Pseudo-octahedral Behaviour

Faraone, V. Ricevuto, R. Romeo, and M. Trozzi, Inorg. Chem., 1969, 8, 2207. G. Faraone, V. Ricevuto, R. Romeo, and M. Trozzi, Inorg. Chem., 1970, 9, 1525. 

There is yet one other factor which can affect the form of the rate law, and that is the nature of the outgoing ligand. The case reported again relates to palladium(ii) rather than to platinum(n), in fast substitution at [Pd dien X]+. Thiocyanate substitution conforms to the pseudo-octahedral pattern, but substitution by selenocyanate follows a two-term rate law. This is claimed to be a unique example of mechanistic control by an outgoing group.  

Most work on pseudo-octahedral behaviour is confined to the determination of rate constants and rate laws, but activation enthalpies and entropies for thiocyanate exchange at alkyl-substituted ethylenediamine complexes of the type [Pt(LL)2(SCN)2] have been evaluated.  

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Pseudo-octahedral behaviour of palladium(ii) complexes has already been discussed in the previous section on platinum(ii). 

Effects of Non-reacting Ligands. The two important effects of non-reacting ligands on the reactivity of platinum(n), as of other square-planar complexes are the trans effect and steric effects of bulky substituents. The latter lead to the so-called pseudo-octahedral behaviour wherein associative attack by the incoming nucleophile is prevented by the bulk of the nonreacting ligand. This results in a purely dissociative mechanism and a rate law... 

The formation in solution of nickel complexes with TV-substituted ethylenediamines has been studied over a long period by many authors.707 Solutions of nickel(II) complexes with TV-substituted diamines often exhibit equilibria between pseudotetrahedral and pseudo-octahedral species. These equilibria are displaced towards the pseudotetrahedral species when the temperature increases. Complexes Ni(TV,TV-Et2en)2X2691 with poorly coordinating anions are thermochromic. This behaviour has been investigated by means of calorimetric and NMR studies.708,709... 

The observation of the pseudo octahedral and pseudo compressed octahedral stereochemistries for the copper(II) ion then arise as an artifact of fluxional behaviour, Fig. 2 (A) for CuL6 chromophores and Fig. 3 (B) for the latter in CuL4X2 complexes. In... 

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