Abstract: Use of a metal complex having the formula: [M(L1)a(L2)b(L3)c]-Xd-Pepe wherein M is a metal selected from osmium, ruthenium, rhodium, rhenium or copper; L1, L2, L are bidentate or tridentate heterocyclic ligands containing O and/or N and may be the same or different; a, b, c are integers between 0 to 3 and may be the same or different and wherein the sum of a+b+c is 2 or 3; X is a functional group for directly or indirectly covalently binding to Pep wherein the functional group for directly covalently binding to Pep is selected from: amine, carboxylic acid, thiol or azide reactive functionalities; Pep is a peptide; d and e are integers between 1 and 3 and are the same and wherein the integers for d and e are equal to or less than the sum of a+b+c; and wherein there is optionally a linker between X and Pep for imaging of a cell or cell derived biological sample.