Ab initio calculations of the vibrational frequencies of W(CO)(5)NH3 in its ground electronic (1)A(1) (b(2)(2)e(4)) and lowest excited state (3)E (b(2)(2)e(3)a(1)(1)) have been performed at the HF level. The calculated frequencies of the nu(CO) bands are in agreement with observed data on experimentally studied W(CO)(5)(amine) molecules. The optimized geometries of the ground and the excited states show that the W-N, W-C-eq, W-C-ax, C-O-eq bonds lengthen and the C-O-ax bond shortens on excitation. Our results resolve an apparent disagreement between the fast time-resolved infrared (TRIR) spectroscopy and the preresonance Raman (PRR) spectroscopy. The unexpected simultaneous lengthening of both W-C-eq and C-O-eq is due to C-O-eq antibonding character in the a(1) orbital which more than offsets its loss from the e. In addition a new band, predicted but as yet unresolved in the TRIR, accounts for the C-O-ax shortening as expected from the PRR W-C-ax lengthening.