An extensive survey of the occurrences and origins of both structural trans-effects (STEs) and kinetic trans-effects (KTEs) in octahedral d-transition metal complexes is presented. This allows the identification of general STE classes into which the majority of common ligands fit: (a) very large STE ligands (STE vs. Cl -\ ca. 0.20 A ,
-, PR 3 , P(OR) 3 , RNH -, RS -, h 1 -thiones. The NO + ligand best illustrates the mutual nature of STEs, since it shows moderate STEs when trans to p-acceptor ligands, negligible STEs when trans to purely s-donor ligands, and inverse STEs when trans to p-donors. STEs can sometimes show a marked dependency upon the electronic properties of the complexed metal centre, e.g. p-accepting RNC and PR 3 ligands generally give moderate STEs, but in d 0 complexes their STEs are weaker than that of Cl -. This may be attributed to an absence of p-back-bonding in such complexes. Also, the STEs of p-donating RN 2-ligands show an extremely wide variation which partially correlates with the metal d-configuration. The relationship between STEs and KTEs depends upon ligand substitution mechanisms, and because such reactions in octahedral complexes are generally dissociatively activated, there is often a close correlation between STEs and KTEs. For example, N 3 -causes very large STEs and KTEs, whilst SO 3