Delta bonding

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... 

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 complex ion octachlorodirhenate(III) ion has short Re Re distance and eclipsed configuration of the chlorine atom Re(III) is a d4 species. The Re Cl bonds may be considered tcrbe dative bonds from Cl ions to Re3+. The formation of one sigma bond, two pi bonds and one delta bond causes the pairing of four electrons in quadruple bond, hence the complex is diamagnetic. 

The Re2Cl4(PMe3)4 and related compounds contain a so-called electron rich triple bond, based on the metal atom. These Re2Clt(PR3)4 species can be reoxidized, stepwise and reversibly ... 

In the case of two-center bonds, a pi bond has a nodal plane that includes the internuclear bond axis, whereas a sigma bond has no such nodal plane. (A delta bond in organometallic or inorganic molecular species has two nodes.) Radicals are classified by analogy into sigma and pi radicals. 

Another example Br4Re-ReBr4, structure III, has two unusual structural features. The bromines attached to the two different metal atoms are not staggered to minimize repulsive interactions, but are eclipsed. Moreover, the rhenium-rhenium bond is very short. These structural features and the magnetic properties have been interpreted as indicating that a quadruple bond links the metal atoms. The bonds include one o--bond, two r-bonds, and a S (delta)-bond. A S bond can be visualized by placing the bonded atoms on a z coordinate axis and having overlap between two orbitals (one from each metal atom), structure IV. 

In 2005, the first metal-metal quintuple bond was proposed for the dichromium compound shown in Figure 10.54, which shows how a second delta bond can form between the d-orbitals of the two metals. 

When two metal atoms are brought together, in principle, one sigma, two pi, and two delta bonds may be formed. 

Cotton EA, Nocera DG (2000) The whole story of the two-electron bond, with the delta bond as a paradigm. Acc Chem Res 33 483 90... 

Falvello LR, Foxman BM, Murillo CA (2014) Fitting the pieces of the puzzle the delta bond. Inorg Chem 53 9441-9456... 

---