Rubredoxin is a small iron sulfur protein involved in biological electron transfer, which is accomplished by changing the oxidation state of the iron atom in the active site. We investigate the possibility of spin-forbidden transitions between the lowest energy electronic states with different spin multiplicities in the rubredoxin active site models [Fe(SCH3)(4)](n) (n = 2-, 1-, 0) using nonadiabatic transition state theory (NA-TST). The equilibrium structures, minimum energy crossing point structures and Hessians were obtained with density functional theory. The spin-orbit coupling (SOC) was calculated with the complete active space configuration interaction method using the two electron spin-orbit Breit-Pauli Hamiltonian. We found several crossings between the lowest energy spin states associated with the changes in Fe coordination. However, only triplet/quintet crossings in [Fe(SCH3)(4)](2-) and[Fe(SCH3)4](0), as well as a quartet/sextet crossing in [Fe(SCH3)(4)](-) are characterized by nonzero first-order SOC responsible for transitions between these spin states. The rates of spin-forbidden transitions in the [Fe(SCH3)(4)](2-) complex are 1 and 2 orders of magnitude higher than the rates in the [Fe(SCH3)(4)](-) and [Fe(SCH3)(4)](0) complexes, respectively. These rate differences are related to a large variation of the SOC between the complexes with different charges, which in turn comes from different molecular orbitals involved in the spin-flip transitions. Finally, we demonstrate that the differences between the NA-TST rates and the rates calculated under the assumption of completely spin allowed transitions could be as large as 4 orders of magnitude. This means that even in qualitative discussions of the reaction mechanisms involving changes in spin states the partially spin-forbidden nature of the transitions between these states must be taken into account.