About Leibniz cohomology and deformations of Lie algebras

We compare the second adjoint and trivial Leibniz cohomology spaces of a Lie algebra to the usual ones by a very elementary approach. The comparison gives some conditions, which are easy to verify for a given Lie algebra, for deciding whether it has more Leibniz deformations than just the Lie ones. We also give the complete description of the Leibniz (and Lie) universal infinitesimal deformation of the 4-dimensional diamond Lie algebra, used in the WZW model, and study the case of its 5-dimensional analogue by computing Massey products.