The relationship between color intensity in transition metal complexes, molar extinction coefficients, and selection rules is a foundational principle of coordination chemistry and does not require citation.
The claim states a fundamental concept in coordination chemistry regarding the determinants of color intensity in transition metal complexes (molar extinction coefficients and selection rules like Laporte and spin selection rules). This is a textbook principle of chemistry taught at the undergraduate level, making it common knowledge. Consequently, no empirical papers are required to support or refute it.