The visible and ultra-violet absorption spectra of many cobaltic ammine, oxalato, carbonato and amino-acido complexes have been determined in solution. The spectrochemical series for the ligands of Co(III) complexes has been redetermined.
The color and the structure of the compounds of a Magnus green salt type, [Pt(amine)4] [PtCl4], have been studied. The dichroism with crystals of [Pt(methylamine)4] [PtCl4] and [Pt(ethylamine)4] [PtCl4] has been quantitatively determined at room temperature in the visible and ultraviolet region by the microscopic method. From the measurements, the unusually deep color of the former has been shown to be ascribed to a direct type of interaction between platinum atoms. No interaction of this sort is present in the crystal of the latter, which is pink in color. It is suggested that the compounds having a formula of [Pt(alkylamine)4] [PtCl4] may generally be classified into two kinds, one being colored green and the other pink. In the crystals of the former there is a direct type of interaction between platinum atoms at the center of the planar complexes, but no interaction of such a sort is operative in the crystals of the latter.
(1) The absorption spectra of co-ordination compounds consist of the first, the second and the third bands as well as those due to ligands themselves. Some of the compounds may lack the first, the third, or both the bands, but none of them the second. (2) Postulating that by absorption of a quantum hν3 (ν3 = the frequency of the third band) a kind of neutralization takes place between the central ion and one of the co-ordinated anion to produce an excited ionogen in the original seat of co-ordination, the following relation has been derived. hν_3=S+P+E-J, where E= electron affinity of the anion, J= ionization energy of the central ion, S= co-ordination energy, and P= work of approach of the anion. The ultra-violet absorption band of solid alkali halide has been explained as a special case of the third band. (3) Besides the numbered bands, a co-ordination compound has generally some more bands due to ligands themselves. Examples of these special bands are given. The end absorption has also been explained. (4) The co-ordination energy, S(=hv2), may be taken as the measure of stability of the co-ordination compound. The order of hypsochromic effect of ligands on the second band gives the order of strength of the co-ordinate linkages between the central ion and the ligands. The order of stability has been determined experimentally. NH_3,NO_2^-, ONO^-, H_2O, NCS^-, OH^-, NO_3^-, Cl^-, CO_3^=, Br^-. (5) Whareas the stability of co-ordination compounds may be defined by S as a rule, the photochemical stability, however, should be given by P–R, R being the activation energy. Both the stabilities were compared with illustrations. (6) The substitution of ligands which proceeds naturally, takes place in the direction, in which the value S increases, or in other words, the second band is displaced towards the shorter wave-lengths. This rule and the spectrochemical series in (4) give the means of explaining as well as finding methods of preparation of various co-ordination compounds.
Dichroisms in the visible and the ultraviolet region have been determined by the microscopic method with cupric formate tetrahydrate, cupric acetate monohydrate, anhydrous cupric propionate and its mono-hydrate. The cupric formate shows an absorption band at about 38–391013/sec. which is considered to be due to the copper ion in combination with the ligands. For this band, electric vector is much more strongly absorbed along the plane of the complex than in the direction normal to the plane. The dichroism and the absorption spectrum of the cupric formate are of the same type as those of most cupric complexes involving no special effect upon the complexes. Cupric acetate monohydrate, anhydrous cupric propionate and its monohydrate show an absorption band of a new kind at about 80×1013/sec., in addition to a band at about 43×1013/sec. which corresponds to the band at about 40×1013/sec. of the cupric formate. The polarization for the band at 80×1013/sec. is the reverse of the polarization for the band at 43×1013/sec. From the appearance of the new band at 80×1013/sec. and the reversal of the polarization property with this band, it has been concluded that the cupric compounds examined in this work, except the formate, consist of dimeric molecules having a structure similar to that of cupric acetate monohydrate. It has also been shown that the water of crystallization has very little effect upon the linkages within the complex molecules. The cupric compounds, except the formate, show in organic solvents a band at 80×1013/sec., involving the dimeric molecules in solution.
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