Exchange reactions bridge/terminal

Zirconium(IV) and hafnium(IV) tetrakis(tetrahydroborates) M(BH4)4 are of interest as extremely volatile, covalent complexes that contain tridentate BH ligands and exhibit rapid intramolecular exchange of bridging and terminal hydrogen atoms. 4,615 These compounds were prepared initially from NaMF5 (M = Zr or Hf) and excess A1(BH4)3 (equation 53),616 but they are obtained more conveniently from the reaction of the anhydrous metal tetrachloride with excess lithium tetrahydroborate (equation 54), either in the solid state617,618 or in the presence of a small amount of diethyl ether.619... 

The [Zn4(SPh)10]2- cluster anion has also been prepared by the reaction of NaSPh with ZnCl2, and H NMR studies have revealed a rapid SPh(bridgmg)-SPh(terminal) interconversion, which is thought to occur via opening of the cluster by successive breaking of two bridging Zn—S bonds.820 A rapid metal exchange reaction in the mixed-metal clusters [M4 Zn (SPh)io]2 (M = Cd, Co or Fe) and [Co4 Cd (SPh)io]2- was also detected. 

The bridge-terminal exchange can be accounted for by this process, where k l pkl, i.e., the rate-determining step is the dissociation of D to SCM and the activation energy for the process must be the dissociation energy corresponding to the process D -> 2 SCM. In order to also account for the reaction with Ga(CH3)3 and In(CH3)3 in cyclopentane and toluene... 

The Zr and Hf tetrahydroborates M(BH4)4 are the most volatile compounds of these elements, with boiling points of 123 and 118°C they are also very sensitive to oxidation and hydrolysis. They can be obtained by the reaction of the chlorides with alkali metal borohydrides or of the complex fluorides NaMFs with aluminum borohydride (the fluorides MF4 do not react), followed by distillation from the reaction mixture. According to low-temperature X-ray data, all the borohydride groups are tridentate in both M(BH4)4 complexes, which have Td symmetry. The same is trae for their substituted M(BH3Me)4 derivatives, that is, the metal atom is 12-coordinate. Proton NMR indicates fast exchange of bridging and terminal protons on the NMR timescale in these fluxional molecules see Fluxional Molecule). 

Whereas at 253 K only the monomeric molecule is detectable, at 283 K a small signal arises from the dimeric species (Fig. 2). At 313 K coalescence is observed between these two species, llirthermore, an intramolecular exchange reaction of the terminal and the bridging phosphanide ligands of the dimer broadens the singlets. The latter process is fast on the NMR time scale. The spectroscopic data are compared in Table 1. For the barium derivative shown in Fig. 3 the bicyclic structure has been proven by the small Vp p coupling of 6.7 Hz at 213 K which leads to a quartet for... 

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