Fareed Bhasha Sayyed and Shigeyoshi Sakaki* Fukui Institute for Fundamental Chemistry, Kyoto University, Nishihiraki-cho, Takano, Sakyo-ku 606-8103, Japan [Introduction] The conversion of carbon dioxide (CO 2 ) to useful chemicals is one of the most escalating areas of research. Aryl halide is one of the best substrates for the synthesis of carboxylic acid from CO 2 . However, a direct CO 2 insertion into aryl halide bond is not easy. In this context, the recently reported Ni- catalyzed direct carboxylation of PhCH 2 Cl with CO 2 is considered efficient CO 2 transformation reaction, where Ni(PCp 3 ) 2 (Cp = cyclopentyl) is employed as a catalyst. Experimental results suggest that the Ni(I) species participates in the CO 2 insertion, as shown in Eq. (1), but the evidence has not been provided. We theoretically investigated this reaction to unravel the reason why Ni(I) species is crucial for this reaction and why Ni(II) is not suitable for the CO 2 insertion; see Eq. (2). (Cp 3 P) 2 Ni 1 (CH 2 Ph) + CO 2 →  (Cp 3 P) 2 Ni 1 (OOCCH 2 Ph) (1) (Cp 3 P) 2 Ni 11 (CH 2 Ph)Cl + CO 2 →  (Cp 3 P) 2 Ni 11 (OCOCH 2 Ph)Cl (2) [Computational Details] All geometries were optimized with the dispersion corrected B3LYP-D functional with BS-I basis set system. In BS-I, Lanl2Tz(f) basis set for Ni, 6-31G(d) for H, C, O, P, and 6- 31+G(d) for Cl were employed. The solvent effect of N,N -dimethylformamide (DMF) was estimated by conductor-like polarizable continuum model (CPCM). A better basis system (BS-II) was used for evaluation of solvent effect. In BS-II, we employed Lanl2Tz(f) basis set for Ni, cc-pVDZ for H, C, O, P, and aug-cc-pVDZ for Cl. The reliability of the B3LYP-D functional is verified here with the oxidative addition to a model complex Ni(PH 3 ) 2 ; Ni(PH 3 ) 2 + PhCH 2 Cl → Ni(PhCH 3 )(Cl)(PH 3 ) 2 . The calculated activation energy agrees well with that of CCSD(T). [Results and Discussion] Geometry and energy changes for the CO 2 insertion into the Ni(I)-CH 2 Ph bond of (Cp 3 P) 2 Ni 1 (CH 2 Ph)  are shown in Figure 1. First, CO 2 coordinates to the Ni(I) centre to form 1 2 - coordinated CO 2 complex 2 , from which CO 2 insertion occurs to afford a carboxylate complex 3 through a four-member transition state TS 2,3 with an activation barrier of 6.6 kcal/mol. Complex 3 isomerises to the most stable η ² -coordinated carboxylate complex (Cp 3 P) 2 Ni 1 ( η ² -O 2 CCH 2 Ph) 4 . The CO 2 insertion into the Ni(II)-CH 2 Ph of (Cp 3 P) 2 Ni l (CH 2 Ph)Cl 5 occurs through a four-member transition state without the formation of a Ni(II)-CO 2 complex, as shown in Figure 2. The activation barrier is 38.8 kcal/mol, which is substantially larger than that into the Ni(I)-CH 2 Ph bond. These results indicate that the Ni(II)-CH 2 Ph is not reactive for CO 2 insertion but the Ni(I)-CH 2 Ph is reactive. We investigated the reasons from the bond energy and the interaction in the transition state. By the insertion reaction, the Ni-C bond is broken and the Ni-OCOR bond (R=CH₂Ph) CO 2 is formed. The Ni(I)-CH 2 Ph bond is calculated to be stronger (42.5 kcal/mol) than the Ni(II)-CH 2 Ph bond (32.1 kcal/mol) but the Ni(I)-OCOR bond (103.3 kcal/mol) is much stronger than the Ni(II)-OCOR bond (79.7 kcal/mol). NBO population analysis of TS 2,3 and TS 5,6 suggest that charge transfer (CT) from the Ni(I)-benzyl moiety to CO 2 much more strongly occurs than that from the Ni(II)-benzyl. The is because the benzyl group is more negatively charged in the Ni(I)-benzyl complex than in the Ni(II)-benzyl group. Thus, it is concluded that the CO 2 insertion into Ni(I)-alkyl bond is thermodynamically and kinetically favourable.