Hapticity change

Either complex can be recovered unchanged from prolonged reflux in xylene, and, in contrast to their pentamethylcyclopentadienyl analogues, neither exhibits any photochemically induced hapticity changes (see Section VIII,E,l). 

Thus, the amount of phosphane added can be used to markedly influence the product distribution when 2-substituted methylenecyclopropanes are used as substrates. A possible explanation of this observation might lie in the ability of a strong donor ligand, such as an alkylphosphane, to prevent a hapticity change of the ring-cleaved methylenecyclopropane from 1/ -coordination (allyl-type) to 1/ -coordination (trimethylenemethane-type), which is indispensable for induction of the scrambling reaction. 

The catalytic activity of [MnRu(CO)6 PhC=C(H)C(H)=N(Pr ) ] for the homogeneous hydrogenation of styrene was explained by the reversible hapticity change of the Mn-coordinated azadienyl cycled... 

A complete analysis of the square scheme is complex since disproportionation and/or other second-order cross-redox reactions have to be taken into consideration. However, the limiting cases of the square scheme are much more tractable. An interesting aspect of the square reaction scheme is that, in principle, it applies to all one-electron processes with reaction steps A+ B+ and A B coupled to the heterogeneous charge transfer. For example, the redox-induced hapticity change, which accompanies the reduction of Ru( j - CeMee), has been proposed [113] to be responsible for the apparently slow rate of electron transfer. That is, the limiting case of an apparent overall Einev process is observed for what in reality is a square scheme mechanism. 

Noteworthy, the arene ligands undergo multiple hapticity changes during the course of the proposed catalytic cycle, which regulates the activity and stability of the catalyst by providing the vacant coordination sites required for substrate coordination while, at the same time, maintains the integrity of the bimetallic entity. 

In Sect. 2.3 it was shown by the example of complexes 1 (2) and 6 (7), that the phenomenon of hapticity change or bond order variation between the transition metal and the aromatic ligands can change the kinetic pathways of the polymerization process and its outcome dramatically. The haptotropic behavior of metallocenes is a known phenomenon in transition organometaUic chemistry and homogeneous catalysis [149-156]. 

Another tricarbonyl species Cr(CO)3(T -l,3,5-cycloheptatriene) has been shown to serve as a photocatalyst for 1,6 H2 addition to cycloheptatriene [71b]. The suggested mechanism involved a photoinduced toTj hapticity change of the coordinated cycloheptatriene allowing access of H2 to the vacated coordination site. 

Heating the complex provokes the ri -Cp to T] -Cp hapticity change, i.e. the formation of ferrocene, resulting from the loss of the two carbonyl ligands. 

The reverse, i.e. hapticity change from r -Cp to r -Cp is usually unlikely given the robustness of the M-(T] -Cp) fragment, but one example is known ... 

The primary photoprocesses in this reaction under low temperature conditions involves CO loss, followed by a hapticity change in the coordinated triene. 

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