Absorption Spectra of Co-ordination Compounds. I

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Abstract
(1) The origins of absorption bands of co-ordination compounds have been discussed. (2) The first band is attributed to electron transitions in the unsaturated transition shell of the central ion. The band, therefore, appears only when the central ion is of a transition element. (3) The shift of the first band by substitution of ligands may be explained by postulating that the electron transitions are more or less depressed by the negative field of the ligand, which may conveniently be represented by P, the energy of approach of the ligand, i.e., the work done by the ligand if it were to approach from infinity to the seat of coordination. The ascending order of P i.e., the sequence of hypsochromic effect, is theoretically NH_3, H_2O, F^-, Cl^-, Br^-, I^-. The empirically obtained order of hypsochromic effect on the first band is & NO_2^-, NH_3, ONO^-, H_2O, NCS^-, NO_3^-, SO_4^-, & OH^-, C_2O_4^-, CO_3^-, S_2O_3^-, Cl^-, CrO_4^-, Br^-. All the experimental rules so far obtained of the shifts of the first bands by substitutions of ligands may be explained by the hypothesis. (4) The first is more or less additive. The complexes which have ligands situated far apart in the spectrochemical series have broader and less symmetrical first bands than those which contain only one kind of ligands. (5) The second band may be attributed to the co-ordination electrons, and is, therefore, the most general characteristic which a co-ordination compound should possess. Postulating that by absorption of a quantum hv2(v2=frequency of the second band) the co-ordinate linkage is broken to produce an excited metastable ligand in the original seat of co-ordination, the following relations have been derived: (Remark: Graphics omitted.) and S=hν2 where Q = heat of formation per co-ordination, R = activation energy, S = co-ordination energy, Φ = lattice energy of the co-ordination compound Φ1’s= lattice energies of the component compounds of first order, and n = co-ordination number.

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