Zintl Anions in the Intermetallic Compounds Yb - American Chemical

Mar 25, 2008 - Carolina State UniVersity, Raleigh, North Carolina 27695-8204 ... of M are completely filled while the frontier levels of M are given b...
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Chem. Mater. 2008, 20, 2751–2756

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Electronic Structure Study of the [Ag-Ag]4-, [Au-Au]4-, and [Hg-Hg]2- Zintl Anions in the Intermetallic Compounds Yb3Ag2, Ca5Au4, and Ca3Hg2: Transition Metal Anions As p-Metal Elements Jürgen Köhler* and Myung-Hwan Whangbo* Max-Planck-Institut für Festkörperforschung, Stuttgart, Germany, and Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204 ReceiVed December 17, 2007. ReVised Manuscript ReceiVed January 24, 2008

The chemical bonding of the M2 dimers (M ) Ag, Au, Hg) present in Yb3Ag2, Ca5Au4 and Ca3Hg2 was examined on the basis of first principles electronic band structure calculations. In these compounds, the 6s- and 5d-block bands of M are completely filled while the frontier levels of M are given by partially filled 6p-block bands such that the transition metal atoms M are best described as anions with electron configuration (5d)10(6s)2(6p)1. Thus, the M2 dimers (M ) Ag, Au, Hg) of Yb3Ag2, Ca5Au4 and Ca3Hg2 are present as [Ag-Ag]4-, [Au-Au]4-, and [Hg-Hg]2- Zintl-anions, respectively, in these compounds in which the M2 dimers have a single bond formed by the pσ-pσ interaction between the 5p and 6p orbitals of M atoms. The anions of late transition metal elements are different from those of late main group elements in their tendency for covalent bond formation.

1. Introduction Late transition metal atoms are rarely considered to form anions, despite the fact that the elements of groups 8 to 11 have electronegativities similar to those of the late main group elements. The electronegativity of Au is only slightly smaller than that of I,1 but it took many years to accept in CsAu2 the existence of the Au- anion,3 which is stabilized by the relativistic effect,4 and in the mean time many aurides have become known.5 In the recently discovered K34In96.19Au8.81 and BaAu0.36In1.64, the Au atoms are present most probably as anions.6 The existence of the isoelectronic Pt2- anion was proposed a long time back7 and the recently discovered platinide Cs2Pt, red and transparent, can in fact be described in terms of Cs+ and Pt2- ions.8 It has also been well-established that transition metal elements can occur as anions in organometallic compounds.9 Some * Corresponding author. E-mail: [email protected] (J.K.); mike_whangbo@ ncsu.edu (M.-H.W.).

(1) For Pauling electronegativities, see: http://www.webelements.com. (2) Sommer, A. H. Nature 1943, 152, 215. (3) (a) Wood, V. E.; Reitz, J. R. J. Phys. Chem. Solids 1962, 23, 229. (b) Liu, T. L. Phys. ReV. 1975, 12, 3008. (c) Hasegawa, A.; Watabe, M. J. Phys. F 1977, 7, 75. (d) Knecht, J.; Fischer, R.; Overhof, H.; Hensel, F. J. Chem. Soc., Chem. Commun. 1978, 905. (e) Wertheim, G. K.; Cohen, R. L.; Crecelius, G.; West, K. W.; Wemick, J. H. Phys. ReV. B 1979, 20, 860. (4) (a) Pyykkö, P. Chem. ReV. 1988, 88, 563. (b) Pyykkö, P. Angew. Chem., Int. Ed. 2002, 41, 1. (5) (a) Pantelouris, A.; Küper, G.; Hormes, J.; Feldmann, C.; Jansen, M. J. Am. Chem. Soc. 1995, 117, 11749. (b) Mudring, A. V.; Jansen, M. Z. Naturforsch. 2001, 56b, 433. (c) Nuss, J.; Jansen, M. Z. Kristallogr.-New Cryst. Struct. 2002, 217, 313. (d) Mattausch, Hj.; Zheng, Ch.; Kienle, L.; Simon, A. Z. Anorg. Allg. Chem. 2004, 630, 2367. (6) (a) Dai, J.-C.; Corbett, J. D. Inorg. Chem. 2006, 45, 2104. (b) Li, B.; Corbett, J. D. Inorg. Chem. 2006, 45, 8958. (7) Goodman, C. H. L. J. Phys. Chem. Solids 1958, 6, 305. (8) Karpov, A. S.; Nuss, J.; Wedig, U.; Jansen, M. Angew. Chem. 20031154966; Angew. Chem., Int. Ed. 2003, 42, 4818. (9) Ellis, J. E. Inorg. Chem. 2006, 45, 3167.

