DOI: 10.1002/chem.201100996 Response to Comment on “A Nickel(II)-Based Radical-Ligand Complex as a Functional Model of Hydrogenase” Sabyasachi Sarkar* [a] In our recent paper, [1] we have reported that a nickel(II) dithiolene complex [PPh 4 ]ACHTUNGTRENNUNG[Ni II ACHTUNGTRENNUNG(L 2 )ACHTUNGTRENNUNG(L 1 C )] (1) [L = 1,2-dicar- bomethoxyethylene dithiolate]electro-catalyzeshydrogen evolution at the lowest achievablereduction potential (E red p = 0.69 V)in CH 3 CN and also in aqueous medium (E red p = 0.71 V) known to date.We reported that1 shows strikingly similarEPR and reduction potentialvaluesob- served with native Ni- containing hydrogenases and that the spectroscopic, X-ray structure and DFT calculationsof 1 show the presence of an S-based radical that promotes hold- ing the proton facilitating its reduction. In his Correspond- ence Dr.Stein [2] about EPR writes,“The spectral form with large g shiftsis characteristic fora transition-metal-based EPR spectrum”. There are exampleswhere a non-EPR- active metalion strongly couples with an adjacent radical (ligand) to show such large g shifts. [3] But Dr. Stein arguesin his Correspondence [2] that the non-innocence ofthe sulfur ligandsin paramagnetic and monoanionic nickel bisthiolene complexes make a definite assignment of the oxidation state of the central metalatom difficult. He has given a more representative notation of these complexes as [NiACHTUNGTRENNUNG( 2.33+ )ACHTUNGTRENNUNG(L 1.66 ) 2 ] . We have already dis- cussed with this author and tried to explain that the difficul- ty encountered in the assignment of a formal oxidation state of the central metal ion in this type of complexes. Based on combined structural and spectroscopic studies several publi- cations [4–7] concluded thatparamagnetic, monoanionic and square-planar nickel bisdithiolene complexes, are bestde- scribed as having delocalized class-III mixed-valent radical anionic ligands coordinated to a closed shell low-spin d 8 cen- tral metal.They can be described by the resonance struc- tures [Ni II ACHTUNGTRENNUNG(L 2 )ACHTUNGTRENNUNG(L C )] $ [Ni II ACHTUNGTRENNUNG(L C )ACHTUNGTRENNUNG(L 2 )] (see,for example, reference [5]). In our formulation we have not differentiated the ligand “L”,which can carry the charges 2or 1 C . and vice versa. In any case, this complex with the proposed spin density distribution as [NiACHTUNGTRENNUNG( 2.33 + )ACHTUNGTRENNUNG(L 1.66 ) 2 ] may not explain the room-temperature magnetic moment of the complex as measured is 1.74 m B . Dr. Stein has used the DFT results of the complex [Ni- ACHTUNGTRENNUNG(mnt) 2 ] as the benchmark for interpreting the chemistry of the present complex but [NiACHTUNGTRENNUNG(mnt) 2 ] neither shows compara- ble electronic or electrochemical behavior nor does it func- tion like the present complex so far to mimic hydrogenase activity.Therefore though using a suitable basis set DFT re- sults may show that these two possess similar electronic dis- tributions,in reality the chemistry displayed by these two are completely different. For the present complex we relied more on its displayed chemistry. Thus the ligand-based radical character has been further envisagedfrom the intense intervalence-charge transfer band (IVCT) at 843 nm,e = 12 300 m 1 cm 1 , the electrochemical reduction potentialE 1 1/2 = 0.341 V, and also from the similarEPR spectra to that of the electro- chemically generated [4] [PdACHTUNGTRENNUNG( Bu bdtC )ACHTUNGTRENNUNG(bpy)] + and the structur- ally characterized [6] [Ni II ACHTUNGTRENNUNG(L Bu )ACHTUNGTRENNUNG(L Bu C )] . The X-ray structural data (in particular, the shortening of C S bond lengths) is merely an additionalpiece of experimental evidence that supports the ligand-based radical nature. We emphasize that this problem to assign the formal oxidation state of the cen- tral metal has been already suggested by Gray et. al and Sawyer et. al long ago. [8, 9] The entire problem has been thor- oughly revisited by the group of Wieghardtand co-work- ers [4, 6–7, 10] and it hasbeen stated thatseveralexperimental findings are essentialto convergea rationaleabout the formal oxidation state of such a complex ion. [10] We conclude that simply by changing the basis set in the DFT calculation may not be a realistic approach to understand the behavior of the complex ion that is present in solution to perform its redox activity. Dr. Stein arguesthat about 90 % of the unpaired spin density resides in the NiS 4 core,the remaining part is delo- calized over the ligand atoms. Does this mean that the NiS 4 core is the central metalatom and the four sulfurs are not part of the ligands ? Since our complex exhibited similar chemical properties to that of the Ni-C active site (E 1/2 values and ESR parame- ters) and also the complex shows other spectroscopic, mag- netic,and X-ray structural properties in tune with a closed shell low-spin d 8 centralmetalcoordinated ligand radical, we have only suggested that the nature of Ni-C state in native hydrogenase of D. gigas may be radical based, and it may be reinterpreted considering the ligand-based oxida- tions.Last but not the least, Volbeda et. al who discovered the crystal structure of the Ni-Fe hydrogenase from D. gigas have also suggested in their paper, [11] which I quote,“Given their proximity to the putatively active Ni ion, one or several [a] Prof. S. Sarkar Department of Chemistry, Indian Institute of Technology Kanpur Kanpur 208016, Uttar Pradesh (India) E-mail : abya@iitk.ac.in Chem. Eur. J. 2011, 17, 15049 – 15050 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 15049 CORRESPONDENCE