According to the invention the use of a spin crossover complex with the general formula [Mx+(Ly)H]AN)zwhereM is a positively charged metal ion,L is a heteroaromatic ligand containing at least one nitrogen atom in the ring system,AN is a negatively charged inorganic anion,x means two or three,y means two or three, andz means two, three or fouris proposed as a contrast agent in magnetic resonance imaging of human or animal bodies.