angle of ~30°. Both the s-cis and the gauche conformers are high energy states, ~15 kJ mol −1 above the ground state and also well above the 2.5 kJ mol −1 corresponding to 300 K. Thus the gauche conformer has only been detected in the gas phase [3] at high temperature or by matrix isolation of the high temperature vapour [4, 5]. However, it is possible to stabilise the s-cis conformer by complexation and the compound [6] C 4 H 6 Fe(CO) 3 (see Figure 2) is an example of this. The vibrational spectroscopy of butadiene has been extensively studied both as the free molecule e.g. [4, 5, 7] and adsorbed on a variety of oxide [8, 9] and metal surfaces [10-16]. Butadiene potentially exhibits many different modes of bonding to a surface [17], the aim of this work is to fully characterise how the spectrum of the s-cis conformer changes on coordination. Stateof-the art computational methods enable transition