years ago, a series of insulating fluorides and semiconducting oxides containing 18-valence-electron octahedral cations [PtIn6]10+ and [IrIn6]9+ were discovered.10 In these cations, the 6s and 5d orbitals of the transition metal atom M () Pt, Ir) act as a reservoir for holding 12 electrons. To a first approximation, the Pt and Ir atoms in the [PtIn6]10+ and [IrIn6]9+ cations have the 6s25d10 configuration with the oxidation states -2 and -3, respectively. In 18-electron HalfHeusler (18eHH) compounds and in similar compounds of late transition elements with graphite- and diamondlike substructures,11 transition metal anions with valence electron configuration (n+1)s2nd10(n+1)p2 behave as p-metal elements as do late main group atoms. There are also covalently bonded homonuclear dimers of transition metal anions with valence electron configuration (n+1)s2nd10(n+1)p1; the Pt2, Cu2, and Au2 dimers in La2Pt2In, La2Cu2In and Yb2Au2In are present as [Pt-Pt]6-, [Cu-Cu]4-, and [Au-Au]4- Zintl anions, respectively.12 Thus, the existence of isoelectronic [Ag-Ag]4-, [Au-Au]4-, and [Hg-Hg]2- Zintl anions in binary intermetallic compounds can be expected. Our search through the crystal structure databases on the Yb/Ag,13 Ca/ Au, and Ca/Hg systems led us to the compounds Yb3Ag2,14 Ca5Au415 and Ca3Hg216 that have Ag2, Au2, and Hg2 dimers, (10) (a) Köhler, J.; Chang, J.-H. Angew. Chem. 20001122077; Angew. Chem., Int. Ed. 2000, 39, 1998. (b) Köhler, J.; Chang, J.-H.; Whangbo, M.-H. J. Am. Chem. Soc. 2005, 127, 2277. (d) Friedrich, H. A.; Köhler, J. Z. Anorg. Allg. Chem. 2001, 627, 144. (e) Köhler, J.; Lee, C.; Whangbo, M.-H. Z. Anorg. Allg. Chem. 2007, 633, 1464. (11) (a) Köhler, J.; Deng, S.; Lee, C.; Whangbo, M.-H. Inorg. Chem. 2007, 46, 1957. (b) Lee, C.; Köhler, J.; Whangbo, M.-H. Z. Anorg. Allg. Chem. 2007, 633, 1464. (12) (a) Whangbo, M.-H.; Lee, C.; Köhler, J. Angew. Chem. 2006, 118, 7627; Angew. Chem., Int. Ed. 2006, 45, 7465. (b) Köhler, J.; Whangbo, M.-H. Solid State Sci. 2008,in press. (13) We have chosen Yb3Ag2 as there is no Ca3Ag2 phase in the Ca/Ag system. (14) Palenzona, A. J. Less-Common Met. 1970, 21, 443. (15) Fornasini, M.-L.; Merlo, F. J. Solid State Chem. 1985, 59, 65.

10.1021/cm703590d CCC: $40.75  2008 American Chemical Society Published on Web 03/25/2008

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Figure 1. Perspective view of the crystal structures of (a) Yb3Ag2 and (b) Ca5Au4. In (a), the yellow and green spheres represent the Ag and Yb atoms, respectively. In (b), the yellow and orange spheres represent the Au atoms, whereas the light and dark blue spheres represent the Ca atoms. The distances of the Ag-Ag and Au-Au dimers are given.

