The success and power of homogeneous catalysis derives in large part from the wide choice of transition metal ions and their ligands. This tutorial review introduces examples where the reactivity of a ligand is completely reversed (umpolung) from Lewis basic/nucleophilic to acidic/electrophilic or vice versa on changing the metal and co-ligands. Understanding this phenomenon will assist in the rational design of catalysts. First the ways ligands and metal ions affect the electronics and sterics in metal complex and catalysts will be examined. The concept of the stereoelectronic map will be examined. Then labelling a metal and ligand with Seebach donor and acceptor labels will identify whether a reaction involving the intermolecular attack on the ligand is displaying native or reactivity umpolung. This has been done for complexes of nitriles, carbonyls, isonitriles, dinitrogen, Fischer carbenes, alkenes, alkynes, hydrides, methyls, methylidenes and alkylidenes, silylenes, oxo/oxide, imide/nitrene, alkylidyne, methylidyne, and azo/nitride. The electronic influence of the metal and co-ligands is discussed in terms of the energy of (HOMO) d electrons. The energy can be related to the pKaLAC (LAC is ligand acidity constant) of the theoretical hydride complexes [H-[M]-L]+ formed by the protonation of a lone pair of d electrons. We will apply this knowledge to selected homogeneous catalytic systems. From Lewis acid-base reactivity we will then look at redox and radical reactivity for selected ligands and homogenous catalysts.