Metal molecules, dissociation energies

Considerable difficulties arise when attempting to apply this expression to polyatomic molecules, owing to possible coupling between different modes of the same symmetry. Nevertheless, the data in Table 9 allow one to estimate the metal-metal bond dissociation energy in [M2X8]" ions (84) to be around 500kJmol, which is a very substantial value, exceeded among homonuclear units only by those of C C and N=N. 

Reviewed here are some of the most important adsorption characteristics of these molecules. Dissociation energies are listed in Table 3.2. On the majority of the catalytically active metals the heats of adsorption increase in the following sequence ... 

The interaction of two alkali metal atoms is to be expected to be similar to that of two hydrogen atoms, for the completed shells of the ions will produce forces similar to the van der Waals forces of a rare gas. The two valence electrons, combined symmetrically, will then be shared between the two ions, the resonance phenomenon producing a molecule-forming attractive force. This is, in fact, observed in band spectra. The normal state of the Na2 molecule, for example, has an energy of dissociation of 1 v.e. (44). The first two excited states are similar, as is to be expected they have dissociation energies of 1.25 and 0.6 v.e. respectively. 

Table 2. Dissociation energies of diatomic transition metal, molecules, Dq(M-M) kj mol-1...

The use of ion cyclotron resonance spectroscopy to measure the proton affinity of a molecule in the gas phase is now well established (for example, Ref.91 ). The application of the technique to transition metal organometallic compounds is a more recent development and some results are shown in Table 22. In all molecules studied so far it is generally observed that the dissociation energy of a cationic... 

The energies required by these processes are composed of the sublimation energy of the metal (Es) and the dissociation energy of the gaseous molecule (fEjf). 

Figure 5.22 Lennard-Jones potential of hydrogen approaching a metallic surface. Far from the metal surface the potential of a hydrogen molecule and oftwo hydrogen atoms is separated by the dissociation energy. The first attractive interaction ofthe hydrogen molecule is the van der Waals force leading to the physisorbed state. Closer to the surface the...

Other similarities with halogens are the presence of diatomic molecules (Au2) in gold vapor, with dissociation energies comparable to those of halogens, or the spontaneous disproportionation of gold metal into Au and Au+, in a melt reaction in the presence of cesium. 

All the other elements except boron, which has a very complicated structure, are metallic, and the only kind of molecular formation known is the occurrence of small concentrations of A2 molecules in the vapours of the alkali metals. Such molecules are analogous to those of hydrogen, but their dissociation energy is small and their existence in the crystalline state is unknown. In the metallic state we have to assume a third kind of bond, which will be dealt with at a later stage. 

We could calculate AH for this if we knew (for the elements) the enthalpy of formation of molecules from their atoms. Some crystalline elements (especially metals) vaporize as monatomic gases, and it is not too difficult to determine their heats of sublimation. Some elements—such as H2, 02, an( Br2—are diatomic gases that dissociate into atoms at high temperature these dissociation energies may also be determined. Table 14-1 also includes the standard enthalpies of formation of a number of atoms these are based on the normal physical form of the element at 25.0°C. For HC1 we find... 

---