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Figure 2. Coordination polyhedron around the M-M dimers in Yb3Ag2, Ca5Au4 (M ) Au), Ca3Hg2 (M ) Hg), and RE2M2In (RE ) rare earth element, M ) Ge, Cu, Au, Pt). The dark blue spheres form a trigonal biprism, and the light blue spheres cap its four rectangular faces. The coordination polyhedra surrounding a M2 dimer are Ag2Yb12 in Yb3Ag2, M2Ca12 (M ) Au, Hg) in Ca5Au4, and Ca3Hg2, and M2RE8In4 in RE2M2In (M ) Ge, Cu, Au, Pt).

respectively. In the present work we probe whether these dimers exist as Zintl anions [Ag-Ag]4-, [Au-Au]4-, and [Hg-Hg]2- on the basis of first principles electronic band structure calculations for these compounds. In the following, results of our calculations are presented and implications of our results are discussed. 2. Electronic Structure Calculations First principles electronic band structure calculations for Yb3Ag2, Ca5Au4, and Ca3Hg2 were performed within the local density approximation17 using the linear muffin-tin orbital (LMTO) method18 encoded in the TB-LMTO-ASA program.19 All our calculations were checked for convergence of energies, orbital moments and magnetic moments with respect to the number of k-points used in the reciprocal-space integrations. Our calculations did not include spin–orbit coupling, and treated the relativistic effects in terms of the scalar relativistic method. Calculations for the electronic band structure of Yb3Ag2 were performed assuming that the Yb 4f orbitals are core states, and each Yb contributes two valence electrons to the Ag framework, i.e., the oxidation state of Yb in Yb3Ag2 is +2. To examine the bonding within the M2 (M ) Ag, Au, Hg) dimers and that between the M2 dimer and the Yb/Ca atoms, respectively, we also performed crystal orbital Hamiltonian population (COHP) analysis,20 which partitions the band-structure energy (i.e., the sum of the Kohn–Sham orbital energies) into contributions between pairs of atomic orbitals. All COHP plots are presented under the convention in which positive and negative values refer to bonding and antibonding interactions, respectively. The crystallographic data of the structures of Yb3Ag2, Ca5Au4, and Ca3Hg2 employed for our calculations are summarized in Table S1 of the Supporting Information.

3. Coordination Environments of Ag2, Au2, and Hg2 Dimers The crystal structure of Yb3Ag2 is isotypic to the Mo2FeB2type structure and can be described in terms of planar YbAg2 layers that alternate with layers of Yb atoms (Figure 1a). The Ag atoms exist as dimers with short Ag-Ag distance (16) Bruzzone, G.; Merlo, F. J. Less-Common Met. 1973, 32, 237. (17) von Barth, U.; Hedin, L. J. Phys. C 1972, 5, 1629. (18) (a) Andersen, O. K. Phys. ReV. B 1975, 12, 3060. (b) Andersen, O. K.; Jepsen, O. Phys. ReV. Lett. 1984, 53, 2571. (c) Andersen, O. K.; Arcangeli, C.; Tank, R. W.; Saha-Dasgupta, T.; Krier, G.; Jepsen, O.; Dasgupta, I. Mater. Res. Soc. Symp. Proc. 1998, 491, 3. (19) Krier, G.; Jepsen, O.; Burkhardt, A.; Andersen, O. K. The TB-LMTOASA Program,version 4.7. (20) Dronskowski, R.; Blöchl, P. E. J. Phys. Chem. 1993, 97, 8617.; see also the Web site http://www.cohp.de.

Figure 3. Total and projected DOS plots calculated for the Yb 6s and 5d orbitals in Yb3Ag2.

