Diazadiene metal tetracarbonyls, DAD M(CO) 4 , (M = Cr, Mo, W; DAD = R-N = CH-CH = NR with aliphatic and aromatic substituents R) show equidistribution of one labelled CO when synthesized from fac-DAD M(CO) 3 (CH 3 CN) as a consequence of intramolecular CO site exchange. The structures of two Mo-complexes (1: R = iso-propyl; 2: R = 2,6-diisopropylphenyl) with DAD's of different bulkiness have been solved by X-ray methods. From the infrared solution spectra, including those of isotopically labelled ( 13 CO, C 18 O) compounds, all of the force constants k 1 , k 2 , k tt , k cc and k cc have been calculated independently. The ratios of interaction force constants deviate sharply from Cotton-Kraihanzel assumptions, perhaps because of the substantial deviation of the iraws-pair of CO groups from linearity (1: 154.5°, 2: 147.9°). The solid state N-Mo, Mo-C and C - O bond lengths are neither in agreement with the generally accepted picture for donor/acceptor influences nor with typical IR data and force constant interpretations. (2 with higher v(CO) and k 1 , k 2 has the longer CO bonds and shorter Mo-C bonds than 1.) The force constant sets have been used to calculate CO frequencies for over 60 labelled compounds with good accuracy (Δv < 2 cm -1 ).