Square pyramid Steric effects

For this reason, it is not even appropriate to state that the square pyramidal nature of the oxometal(IV) porphyrins, (e.g., TiO(OEP) ([22a], 75, 76), VO(OEP) (55), or MoO(OEP) (55)) is caused by the very strong tram effect of the multiply bound oxygen. In TiO(OEP) [23a, the TiIV ion is exoplanar by 53 pm (75), and any addition of a tram ligand L will produce elongated and therefore weak Ti-L bonds in this system. The preference of square pyramidal coordination is therefore thought to be of steric origin. 

The corresponding palladium compound (157) must be formulated as [Pd(TA)Br]+Br. The cation is essentially square-planar, but the bromine atom is displaced 10° out of the arsenic-palladium plane. Such a distortion is very unusual for palladium, and may be due to a steric effect of the alkyl chains in the arsine ligand. This might similarly account for the distortion of the nickel complex from a square pyramidal shape. 

RuHCl(PPh3)3 itself has a distorted square pyramidal structure (Figure 1.25) but the hydride ligand has less steric effect and the structure has been described as a capped tetrahedron (Ru-P 1.7 A, Ru-Cl 2.421 A, Ru-P 2.206-2.329 A j/(Ru—H) 2020cm ) [83]. The Ru-Cl stretching frequency in the IR spectrum of RuHCl(PPh3)3 is at 282cm , compared with... 

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