Quadruple bond formation

The empirical valence bond model has shown good predicting power if one defines the bond multiplicity to be two for compounds such as CePd or BaPd and three for compounds such as LaRh, Ylr, or RhTh. The limitations of the model could also be shown (40) when applied to assumed quadruple bond formation, as in RuV or ThRu or to double bond formation as in CePt or ThPt. The case of ThRu (41) shows that quadruple bonding is approached where suitable valence states are available, but not fully achieved, apparently because of the directional requirements of the bonding in a diatomic molecule. The examples of CePt and ThPt (40) show the calculated values that have been based on an assumed double bond to be too low. Platinum has no suitable valence state for triple bond formation, but apparently forms triple bonds with other... 

Metal-metal bond formation for the transition metals shows similar bonds to those for the representative elements with single bonds, double bonds and triple bonds. However, transition metals also have the capability for quadruple bond formation with 8 bond formation. The inner transition metals are more limited in metal-metal bond formation. 

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 ... 

Based on Lewis theory, one would expect C2 to have the greater bond energy due to the formation of quadruple bond, CasC vs. Li—Li. The molecular orbital diagrams and bond orders are as follows. 

Mo2(H20)8+ is one of only three dimeric aqua ions with no bridging ligands (Hgf and Rh are the others). The structure 5 is eclipsed with 6-bond formation and quadruple metal-metal bonding. A rapid pre-equilibrium involving substitution of anion into the axial (end) waters. 

The reactions discussed in this section include those in which monomeric reagents are converted into dimeric species containing quadruple bonds. Later sections of this review consider reactions in which substitution or redox processes occur with retention, alteration, or cleavage of the metal—metal multiple bond. In very general terms, it is the initial formation of the quadruple bond that is common to the reactions below, and the number and variety of... 

In the deformation density in Fig. 17c, no charge accumulation is observed at the center of the Fe—Fe single bond. The lack of significant feature in the deformation density may be related to the small bond overlap population between the two Fe atoms given by the theoretical calculations. In view of the diffuse manner of charge accumulation upon a metal-metal bond formation as described in the Cr—Cr and Mo—Mo quadruple bonds (Section III,B), it cannot be decided... 

Overlap of d orbitals leading to the formation of a quadruple bond between two metal atoms. Note that the z axis of each metal atom is taken to point toward the other, such that if a right-handed coordinate system is used for the atom on the left, a left-handed coordinate system must be used for the atom on the right. 

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. 

Mathematically, there is no restraint on the formation of a quadruple bond between two carbon atoms however, the energy constraints on such a combination, along with other considerations make such a molecule if not impossible, so highly unlikely that it need not be considered further. The existence of quadruple-bonds between pairs of atoms, other than carbon, are known and the nomenclature that is proposed must be able to canonically name such moieties. 

An attempt to estimate the Re-Re quadruple bond energy in the [Re2Br8]2 ion using the heat of formation of the cesium salt and estimates of the lattice energy and other ancillary quantities has lead to a result of 100 12 kcal/mol (39). 

The isolobal approach was used in the previous section to describe the formation of metal-metal bonds. These bonds differ from others only in the use of d orbitals on both atoms. In addition to the usual ct and n bonds, quadruple bonds are possible in transition metal compounds. Furthermore, bridging by ligands and the ability to form cluster compounds make for great variety in structures containing metal-metal bonds. 

Figure 23.12 Simplified MO diagram showing the formation of an M-M quadruple bond in M Ls systems of d" metal ions giving a ground configuration of (the along with pj, p, and s orbitals of...

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