Trans-effects in octahedral transition metal complexes

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 Angstrom): SiR3-, NO-, N3-, O2-, S2-, RC3-; (b) large STE ligands (ca. 0.20 > STE vs. Cl- > ca. 0.10 Angstrom): H-, R-, eta(1)-alkenyl, eta(1)-Ph, RCO-, RN2-; (c) moderate STE ligands (ca. 0.10 Angstrom > STE vs. Cl- > 0.00 Angstrom): CO, CN-, CNR, eta(1)-acetylide, R2C, NO2-, NS+, RN2+, SO32-, RSO2-, PR3, P(OR)(3), RNH-, RS-, eta(1)-thiones. The NO+ ligand best illustrates the mutual nature of STEs, since it shows moderate STEs when trans to pi-acceptor ligands, negligible STEs when trans to purely sigma-donor ligands, and inverse STEs when trans to pi-donors. STEs can sometimes show a marked dependency upon the electronic properties of the complexed metal centre, e.g. pi-accepting RNC and PR3 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 pi-back-bonding in such complexes. Also, the STEs of pi-donating RN2- 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, N3- causes very large STEs and KTEs, whilst SO32- gives moderate STEs and large KTEs. Since both of these ligands cause STEs primarily via powerful electron donation, the ground state destabilisations implied by STEs are likely to be accompanied by stabilisation of the electron-deficient five-co-ordinate transition states. By contrast, pi-acceptor ligands such as CO or RNC generally exert moderate STEs, but cause pronounced delabilisation of trans metal-ligand bonds due to destabilisation of transition states. (C) 2000 Elsevier Science S.A. All rights reserved.