of 258 pm. In the crystal structure of Ca5Au4 planar CaAu(1)2 layers, which are isostructural with the YbAg2 layers of Yb3Ag2, alternate with Ca4Au(2)2 slabs (Figure 1b). The Au atoms are also present as Au2 dimers with short interatomic distance, i.e., Au(1)-Au(1) ) 285 pm and Au(2)-Au(2) ) 296 pm. The interdimer distances, Ag · · · Ag and Au · · · Au, respectively, in both compounds are greater than 360 pm, so that the dimers are discrete. Each Ag2 dimer in Yb3Ag2 and each Au2 dimer in Ca5Au4 is surrounded by 12 atoms (Figure 2). Eight of the twelve Yb atoms surrounding each Ag2 dimer in Yb3Ag2 and eight of the twelve Ca atoms surrounding each Au2 dimer in Ca5Au4 form a trigonal double prism sharing a rectangular face. The rectangular faces of the trigonal double prism are capped by four Yb atoms in Yb3Ag2, and by four Ca atoms in Ca5Au4. Such a coordination polyhedron is typically found for metal-metal bonded M2 dimers in intermetallic compounds, e.g., RE2M2In (RE ) rare earth element, M ) Ge, Cu, Au, Pt)21,22 that crystallize in the Mo2FeB2 type structure. Within the Ag2Yb4Yb8 units of Yb3Ag2 the Ag-Yb distances to the capping Yb are 308 pm, while Ag-Yb distances to the bridging and terminal Yb atoms are 291 and 301 pm, respectively. Within the Au2Ca12 units of Ca5Au4, the CaAu(1) distances range from 295 to 350 pm, and the Ca-Au(2) distances from 303 to 349 pm. (21) (a) Choe, W.; Miller, G. J.; Levin, E. M. J. Alloys Compd. 2001, 329, 121. (b) Mishra, R.; Hoffmann, R.-D.; Pöttgen, R. Z. Naturforsch. B 2001, 56, 239. (c) Pöttgen, R. Z. Naturforsch. B 1994, 49, 1525. (d) Kaczorowski, D.; Rogl, P.; Hiebl, K. Phys. ReV. B 1996, 54, 9891. (22) For a detailed list of known RE2M2In compounds, see: Kalychak, Ya. M.; Zaremba, V. I.; Pöttgen, R.; Lukachuk M.; Hoffmann, R.-D.; Rare Earth-Transition Metal-Indides. In Handbook on the Physics and Chemistry of Rare Earths, Schneider, K. A., Jr., Pecharsky, V. K., Bünzli, J.-C., Eds.; Elsevier: Amsterdam, 2005; Vol. 34, pp 1–124.

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Figure 4. Dispersion relations of the energy bands calculated for Yb3Ag2 with the fat band representation for (a) the 5s orbital contributions of Ag, (b) the 5p orbital contributions of Ag, and (c) the 4d orbital contributions of Ag.

Ca3Hg2 is isotypic to Yb3Ag2 and crystallizes also in the Mo2FeB2-type structure (Figure 2a). Characteristic building units are planar CaHg2 layers, which are isostructural with the YbAg2 layers of Ca5Au4 and the CaAu(1)2 layers of Ca5Au4 and alternate with Ca2 layers. The Hg2 dimers have a short Hg-Hg distance (300 pm), but the interdimer Hg · · · Hg distances are much longer (>360 pm). The coordination polyhedron of the 12 Ca atoms surrounding a Hg2 dimer (Figure 2) is more regular in shape and larger in size than the corresponding polyhedron of Ca5Au4; the Hg-Ca distances are 325 pm to the eight Ca atoms forming the double prism, and 336 pm to the four capping Ca atoms. Figure 5. COHP plots calculated for the Ag-Ag interactions in Yb3Ag2.

4. Bonding and Oxidation States of Ag2, Au2, and Hg2 Dimers 4.1. Ag2 Dimer. The projected density of states (DOS) for the Yb 6s-, Yb 5d-, Ag 5s-, and Ag 5p-orbital contributions are shown in Figure 3, and the dispersion relations of the energy bands calculated for Yb3Ag2 are presented in Figure 4, with the fat-band representation for the Ag orbital contributions. Below the Fermi level, there are some Yb 6sorbital contributions (Figure 4) and the Yb 5d-orbital contributions are substantial (the Yb 6p-orbital contributions lie above 4 eV). The Ag 5s-orbital contributions appear in two regions, i.e., between -6.5 and -6.0 eV and between -4.0 and -2.0 eV. The nearly dispersionless Ag 4d-block bands occur in between the two Ag 5s-block bands. The Ag 5s- and 4dblock bands are completely filled so that the Ag 5s and 4d levels can be considered as pseudocore levels. The Ag 5pblock bands (above -2.0 eV) are well separated from the Au 6s- and 5-block bands and are partially filled. The Ag and Yb atoms of Yb3Ag2 show no s/p hydridization because their ns and np levels are well-separated. In general, the electronic structure of the Ag2 dimer in Yb3Ag2 is quite similar to that of the Cu2 and Pt2 dimers in La2Cu2In and La2Pt2In, respectively, reported earlier.12 As can be seen from the COHP curve for the Ag-Ag interactions (Figure 5), the two Ag 5s-block bands represent the bonding and antibonding interactions between the Ag 5s orbitals in each Ag2 dimer. Since both the Ag 5s-block bands are occupied, their bonding and antibonding effects are compensated. The Ag 5p-block bands show bonding interactions between the Ag 5p orbitals of each Ag2 dimer. Thus, by

