Complexes with unusual coordination numbers

The nature of the bonding, particularly in CO, has excited much attention because of the unusual coordination number (1) and oxidation state (-f2) of carbon it is discussed on p. 926 in connection with the formation of metal-carbonyl complexes. 

There are examples where the metal ion may form high coordinate species with an unusual coordination number, for example, [Fe(H20)9]+ (97). The ability to form highly coordinated metal complex ions suggests that under suitable conditions, involving a high concentration of the ligand molecules, coordinative saturation may take place. 

Excessive alkyl substitution on the N-donor atoms can result in the isolation of amine complexes with unusually low coordination numbers, e.g. Cr111(NPr12)3.135... 

If you were given the task of attempting to prepare a complex with an unusual coordination number, which coordination numbers should you seek to avoid, and why ... 

Coordination Compounds. A large number of indium complexes with nitrogen ligands have been isolated, particularly where Ir is in the +3 oxidation state. Examples of ammine complexes include pr(NH3)3] " [24669-15-6], prCl(NH3)] " [29589-09-1], and / j -pr(03SCF3)2(en)2]" [90065-94-4], Compounds of A/-heterocychc ligands include trans- [xCX py)][ [24952-67-8], Pr(bipy)3] " [16788-86-6], and an unusual C-metalated bipyridine complex, Pr(bipy)2(C, N-bipy)] [87137-18-6]. Isolation of this latter complex produced some confusion regarding the chemical and physical properties of Pr(bipy)3]3+ (167). 

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