Groups VI and VII

Mixed-ligand Carbonyls.—Groups VI and VII. Continuing kinetic studies of the 

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The most optimistic response to this situation is to claim that the force constant — t-bond order relationship is still valid, but that the reference points need to be changed V(CO)e itself is then a possible reference compound (76). The relationship can then only be quantified by using calculated orbital populations for the reference species, and can only be tested by more extended comparisons between calculated bond order and observed force constant. Precisely this test has been apphed to a whole group of substituted and unsubstituted octahedral carbonyls of groups VI and VII, the substituents in every case being hahde (77). The data used in fact were not force constants, but Cotton-Krainhanzel parameters this does not actually matter, since no reference molecules were used at all. Excellent agreement was found with an expression. 

The occurrence of stable carbene complexes of Groups VI and VII is well documented (87, 96, 202), but the 77-cyclopentadienyliron carbene carbonyl cations (103, 151) appear unstable. 

It is difficult for the elements of group VI and VII to act as central atoms in n complexes. Vacant orbitals or coordination sites are generally necessary for the stabilization of a n complex. This would require high oxidation states of the group VI or VII elements, as illustrated by the structures V, W, and X for a series of cyclopentadienylsulfur complexes. It is therefore not surprising that attempts to generate complexes of this type so far have been unsuccessful (259). 

The majority of organic reactions that have been developed to date for group VI and VII transition metal n bases are with the system TpRe(CO(L). The limited number of examples of reactions with these systems compared to that of the pentaammineosmium system reflects their more recent discovery. 

Sweigart DA (1973) Lone pair orbital in group VI and VII hydrides. J Chem Educ 50 322... 

Many stable alkyl and aryl derivatives of transition metals contain TT-acceptor ligands such as CO or CsHs". Compounds of the types RTiCl(CsHs)2. RMo(CO)3CsH5, and RMn(CO)s are moderately stable to air in the solid state and sometimes in solution. The Group VIII compounds such as RFe(CO)2CsHs and RCo(CO)4 are less stable than those of Groups VI and VII but are much more stable than the simple alkyl... 

Some inhibitors are reduced on the surface to yield secondary products that are themselves the active inhibitors. In strong mineral acids, elements from Groups VI and VII tend to become protonated, a necessary prerequisite for many reduction reactions. Such is the case for triphenyl benzyl phosphonium chloride, which forms triphenyl phosphine, and triphenyl arsenic oxide, which undergoes protonation (permitting it to dissolve) and forms triphenyl arsine on the surface. Some sulfonium salts, e.g., tribenzylsulfonium hydrogen sulfate, and dibenzylsulfoxide also can be reduced by iron in HCI. 

IV and V. Chain transfer to BPO also forms structure IV. Termination of the propagating radicals by disproportionation gives end groups VI and VII. Termination by combination forms structure VIII in the middle of the polymer chain. Table 7.3 gives the numbers of end groups per chain for the polymerization of PMMA with BPO in toluene at 100°C. 

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