regarding the Ag-Ag bond of each Ag2 dimer as a 5pσ-5pσ single bond, the electron configuration (5s)2(4d)10(5p)1 can be assigned to each Ag atom, which corresponds to an Ag2anion. As a consequence, each Ag2 dimer in Yb3Ag2 exists as a Zintl anion [Ag-Ag]4-, which is isoelectronic with the [Pt-Pt]6- anion found in La2Pt2In.12 To a first approximation, the charge balance for Yb3Ag2 can be written as (Yb2+)3[Ag-Ag]4-(e-)2 or as (Yb3)4+[Ag-Ag]4-. The latter description is consistent with the observation that the Yb 6s and 5d states are also slightly occupied. 4.2. Au2 Dimer. The bonding situation of the Au2 dimers in Ca5Au4 is rather similar to that of the Ag2 dimers in Yb3Ag2 (Figure 6). To begin with, the Au(1) and Au(2) atoms have nearly the same electronic structure due to their similar local chemical environments. The nearly dispersionless Au 5d bands, present between -5.0 and -4.0 eV, are completely filled, so that they can be considered as semicore states in this case as well. The Au 6s bands occur mainly in two regions, one below the Au 5 bands (i.e., between -6.2 and -5 eV) and the other above the Au 5d bands (i.e., between -4 and -3 eV). The Au 6p bands appear largely between -2.0 and +4 eV, and only the bottom portion is occupied hence giving rise to the metallic property of Ca5Au4. Figure 7a-c shows that there is a small Au 6s/5d hybridization, whereas the energy regions of the Au 6s and Au 6p states are well-separated, leading to no Au 6s/6p hybridization. As expected, the plots of the projected density of states (DOS) presented in Figure 8 show that the Au 6s and 5d block bands are fully occupied, and the Au 6p bands are partially occupied. In these projected DOS plots, the Au 6s and Au 6p contributions to the DOS appear negligibly small

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Figure 6. Dispersion relations of the energy bands calculated for Ca5Au4 with the fat-band representation: (a) the 6s orbital contributions of Au(1), (b) the 6p orbital contributions of Au(1), (c) the 5d orbital contributions of Au(1), (d) the 6s orbital contributions of Au(2), (e) the 6p orbital contributions of Au(2), and (f) the 5d orbital contributions of Au(2).

Figure 7. Projected DOS plots calculated for (a) the Au 6s and 6p orbitals and (b) the 5d orbitals of Ca5Au4.

compared to the Au 5d contributions. This is due to the artifact of constructing projected DOS plots, since they consider only the orbital contributions from the muffin-tin spheres representing the atoms and neglect those from the intersphere region. Within a given muffin-tin sphere, a contracted orbital (e.g., Au 5d) has a greater contribution than does a diffuse orbital (e.g., Au 6s or Au 6p) even if the two orbitals orbitals contribute equally to the energy bands. The COHP plots calculated for the Au-Au and Ca-Au interactions are presented in Figure 9. The Au 6s bands below the Au 5d bands show Au-Au bonding interactions and hence represent the bonding states between the Au 6s orbitals, while the Au 6s bands above the Au 5d bands show Au-Au antibonding interactions and hence represent the antibonding states between Au 6s orbitals. For both Au(1) and Au(2), the Au-Au interactions are bonding in the Au 6p block bands below the Fermi level. To a first approximation, therefore, the Au-Au bond of each Au2 dimer in Ca5Au4 is a single bond formed by the pσ-pσ interaction of the Au 6p orbitals. Then, each Au atom can be considered to have the electron configuration (6s)2(5d)10(6p)1 corre-

