Square-pyramidal complexes Substitution

If the BF2 groups in Ni(dmg-BF2)2 are substituted by BPh2 units (122), the complex also adopts the saddle-shaped conformation of type D (Fig. 32), in which the two dimethylglyoxime fragments are bent down from the N4 plane with a dihedral angle of about 27° between the two least-squares planes of the dioxime units. The coordination geometry around the nickel ion is distorted square-pyramidal, but there are no intermolecular Ni Ni interactions [167]. 

Iron(III) complexes have also been prepared with 2-acetylpyridine N-phenylthiosemicarbazone [142], 14. Three have been formulated as square pyramidal [Fe(14-H)A2] (A = Cl, NCS and NOj) based, in part, on molar conductivities of 37-62 ohm cm mol . Their ESR spectra in frozen DMF are essentially the same as for other Af-substituted species, which likely indicates the presence of [FeL2] ions. More recently [138], [Fe(14-H)2]C104 has been isolated and its solid ESR spectrum is reported to be axial with g > g - Table 1 summarizes the g-values of iron(III) heterocyclic thiosemicarbazone complexes. 

Under (i) the square and pyramidal complexes are often easier to substitute than the octahedral complexes for the obvious reason that they have open residual coordination sites, looking upon all the complexes as derived from an octahedron. The mechanism of substitution can then be the typical organic Sn2 attack. More usually the reactions of complex ions proceed by predissociation, SnI, so that the important consideration is that c and d should be at least relatively good leaving groups. 

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