The Nature of the Gallium-Gallium Triple Bond Yaoming Xie, † R. S. Grev, ‡ Jiande Gu, †,§ Henry F. Schaefer III,* ,† Paul v. R. Schleyer, † Jianrui Su, ⊥ Xiao-Wang Li, ⊥ and Gregory H. Robinson ⊥ Contribution from the Center for Computational Quantum Chemistry and Department of Chemistry, UniVersity of Georgia, Athens, Georgia 30602, and Department of Chemistry, UniVersity of Kentucky, Lexington, Kentucky 40506 ReceiVed NoVember 17, 1997. ReVised Manuscript ReceiVed March 3, 1998 Abstract: To simulate and help interpret the nature of the newly synthesized Ga 2 R 2 Na 2 molecule with bulky groups, ab initio and density functional quantum mechanical methods were applied to study the structures and bonding of the model [HGaGaH] 2- , [H 2 GaGaH 2 ] 2- , and [H 3 CGaGaCH 3 ] 2- dianions, as well as the neutral Na 2 [H 2 GaGaH 2 ], Na 2 [H 3 CGaGaCH 3 ], Ga 2 H 2 , and Ga 2 H 4 species. Basis sets of triple- plus double polarization quality augmented with diffuse functions were employed. No general bond lengthsbond order relationship is found. Bending from linearity of the acetylene analogues increases the GaGa separation more than the bond order is decreased. The GaGa bonding in the experimental molecule is concluded to be between triple and double in character despite the relatively long bond length. Introduction Recently a gallyne Na 2 [Mes* 2 C 6 H 3 -GatGa-C 6 H 3 Mes* 2 ] (Mes* ) 2,4,6-i-Pr 3 C 6 H 2 )(1) was synthesized and characterized as the first triple bond between main group 13 metals. 1 However, the formal assignment of a -GatGa- triple bond has been questioned, since the bond length is only marginally shorter than that of some known Ga-Ga single bonds. 2 Indirect support for a triple bond comes from previous theoretical studies on similar systems, such as HSitSiH, 3 HGetGeH, 4 and RSitSiR (R ) bulky aryl substituent). 5 However, [R-GatGa-R] 2- is the first known example among heavier main group metals that has been realized experimentally, and no prior theoretical study has been reported. Herein we report a theoretical analysis of the electronic structure of model dianions [H-GatGa-H] 2- and [H 3 C-GatGa-CH 3 ] 2- , as well as the related neutral molecules Na 2 [HGatGaH] and Na 2 [H 3 CGatGaCH 3 ], to better understand the bonding between Ga atoms. We also compare results using the same methods on neutral HGaGaH and H 2 GaGaH 2 , as well as the H 3 GaGaH 3 2- and H 2 GaGaH 2 2- dianions, which possess double or single Ga- Ga bonds. Methods Geometries were fully optimized at the self-consistent field (SCF) and the density functional theory (DFT) levels of theory. In this paper, the DFT method we employed is B3LYP, Becke’s three parameter hybrid exchange functional 6 and the Lee-Yang-Parr nonlocal cor- relation functional. 7 B3LYP is a hybrid Hartree-Fock/density func- tional theory (HF/DFT) approach. The coupled-cluster with single and double excitation (CCSD) method was also used to investigate the effect of electron correlation on the geometry of [H-GatGa-H] 2- . The basis sets were of triple- (TZ) quality augmented with two sets of d-polarization functions (+2P) augmented with diffuse functions. For Ga, the TZ functions are from Dunning’s 14s11p5d primitive basis set contracted to 10s8p2d. 8 For C, the TZ part is from Dunning’s (10s6p/5s3p). 9 All these basis sets were augmented with one diffuse s and one set of p diffuse functions as well as two sets of d-polarization functions. The exponents of the diffuse functions were Rs(Ga) ) 0.01838, Rp(Ga) ) 0.01472, and Rs(C) )Rp(C) ) 0.04380. 10 The exponents of the polarization functions were Rd(Ga) ) 0.216, 0.068, Rd(C) ) 1.50, 0.375. For H, Huzinaga’s 5s primitive set 11 was contracted to 3s, and then augmented with one s diffuse function Rs- (H) ) 0.03016 and two sets of p-polarization functions Rp(H) ) 1.50, 0.375. The technical description of this final basis set is Ga(15s12p7d/ 11s9p4d), C(11s7p2d/6s4p2d), and H(6s2p/4s2p). Analytic gradient methods were used for geometry optimizations. 12-14 Harmonic vibrational frequencies were determined via analytic second derivative methods. 15,16 Computations were carried out with the Gaussian 94 17 and PSI 2.0.8 programs. 18 † Center for Computational Quantum Chemistry, University of Georgia. ‡ Department of Chemistry, University of Kentucky. § Permanent Address: State Key Laboratory of Drug Research, Shanghai Institute of Materia Medica, Academia Sinica of Sciences, Shanghai 200031, P. R. China. ⊥ Department of Chemistry, University of Georgia. (1) Su, J.; Li, X.-W.; Crittendon, R. C.; Robinson, G. H. J. Am. Chem. Soc. 1997, 119, 5471. (2) Dagani, R. Chem. Eng. News 1997, 75, 9. (3) Grev, R. S.; Schaefer, H. F. J. Chem. Phys. 1992, 97, 7990. (4) Grev, R. S.; DeLeeuw, B. J.; Schaefer, H. F. Chem. Phys. Lett. 1990, 165, 257. (5) Kobayashi, K.; Nagase, S. Organometallics 1997, 16, 2489. (6) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (7) Lee, C.; Yang, W.; Parr, R. 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