sponding to an Au2- anion, so that each Au2 dimer in Ca5Au4 exists as a Zintl anion [Au-Au]4- and hence the charge balance for Ca5Au4 is written as (Ca2+)5([Au-Au]4-)2(e-)2 or as (Ca5)8+([Au-Au]4-)2. The latter description is consistent with the observation that the Ca 4s states, which occur in the same energy regions as the Au 6s and 6p block bands, are also slightly occupied (see Figure S1 of the Supporting Information). The COHP plots of Figure 9c show that, for the interactions between the Au 6s and Ca 4s orbitals, the effect of the occupied bonding states is nearly canceled out by that of the occupied antibonding states. For the interactions between the Au 6p and Ca 4s orbitals, only the bonding states are occupied hence stabilizing the structure of Ca5Au4. (The bands based on antibonding Au 6p - Ca 4s interactions are found above +7.5 eV, see Figure S2 of the Supporting Information.) 4.3. Hg2 Dimer. The dispersion relations of the energy bands calculated for Ca3Hg2 are presented in Figure 10. The Hg 5d-block bands are about -8 eV below Fermi level and are nearly dispersionless showing that they act as a reservoir for holding 10 electrons, i.e., they are pseudocore states. The Hg 6s-block bands lie above the Hg 5d-block bands (i.e., between -7.5 and -4 eV). The Hg 6p-block bands are dispersive and are partially filled. In contrast to the case of Ca5Au4, however, there is no Hg 6s/5d hybridization, and the Hg 6s/6p hybridization is negligibly small. The COHP plots calculated for the Hg-Hg and Ca-Hg interactions are given in Figure 11. As in the case of Ca5Au4, there are significant pσ-pσ bonding interactions between the Hg 6p orbitals below the Fermi level. The Ca-Hg interactions of Ca3Hg2 are rather strong and comparable in magnitude to the Hg-Hg interactions. For the interactions between the Ca 4s and Hg 6s orbitals, the effect of the occupied bonding states is nearly canceled out by that of the occupied antibonding states. For the interactions between

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Figure 8. COHP plots calculated for the Au-Au and Ca-Au interactions in Ca5Au4.

Figure 9. Dispersion relations of the energy bands calculated for Ca3Hg2 with the fat-band representation: (a) the Hg 6s-orbital contribution, (b) the Hg 6p-orbital contribution, and (c) the Hg 5d orbital contribution.

Figure 10. COHP plots calculated for the Hg-Hg and Ca-Hg interactions in Ca3Hg2.

Hg-Hg bond of each Hg2 dimer is a single bond formed by the pσ-pσ interaction between the Hg 6p orbitals, each Hg atom has the electron configuration (6s)2(5d)10(6p)1, each Hg2 dimer in Ca3Hg2 exists as a Zintl anion [Hg-Hg]2-, and the charge balance for Ca3Hg2 can be written as (Ca3)2+([Hg-Hg]2-). Obviously, the Ca atoms of Ca3Hg2 are less oxidized compared to those of Ca5Au4, which is consistent with what is expected from the fact that Au is more electronegative than Hg. The description of the HgHg dimers in Ca3Hg2 in terms of Zintl anions may seem unusual, but the analogy with the Au-Au dimers is obvious. Here it should be mentioned that another Hg-based Zintl anion with p-p bonding has been discussed for the mercuride Na3Hg2,23 which can formally be described as (Na+)6[Hg4]6containing square planar [Hg4]6- cluster units embedded in a sea of Na+ cations.24 5. Discussion

Figure 11. Oxidation states and valence electron configurations of the anions associated with late transition metal and main group elements.

the Ca 4s and Hg 6p orbitals (see Figure 11), only the bonding states are occupied thereby stabilizing the Ca3Hg2 structure. (The bands based on antibonding Hg 6p - Ca 4s interactions are found above +5 eV, see Figure S3 of the supporting information.) Thus, to a first approximation, the

It is important to note differences between the electronic structures of the Ag2, Au2, and Hg2 dimers. In the Ag2 dimer, the valence 4d states lie in between the σ-bonding and σ-antibonding states of the valence 5s orbitals, which means that the Ag 5s and 4d levels of the Ag2- ion are similar in energy. In the Au2 dimer, the bonding situation is similar and even more pronounced. The valence 5d states lie also (23) (a) Nielson, J. W.; Baenziger, N. C. Acta Crystallogr. 1954, 7, 277. (b) Deiseroth, H. J. Prog. Solid State Chem. 1997, 25, 73. (c) Tkachuk, A. V.; Mar, A. Acta Crystallogr., Sect. E 2006, 62, 29. (24) Kuznetsov, A. E.; Corbett, J. D.; Wang, L.-S.; Boldyrev, A. I. Angew. Chem., Int. Ed. 2001, 40, 3369.

