The investigation of nucleation during solidification on an atomic scale is challenging for both experimental and theoretical methods due to the involved time and length scales. Since nucleation time scales are many orders of magnitude larger than the ones achieved by classical molecular dynamics, we employ transition path sampling, an atomistic simulation method that generates an ensemble of dynamical trajectories between the liquid and the solid state. From the path ensemble, thermodynamic and kinetic properties such as the free energy barrier and the nucleation rate are calculated. We study homogeneous nucleation in Mo at various undercoolings and compare the effect of the interatomic potential on the calculated properties. Furthermore, we employ transition path sampling for analysis of the nucleation mechanisms by studying the polymorphs that are generated during the initial stages of nucleation and growth. The crystal core originates in a region of pre-ordered liquid, with BCC being the most abundant phase throughout nucleation. We further compare the nucleation mechanisms in molybdenum and nickel to reveal that the final bulk structure emerges at an early stage of nucleation.