The colour of chelates of boron. An X‐ray structural investigation of bis(4‐methylphenyl)boryl and 9‐borabicyclo‐[3.3.1]nonyl acetylacetonates

The molecular structures of the colourless bis(4‐methylphenyl)boryl and the orange coloured 9‐borabicyclo[3.3.1]non‐9‐yl acetylacetonates (4 and 5, respectively) have been determined by X‐ray analysis. The CO bond lengths in 4 are significantly (about 0.029 Å) longer than in 5. It is suggested that variation of the π bond order in the CO bonds is responsible for the shifts in the electronic spectra observed for the various chelates of 1,3‐diketones and related compounds.