Substitution of Ligands at the Transition Metal

This process, represented as mode 4 in Fig. 2, is exemplified by the reactions in Table XVI. It may be further subdivided according to the nature of the ligands. 

The most studied reaction involves the replacement of carbonyl groups by tertiary phosphines. This proceeds most readily in the case of cobalt derivatives, manganese compounds must usually be heated, and iron compounds require irradiation (except in the case of entry 13). 

It had been suggested some years ago (150) on the basis of kinetics measurements that substitution of Ph eMn(CO)s by phosphines led initially to the cis isomer with bulky phosphines, this then rearranged to the trans species. A similar effect has now been demonstrated for PhsSiCo(CO)4 (227) (entry 24) by infrared methods an equatorially substituted compound (XXVI) is apparently first formed, and this slowly rearranges to the axially substituted product (XXVII). 

HjSiColCOla/PPhs ClsSiColCOla/PRs (R = Me, Et, F) MeaSiCoiCOla/PPha/Zli Cl3SiCo(CO)a/P(n-Bu3) 

In the case of disubstituted cobalt derivatives, it is believed that representatives of the three isomers (XXVIII)-(XXX) can be prepared by reaction (86) (entry 25), reaction (87) (entry 31), and reaction 88 (Table IV, entry 33). 

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