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Valence correlation consistent and augmented correlation consistent basis sets have been determined for the third row, main group atoms gallium through krypton. The methodology, originally developed for the first row atoms, was first applied to the selenium atom, resulting in the expected natural groupings of correlation functions ͑although higher angular momentum functions tend to be relatively more important for the third row atoms as they were for the second row atoms͒. After testing the generality of the conclusions for the gallium atom, the procedure was used to generate correlation consistent basis sets for all of the atoms gallium through krypton. The correlation consistent basis sets for the third row main group atoms are as follows: cc-pVDZ: (14s11p6d)/͓5s4p2d͔; cc-pVTZ: (20s13p9d1 f)/͓6s5p3d1 f ͔; cc-pVQZ: (21s16p12d2 f 1g)/ ͓7s6p4d2 f 1g͔; cc-pV5Z: (26s17p13d3 f 2g1h)/͓8s7p5d3 f 2g1h͔. Augmented sets were obtained by adding diffuse functions to the above sets ͑one for each angular momentum present in the set͒, with the exponents of the additional functions optimized in calculations on the atomic anions. Test calculations on the atoms as well as selected molecules with the new basis sets show good convergence to an apparent complete basis set limit.

Highly efficient electrocatalysts for both hydrogen evolution and hydrogen oxidation have been designed, synthesized, and characterized. The catalysts in their resting states are air-stable, mononuclear nickel(II) complexes containing cyclic diphosphine ligands with nitrogen bases incorporated into the ligand backbone. X-ray diffraction studies have established that the cation of [Ni(P(Ph)(2)N(Ph)(2))(2)(CH(3)CN)](BF(4))(2), 6a, (where P(Ph)(2)N(Ph)(2) is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane) is a trigonal bipyramid with bonds to four phosphorus atoms of the two bidentate diphosphine ligands and the nitrogen atom of an acetonitrile molecule. Two of the six-membered rings formed by the diphosphine ligands and Ni have boat conformations with an average Ni- - -N distance to the two pendant bases of 3.4 A. The cation of [Ni(P(Cy)(2)N(Bz)(2))(2)](BF(4))(2), 6b, (where Cy = cyclohexyl and Bz = benzyl) is a distorted square planar complex. For 6b, all four six-membered rings formed upon coordination of the diphosphine ligands to the metal are in the boat form. In this case, the average Ni- - -N distance to the pendant base is 3.3 A. Complex 6a is an electrocatalyst for hydrogen production in acidic acetonitrile solutions, and compound 6b is an electrocatalyst for hydrogen oxidation in basic acetonitrile solutions. It is demonstrated that the high catalytic rates observed with these complexes are a result of the positioning of the nitrogen base so that it plays an important role in the formation and cleavage of the H-H bond.

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Studies of the role of proton relays in molecular catalysts for the electrocatalytic production and oxidation of H 2 have been carried out. The electrochemical production of hydrogen from protonated DMF solutions catalyzed by [Ni(P 2 Ph N2 Ph )2(CH3CN)](BF4)2, 3a (where P 2 Ph N2 Ph is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane), permits a limiting value of the H 2 production rate to be determined. The turnover frequency of 350 s ؊1 establishes that the rate of H2 production for the mononuclear nickel catalyst 3a is comparable to those observed for Ni-Fe hydrogenase enzymes. In the electrochemical oxidation of hydrogen catalyzed by [Ni(P 2 Cy N2 Bz )2](BF4)2, 3b (where Cy is cyclohexyl and Bz is benzyl), the initial step is the reversible addition of hydrogen to 3b (K eq ‫؍‬ 190 atm ؊1 at 25°C). The hydrogen addition product exists as three nearly isoenergetic isomers 4A-4C, which have been identified by a combination of one-and two-dimensional 1 H, 31 P, and 15 N NMR spectroscopies as Ni (0) catalysis ͉ hydrogen oxidation ͉ hydrogen production

Thermodynamic data for hydride complexes of the general formula [HNi(P2 RN2 R‘)2](BF4), 3, which have been reported previously to function as effective catalysts for the electrochemical oxidation or production of hydrogen, have been determined. Values of ΔG°H+, ΔG°H •, and ΔG°H- have been determined for 3a where R = Cy, R‘ = Bz, for 3b where R = R‘ = Ph, and for the new complex 3c, R = Ph, R‘ = Bz. In addition, the ΔG values for the heterolytic addition of hydrogen to the Ni(II) precursor complexes [Ni(P2 RN2 R‘)2](BF4)2, 1a−c, have been determined experimentally or calculated. The data are useful for understanding the factors that control the catalytic activities observed for these complexes and for the design of additional catalysts.

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