Despite the fact that chemists frequently draw electron density distributions, there are few good methods for measuring this quantity, which has contributed to many longstanding controversies in chemistry. In this paper, we report the first application of element specific L absorption spectra and the "white light" sum rule to a series of nickel complexes, with wide oxidation state range from Ni-I to Ni-IV. Nickel L edge X-ray absorption spectroscopy is turning out to be an excellent quantitative probe of 3d-vacancies.