Complex stability pendant donor groups

As coordinatively unsaturated species, four-electron donor stibinidenes and bismuthinidenes readily form adducts with neutral and anionic donors. The stabilizing effect of these interactions has been exploited to prepare otherwise unisolable pnictinidene complexes by addition of Lewis bases [227]. This Lewis base stabilization can also originate from a pendant donor group [30, 62, 233]. This last... 

Addition of pendant donor groups increases the coordination number of the ligand-metal complex. Hence, consideration of the ability of the target ion to accommodate the increased coordination number is necessary. For example, Hancock and Martell (13) review the effect of addition of four N-acetate pendant groups to tetraazamacrocycles. Metals such as Ca that can accommodate a coordination number of eight experience a dramatic increase in complex stability, whereas metals that cannot accommodate the increased coordination (e.g. Cu ) experience a decrease in complex stability. 

L14 incorporates a further aminoethyl pendant arm as a potential donor group, but its Cu11 complex displays a somewhat lower stability than with L11, but at the same time, a much higher tendency to form a monoprotonated [Cu(HL14)]3+ complex (Table 1). This behaviour can be ascribed to the presence in [Cu(L14)]2+ of an uncoordinated aminoethyl pendant arm which can be readily protonated in aqueous solution. 

Ligands with three donor groups may bind to three sites of a metal as a tridentate ligand, although didentate chelation with one donor group pendant and unbound is also observed. Complex stability is usually enhanced by coordination of aU three donor groups. Examples appear below in Figures 11,13, and 14. 

Complex stability for macrocyclic carriers is determined by a number of factors including donor atom type, macrocyclic ring size, chelate ring size, pendant group characteristics, steric considerations, and degree of preorganization. Because complex stability is so crucial to carrier effectiveness, we examine a few of these factors below. 

Additional N/O donor pendant groups were shown to form spirocyclic compounds. In this work, six membered 1,3,2 oxazaphosphorine rings (56) were formed. Scheme 26. Stabilization of these new complexes was provided by replacement of the chlorines with either pyrroline. 

---