Z={{-1/Sqrt[2],I/Sqrt[2]},{I/Sqrt[2],-1/Sqrt[2]}};
U={{Cos[θ/2],-E^(I λ)Sin[θ/2]}, {E^(I φ)Sin[θ/2],E^(I(λ+φ))Cos[θ/2]}};
Reduce[Z==U,{θ,φ,λ}]