Abstract
The bond-types of the complexes are reviewed in relation with their nature in solution. The complexes are classified into two groups. One is the complexes of ionic bond and the other the complexes of covalent one (the penetrating complexes). About these two types of the complexes are referred to the following subjects. (1) The difference of the stabilities between them was emphasized and explained neatly from a view-point of the ligand field theory and of the hybridization orbital method. (2) The absorption spectra of the six-coordinated dG complexes (Oh) were described simply, with regards to the bonding in the complexes. (3) The two mechanisms of the electrode reactions were tentatively proposed for the ionic and the covalent complexes. Namely, the dropping mercury electrode can be regarded as a kind of donor (the ligand), of which coordinating ability becomes stronger the applied potential increases gradually. On this consideration, the electrode process for the ionic complexes is considered to take place through SN1 reaction and the process for the covalent complexes proceeds through SN2 reaction, respectively. As an example of the former, the mechanism for the electrode processes of amminenickel(ll) system is given schematically (Fig. 5). The mechanism of the latter case is considered as being most rprobable for the polarographic reduction of dicyano-bis-ethylenediamine cobalt (lll) ion, judging from the thermodynamic data of the electrode reaction (Fig.7). The mechanism of SN2 reaction for the covalent complexes is generally supported by the facts that most penetrating complexes are not suffered from any change of the structure through net electrode reaction and the value of activation energy is not so large, compared with that of a bond energy. For the mixed liganded complexes the choice of the two mechanisms (SN1 or SN2) depends upon the weakness of the bond in its coordination sphere, since the weakest bond is naturally expected to be attacked with “a donor atom of mercury”.
