Fluxionality

The theoretical determination of cluster stability requires some care. The HF method severely underestimates the cluster binding energy. Correlation accounts for about 50 % of the stability in Li clusters and 90 % of the stability in Na and K clusters. [25] This makes it difficult, if not impossible, to treat larger aggregates (especially of transition metals) by means of wavefunction based methods. Alternatives are provided by density functional methods or, with some caution, by well 

2 Electronic Structures of Metal Clusters and Cluster Compounds 

Gas phase clusters undergo fragmentation when their internal temperature is high enough. The theoretical study of such fragmentation processes, M -  

Figme 2-3. Binding energy per atom (DJn) for AI clusters versus cluster size as measured by the parameter The solid line connects the atomic with the bulk limits. The dash-dotted line represents a least squares fit of the computed DJn values. Adapted from [IDS]. 

For the special case where the dissociation involves a single metal atom, Mj, one obtains 

Lipscomb, W. N. NMR Studies of Boron Hydrides and Related Compounds, Benjamin New York, 1969. 

Wrackmeyer, B. Nuclear Magnetic Resonance Spectroscopy of Boron Compounds, Springer Berlin, 1978. 

Spectroscopic Properties of Inorganic and Organometallic Compounds, The Chemical Society London, yearly, 1968-present. 

Gmelin Handbuch, New Supplement Series, Boron Compounds Springer Berlin, 1977 ff Caveat, note that in some sections of this work the [Bp3(OEt2)] =zero, high frequency positive, chemical shift convention is not universally applied, 

Wrackmeyer, B, Koster, R. in Organoborverbindung III (Houben-Weyl Methoden der Organischen Chemie, Volume XIII/3c), Koster, R., Ed. Georg Thieme Stuttgart and New York, 1984, Sec. D, pp. 377-611. 

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Figure A3.12.5. A model reaction coordinate potential energy curve for a fluxional molecule. (Adapted from [30].)...

In the above discussion it was assumed that the barriers are low for transitions between the different confonnations of the fluxional molecule, as depicted in figure A3.12.5 and therefore the transitions occur on a timescale much shorter than the RRKM lifetime. This is the rapid IVR assumption of RRKM theory discussed in section A3.12.2. Accordingly, an initial microcanonical ensemble over all the confonnations decays exponentially. However, for some fluxional molecules, transitions between the different confonnations may be slower than the RRKM rate, giving rise to bottlenecks in the unimolecular dissociation [4, ]. The ensuing lifetime distribution, equation (A3.12.7), will be non-exponential, as is the case for intrinsic non-RRKM dynamics, for an mitial microcanonical ensemble of molecular states. 

The volatile, air-sensitive Hquid species (CH3)2AlB3Hg and (CH3)2GaB3Hg are prepared by the direct reaction of the corresponding main group metal hahde and salts of the [B3Hg] ion, in the absence of solvent (178). The reaction of (CH3)2AlB3Hg and A1(BH 3 results in the species (BH 2AlB3Hg. These small metallaboranes are fluxional in solution and have limited thermal stability at room temperature. 

In another fluxional process involving ruthenium instead of rhodium, it has been shown that the rate-controlling step is the complex dissociation and that the ligand exchanges between the two annular nitrogen atoms by an intermolecular process. 

In solution, the phenyl derivative 12.29 (R = Ph) is fluxional. The mechanism of the fluxional process has been shown by an N NMR investigation of a partially N-labelled sample, Le., PhCN2 N3S3 ( N = 99% N), to involve a series of 1,3-nitrogen shifts (Section 4.8.4). Thermolysis or photolysis of 12.29 generates the corresponding 1,2,3,5-dithiadiazolyl radicals [RCNaSa]". ... 

The open face comprises a fluxional system involving the two atoms and the endo-Hi atom of the BH2 group)... 

AI(BH4)3 was the first fluxional compound to be recognized as such (1955) and its thermal decomposition led to a new compound which was the first to be discovered and structurally characterized by means of nmr ... 

This binuclear complex is also fluxional and has the structure shown in Fig. 7.4a. A1(BH4)3 reacts readily with NMe3 to give a 1 1 adduct in which Al adopts the unusual pentagonal... 

XeFs 49.5 Distorted octahedral (fluxional) Pentagonal bipyramidal or capped oct edral (1)... 

In addition to acting as an rf ligand, CgHg can coordinate in other modes, " some of which are illustrated in Fig. 19.33. Many of these complexes show fluxional behaviour in solution (p. 935) and the distinction between the various types of bonding is not as clear-cut as implied by the limiting structures in Fig. 19.33. 

Fe(CO)s is a highly toxic substance discovered in 1891, the only previously known metal carbonyl being Ni(CO)4. Like its thermally unstable Ru and Os analogues, its structure is trigonal bipyramidal (Fig. 25.10a) but its C nmr spectrum indicates that all 5 carbon atoms are equivalent and this is explained by the molecules fluxional behaviour (p. 914). 

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