Computational Details
All DFT and MP2[8] geometry optimizations and vibrational calculations were carried out using the Gaussian 09 program package.[9] As reported previously,[6c] both complexes 1 and2 were calculated to be the singlet ground state. Therefore, only singlet state was considered for the complexes involved in this system. The 6-31G* basis set was used for C, H, N and S atoms, and Fe atoms were treated by 6-311G* basis set. Each optimized structure was analyzed by harmonic vibrational frequencies obtained at the same level and characterized as a minimum (Nimag = 0) or a transition state (Nimag= 1). The transition state structures were shown to connect the reactant and product on either side via intrinsic reaction coordinate (IRC) following. Some relaxed potential energy scans of 18 functionals were performed with only the scanned N−N distance from 0.500 Å to 2.500 Å with stepsize 0.050 Å. The most reliable available levels, the domain-based local pair natural orbital coupled cluster method with single and double excitations and perturbative triple corrections (DLPNO-CCSD(T)[10,11]/6-311G*(Fe)/6-31G*(C, H, N, S) in conjunction with the correlation fitting auxiliary basis set TZV/C for the single-point energy calculations were performed for the species at the MP2 optimized geometries in ORCA Program[12] as the benchmark data. Considered the calculation of DLPNO-CCSD(T), the model compounds CpFe(μ -SMe)2(μ -NHNH3)FeCp (Cp = η 5-C5H5,1m ) and CpFe(μ -SMe)2(μ -NH)FeCp (2m ) (Figure 1) were adopted in this systems. The ONIOM calculations[13] were carried out for 1and 2 to validate the rationality of simplified model (in Supporting Information).