z Electro, Physical & Theoretical Chemistry Comprehensive Understanding of Bi-functional Behavior of PNP-Pincer Complexes Towards the Conversion of CO into Methanol and CO 2 :ADFTApproach Tamalika Ash, Tanay Debnath, and Abhijit K. Das* [a] The conversion mechanisms of CO into methanol through hydrogenation and CO 2 through hydrolysis catalyzed by (PNP) RuH 2 CO (1) pincer complex have been explored employing density functional theory (DFT). For both the reactions, we have identified two pathways. In pathway-I, at first the CO takes part in the reaction by interacting with the Ru-center of pincer complex and after that the H 2 /H 2 O molecule participates in the reaction, whereas in pathway-II, the generation of hydrogenated/hydrolyzed pincer from initial pincer via the assistance of H 2 or H 2 O molecule occurs first, followed by the reaction with CO. In case of hydrogenation, for both the pathways, CO is first converted to HCHO through hydro- genation which further undergoes hydrogenation to form methanol. In pathway-I, the reaction of HCHO with H 2 in presence of dehydrogenated pincer leads to produce meth- anol, whereas for pathway-II the hydrogenated pincer formed at the beginning of this pathway carries out the aforemen- tioned conversion. Further in presence of methanol, CO is catalytically converted to methyl formate. On the other hand, in case of hydrolysis, the conversion of CO to CO 2 can be achieved in two ways, where 2 nd step, i.e. formic acid to CO 2 via formate ion formation follows similar mechanism for both the pathways. Overall, from our study it is revealed that the Ru- pincer complex acts as a promising homogeneous catalyst for converting CO to various organic chemical commodities. 1. Introduction Carbon monoxide is a well-known green house gas. It is toxic to hemoglobic animals when the concentration is above ∼ 35 ppm. [1] In presence of CO, hemoglobin gets converted into carboxyhemoglobin through formation of metal-carbonyl bond, which is ineffective for delivering oxygen into tissues. [2] Apart from the toxic character, the vast application of CO in the chemical industry should also be noted in this regard. CO, in the form of syn-gas and water-gas, is highly utilized as the feedstock of various organic chemical commodity production, such as formaldehyde, methanol, methane, formic acid etc. in the industrial sector. CO, being π-acidic ligand, shows some unique bonding pattern. In case of transition metal complexes, CO forms bond using synergic π-back bonding. The formation of σ-bond takes place because of the overlap of nonbonding (or weakly anti- bonding) sp-hybridized electron pair on carbon with a blend of d-, s-, and p-orbitals on the metal, whereas π-bond arises due to the overlap of filled d-orbitals on the metal with a pair of π- antibonding orbitals projecting from the carbon center of the CO. On the other hand, during the hydrogenation and hydrolysis of CO, the simultaneous transfer of proton and hydride/hydroxide occurs to the same carbon-center of CO. Hence, whether the metal-carbonyl bond formation or the nucleophilic/electrophilic attack to CO is considered, both of them are observed to be a mono-centric chemical processes. This unique behavior of CO differentiates it’s bonding pattern from that of other unsaturated molecules. Experimentally, the hydrogenation and hydrolysis of CO without the assistance of any catalyst are difficult to achieve. Thus, a variety of heterogeneous [3–29] and homogeneous [30–34] catalytic processes have been adopted till date to convert CO containing gases (syn-gas and water-gas) into the aforemen- tioned organic molecules. In 1921, Patart first reported the synthesis of methanol from CO and H 2 . [35] In 1989, Inoue et al. [4] succeeded to perform the hydrogenation of CO 2 and CO under 10 bar in presence of supported Rh-catalysts, where they obtained higher hydrocarbons and ethanol as products. Very recently, Studt et al. [15] reported the hydrogenation of CO mediated by Cu  Ni bimetallic surface from both the exper- imental as well as theoretical perspectives. Santiago-Rodriguez et al. [24] investigated the catalytic performance of metal (Ga, Mg and Ti)-doped Cu(111) surfaces in the hydrogenation of CO 2 and CO employing DFT methods. Applicability of homoge- neous catalysis for the conversion of CO is also one of the pivotal interests now-a-days. Dombek studied the synergistic behavior of homogeneous ruthenium-rhodium catalysts for the hydrogenation of CO to ethylene glycol. [31] From his study, it is apparent that homogeneous solution of both ruthenium and rhodium show excellent selectivity for CO reduction at pressures from 400 to 850 atm and temperatures from 190° to 240° C. Miller et al. [32] examined the homogeneous hydro- [a] T. Ash, Dr. T. Debnath, Prof. A. K. Das School of Mathematical and Computational Sciences, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India Tel.: + 91-33-2473-4971(ext.1257) Fax: 91–33-24732805 E-mail: spakd@iacs.res.in Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.201901767 Full Papers DOI: 10.1002/slct.201901767 10777 ChemistrySelect 2019, 4, 10777–10786 © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim