In a process for the hydroformylation of ethylenically unsaturated compounds, at least one complex of a metal of transition group VIII. with at least one phosphorus-, arsenic- or antimony-containing compound as ligand is used as hydroformylation catalyst, where the compound used as ligand in each case comprises two groups which comprise a P, As or Sb atom and at least two further hetero atoms and are bound to a xanthene-like molecular framework. New compounds of this type and catalysts which comprise at least one complex of a metal of transition group VIII. with at least one such compound as ligand are also provided.