept rehes on a multi-configurational wavefunction and takes into account the effect of electron correlation involving the antibonding orbitals. There are various ways of quantifying bond orders [12-14]. The Natural Bond Orbital (NBO) valence and bonding concepts are also extensively used in the analysis of multiple bonds. NBO, like EBO, is based on a quantum mechanical wavefunction. The NBO description of a bond can be derived by variational, perturbative, or density functional theory (DFT) approximations of arbitrary form and accuracy [15]. 

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