Infrared laser spectroscopy and ab initio calculations are reported for cyanoacetylene bound to magnesium clusters of varying size, formed and observed in the superfluid helium nanodroplets. Ab initio calculations of the binary HCCCN-Mg potential energy surface reveal three minima. The corresponding isomers are observed experimentally, owing to the unique "growth" conditions provided by liquid helium. The cyanoacetylene is also observed to bind at both the nitrogen and hydrogen ends to larger magnesium clusters in agreement with the minima found in the corresponding ab initio calculations. The infrared spectra show rotational structure that can be fit by the corresponding gas-phase Hamiltonian, using modified constants that account for the effects of the helium solvent. Stark spectroscopy is used to measure the dipole moments of these complexes, providing further insights into the nature of the associated interactions.