From a simple time-dependent tight-binding model we calculate the influence of molecular adsorbates on absorption spectra of spherical metallic nanoparticles. The model allows arbitrary coupling to the surface as well as changing the molecular excitation characteristics through its HOMO-LUMO gap. Adsorbates cause shifting and widening of the plasmon resonance peak for small coupling and even splitting of the resonance in two different excitations located on the particle surface and the adsorbate layer, respectively, for strong couplings. (C) 2010 Elsevier B. V. All rights reserved.