Gold (I) hydroxide complexes of the form Z—Au—OH and digold complexes of the form Z—Au-(μOH)—Au—Z where groups Z are two electron donors are provided. The groups Z may be carbenes, for example nitrogen containing heterocyclic carbenes (NHCs), phosphines or phosphites. The complexes can be used as catalysts, for example in reactions such as hydration of nitriles, skeletal arrangement of enynes, alkoxycyclization of enynes, alkyne hydration, the Meyer-Shuster reaction, 3,3′ rearrangement of allylic acetates, cyclization of propargylic acetates, Beckman rearrangements and hydroamination. The complexes can be used in medicine, for example in the treatment of cancer.