Fluxional, Rotational, and Conformational Molecules

Molybdenum and Tungsten.—Fluxional, Rotational, and Conformational Molecules. [(A -C5H5)Mo(CO)3—Mo(CO)a(CNR)(A -C5H5)] is fluxional. In the low-temperature limit cis- and /rmw-forms are detected, whilst brid -terminal interconversion of nitrile and carbonyl groups is observed in the high-temperature limit. There have been a number of reports on (pyrazolyl)-borato RnB(pz) complexes of molybdenum. N.m.r. measurements on [ B(pz)4 Mo(CO)3(A -CbH5)], [ HB(pz)3 Mo(CO)a(/i -C6H5)], and 

Examples of flip and rearrangement processes of co-ordinated allyl have appeared in this section. The former is apparent in an extensive investigation into the effects of allyl substituents on the configuration of the allyl in I(/i -C6H5)(A -allyl)Mo(CO)2] and [(/z -indenyl)(/z -allyl)Mo(CO)2] and some tungsten analogues. Anisotropy effects on chemical shifts of the allyl protons in the indenyl complexes allow conclusive identification of (8) and (9). 

Equilibrium constants and thermodynamic data are summarized in Table 1. Substitution generally favours the type of conformation shown in (9), although 2-substitution [as in (8) and (9)] specifically favours (8). The presence of a cyclopentadienyl ring rather than an indenyl displaces equilibria in the 

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Cobalt.—Fluxional, Rotational, and Conformational Molecules. A modification of the tetrahedral tunnelling or jump mechanism mentioned above arises from the fluxional behaviour of the series [XCo P(OR)3 4] X = (non-rigid... 

There are two basic types of chemical processes reversible (leading to, and maintaining, an equilibrium mixture) and irreversible (proceeding in one direction, i.e., to completion). NMR methods can be used to study both types of processes. A common example of an irreversible process is a chemical reaction with a free energy more negative than ca. 20 kJ/mol. Reversible processes include interconversions of conformations by rotations around single bonds, interconversions of fluxional molecules and valence isomers, and proton transfers, to name a few. 

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