The invention relates to quaternary ammonium multi-dentate mono-, bis-, tris- and tetrakis-phosphonate compounds, processes for preparing quaternary ammonium multi-dentate mono-, bis-, tris- and tetrakis-phosphonate compounds, antimicrobial coating compositions comprising quaternary ammonium multi-dentate mono-, bis-, tris- and tetrakis-phosphonate compounds and method of treating a surface with said compositions to provide a durable, antimicrobial-treated surface.