J. Am. Chem. SOC. zyxwvu 1986, 108, zyxwvu 7893-7897 7893 Table IX. Energy Differences (kcal/mol) for Y- zyxwvutsr + SiH4 - YH + SiH3- E[MP2/ E[MP4/ Y 6-31++G(d,p)] 6-31 1++G(3df,2pd)la H -27.2 -29.5 -48.2 -46.8 -32.3 CH3 -31.3 -17.4 NH2 OH -12.2 F +11.9 +3.0 SiH3 0.0 0.0 PH2 +7.1 +6.6 SH +25.2 f23.1 CI +46.5 +41.7 a For third-row substituents this basis set is 6-31++G(3df,2pd). bound ion-dipole complexes with Y = C1, SH are also quite stable relative to the products in reaction 3. This may be explained in terms of reaction 4, the energetics for which are summarized in (4) Y- + SiH4 - YH + SiH3- Table IX. This reaction is highly endothermic for Y = CI, SH, and the exothermicity increases with decreasing electronegativity for second-row substituents. For Y = H and CH3 the exother- micity of reaction 4 is sufficient to more than balance the exo- thermicity of reaction 2, resulting in an exothermic reaction 3. The trends displayed in Table zyxwvutsr VI11 reflect the greater ability of more electronegative and more diffuse species to accommodate a negative charge, as well as the strength of SiY bonds when Y is electronegative. Finally, note that the two levels of theory presented in Table VI11 for reaction 4 agree to within a few kcal/mol. IV. Conclusions The main conclusions to be drawn from this work are the following: (1) The highest level of theory is able to predict gas-phase acidities with an accuracy of 2-3 kcal/mol. The use of a smaller, more tractable basis set and second-order perturbation theory results in very little loss of accuracy. (2) The MNDO method, corrected for experimental heats of formation of the reference anions, reproduces all of the important trends in gas-phase acidities and provides a reasonable quantitative estimate of these values as well. (3) Both ab initio and semiempirical calculations agree that substituents from the second row of the periodic table are much better able to stabilize pentacoordinated silicon anions relative to silane than are substituents from the third row. Indeed, no stable structures were found with SiH,-, SH-, or C1- in the equatorial positions, and the axial substituents are only marginally stable as charge-dipole complexes. (4) H- is more stabilizing than the substituents from the third row, but less than those from the second row. (5) MNDO does a good job of predicting which substituents stabilize pentacoordinated silicon anions, but not which isomers will be most stable. (6) For the electronegative substituents Y = F, CI, and SH, the negative charge prefers to reside on Y rather than on SiH3-; that is, the abstraction reaction 4 is endothermic. Consequently, the corresponding SiH4Y- complexes are found to be stable with respect to SiH< + HY. The O H substituent is also stable to HY extrusion, because the exothermicity of reaction 4 is less than that for reaction 2. Acknowledgment. This work was supported in part by grants to M.S.G. (CHE83-09948) and R.D. (CHE83-13826) from the National Science Foundation and to M.S.G. from the Air Force Office of Scientific Research (82-0190), and the donors of the Petroleum Research Fund, administered by the American Chemical Society. All of the ab initio calculations were performed on the IBM 3081/D computer at North Dakota State University, and the computer time made available by the NDSU Computer Center is gratefully acknowledged. Registry No. H1, 1333-74-0; CH,, 74-82-8; NH,, 7664-41-7; H20, 7732-18-5; HF, 7664-39-3; SiH,, 7803-62-5; PH,, 7803-51-2; H2S, 17655-31-1; OH-, 14280-30-9; F, 16984-48-8; SiH3-, 15807-96-2; PH2-, 13937-34-3; SH-, 7783-06-4; Cl-, 16887-00-6; SiH5-, 41650-16-2; axi- al-SiH4CH3-, 104911-61-7; equatorial-SiH,CH,-, 105015-58-5; axial- SiH,OH-, 73085-3 1- 1; equatorial-SiH,OH', 78853-67-5; axial-SiH,F, 73173-71-4; equatorial-SiH,F, 10501 5-59-6. Supplementary Material Available: Tables of total 6-3 1 ++G- (d,p) energies and 6-3 1 l++G(3df,2pd) energies for reference compounds (2 pages). Ordering information is given on any current masthead page. 7783-06-4; HCI, 7647-01-0; H-, 12184-88-2; CHj-, 15194-58-8; NH2-. Theoretical Evidence for Two Geometrical Isomers of Ago2+ F. Illas,*t J. Rubio,l J. M. Ricart,l and P. L. Cabots Contribution from the Departament de Fhico Qujmica Aplicada, Facultat de Farmacia, Uni_versitat-deBarcelona, Avda.-Diagonal zyxwvut s/n 08028 Barcelona, Spain, the Departament de Quimica Fisica, Facultat de Quimica de Tarragona, Universitaf de Barcelona, Pca. Imperial Tarraco s/n, 43005 Tarragona, Spain, and Departament de Quimica Fisica, Facultat de Quimica, Universitat de Barcelona, Avda. Diagonal, 647, 08028 Barcelona, Spain. Received October 8, 1985 Abstract: With use of nonempirical pseudopotentials and moderately large basis sets, SCF and CI calculations show that Ago, presents two nearly degenerate structures of C2" and zyxwvu C, symmetry, respectively. The ground state of these structures is found to be a 'A2 and a 'A". While the SCF and the CI description are almost coincident, the effect of electron correlation on the geometrical parameters is found to be quite important. Because of its relevance in the silver-catalyzed oxidation re- actions, a considerable amount of experimental work has been 'A contribution from the 'Grup de Ouhica Ouintica de 1'Institut d'Es- devoted to the isolation and characterization of the reaction products formed thorugh molecular oxygen and atomic silver interactions in rare gas matrices. Of Darticular interest is the AEO, molecule which is onlv stable . - tudis Catalans". *Author to whom correspondence should be addressed. * Departament de F&o Quiqica Aplicada. Departament de Q$mica Fisica. Departament de Quimica Fisica. at low'temperatures and has to ibtained by means of c;yogenic techniques and isolated in an inert matrix. However, from the different experiments carried out to determine the molecular structure of this molecule two distinct geometrical arrangements 0002-7863/86/1508-7893$01.50/0 0 1986 American Chemical Society