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in between the σ-bonding and σ-antibonding states of the valence 6s orbitals and Au 6s and 5d levels of the Au2- ion are even closer in energy compared to the Ag case. In the Hg2 dimer, both the σ-bonding and the σ-antibonding states of the valence 6s orbitals lie above the valence 5d states. This indicates that the valence atomic orbitals change from the sequence 6s e 5d < 6p in compounds of Pt3- and Au2to the sequence 5d < 6s < 6p in compounds of Hg-. This change in the valence orbital sequence would lead to differences in the chemistry of mercurides, aurides, and platinides. In intermetallic compounds of Hg and electropositive elements, Zintl-like anionic units are rare and extended Hg networks of multicenter bonds are mostly found, namely, the Zintl anion formation is less pronounced for Hg than for Au. This reflects the fact that the 6s orbital lies closer in energy to the 6p orbital for Hg than for Au, so the s/p hybridization is more favorable for Hg thereby leading to Hg networks with multicenter bonds. As already pointed out elsewhere,12 late main group elements can exist as isolated closed-shell anions with valence electron configuration ns2np6, and form covalent bonds to satisfy the octet rule when their valence p-shell is partially empty. For instance, the valence electron configuration ns2np5 leads to homonuclear dimers, as found for halogen molecules X2 (X ) F, Cl, Br, I) and the Zintl anion [Ge-Ge]6- of RE2Ge2In,21 to name a few. Late transition metal atoms can exist as isolated pseudoclosed-shell anions with electron configuration (n+1)s2nd10 and form covalent bonds when their valence p-shell is partially occupied.6 For example, the electron configuration (n+1)s2nd10(n+1)p1 leads to homonuclear dimers as found for RE2M2In (M ) Pt, Cu, Au),12 Ca5Au4, and Ca3Hg2. The electron configuration (n+1)s2nd10(n+1)p2 leads either to four-coordinate heteronuclear covalent bonding as found for diamond-like 18eHH compounds11 or to three-coordinate heteronuclear covalent bonding as found for the graphite-like alternative structures of 18eHH compounds.11b The electron configurations for the probable anions of late main group and late transition metal elements, which have comparable bonding capability, are represented in Figure 11. In this simplified presentation, it should be understood that polarization effects of electropositive elements surrounding such anions will induce a slight charge removal from the valence (n+1)p

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orbitals for the main group anions, but a slight charge transfer to the valence (n+1)p orbitals for the late transition metal anions. 6. Concluding Remarks Our analysis of the electronic structures of Yb3Ag, Ca5Au4 and Ca3Hg2 reveals that their transition metal atoms exist as anions with electron configuration (5d)10(6s)2(6p)1. The Ag2, Au2 and Hg2 dimers are present as the [Ag-Ag]4-, [Au-Au]4-, and [Hg-Hg]2- Zintl-anions, respectively, and the Ag-Ag, Au-Au and Hg-Hg bonds are single bonds formed primarily by the pσ-pσ interaction between the 6p orbitals of M () Ag, Au, Hg). The interdimer interactions are negligible, and the bonding interactions between the Ca 4s and M 6p orbitals (M ) Au, Hg) in Ca5Au4 and Ca3Hg2 and those between the Yb 6s/5d and Ag 5p orbitals in Yb3Ag2 make the M2 dimers form the three-dimensional networks of Ca5Au4, Ca3Hg2, and Yb3Ag2. The valence atomic orbitals have the sequence 6s e 5d < 6p for Pt3- and Au2-, and the sequence 5d < 6s < 6p for Hg-. This change in the valence orbital sequence should have consequences on the chemistry of the late transition metals when going from Pt to Au to Hg. The anions of late main group elements form covalent bonds when their p-shells are partially empty. In contrast, the anions of late transition metal elements do when their p-shells are partially filled. The oxidation states of the Ag, Au, and Hg atoms in Yb3Ag2, Ca5Au4, and Ca3Hg2 are best described as -2, -2, and -1, respectively. This assignment provides the prediction that the frontier orbitals of Yb3Ag2, Ca5Au4, and Ca3Hg2 are given primarily by the Ag 5p, Au 6p and Hg 6p orbitals, respectively. Extension of our work to other intermetallic compounds with late transition metal anions is in progress. Acknowledgment. The authors thank O. Jepsen for help with the LMTO program. M.-H.W. thanks the financial support from the Office of Basic Energy Sciences, Division of Materials Sciences, U.S. Department of Energy, under Grant DE-FG0286ER45259. Supporting Information Available: Table S1, and Figures S1-S3 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. CM703590D