Axial dissociation

The kinetics and mechanism of chloride and bromide substitution of diaquonickel(III) complexes with several ligands of the cyclam type have been studied. The data reveal a reversible process yielding [NiL(H20)X]2+ and are consistent with a dissociative mechanism, sterically more demanding axial groups substantially increasing the rate. In addition, a rearrangement step is observed subsequent to the formation of the monohalide complex.146... 

The sonolysis of Mn2(CO)10 makes for an interesting comparison (186), since either metal-metal (as in photolysis) (187) or metal-carbon (as in moderate temperature thermolysis) (188) bond breakage could occur. Ligand substitution will occur from either route producing the axially di-substituted Mn2(CO)8L2. Using benzyl chloride as a trap for the possible intermediacy of Mn(CO)5, the sonochemical substitution of Mn2(CO)10 has been shown to follow the thermal, rather than the photochemical, pathway of dissociative CO loss. 

In the absence of 13C hyperfine interaction measurements, it is not clear to what extent dissociative electron capture with CO loss occurs for such molecules. It is entirely possible, for example, that electron capture by M(CO)sI results in loss of the axial CO ligand, yielding the radical M(C0)4I2-. If so, it is noteworthy that CO is lost in preference to I-. 

Scans based on resonant ejection may either be carried out in a forward, i.e., from low to high mass, or a reverse manner. This allows for the selective storage of ions of a certain m/z value by elimination of ions below and above that m/z value from the trap. Thus, it can serve for precursor ion selection in tandem MS experminents. [156,158] Axial excitation can also be used to cause collision-induced dissociation (CID) of the ions as a result of numerous low-energy collisions with the helium buffer gas that is present in the trap in order to dampen the ion motion. [150,156] A substantial increase of the mass range is realized by reduction of both the RF frequency of the modulation voltage and the physical size of theQIT. [154,159,160]... 

D. R. Prasad, T. Ramasami, D. Ramasamy and M. Santappa, Inorg. Chem. 21, 850 (1982) — These unusually rapid reactions of Cr(III) are promoted by lengthening (distortion) of the axial water and thus enhancing the dissociative charaeter. They are not typical of Cr(III) behavior. 

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