Late transition metal anions acting as p-metal elements
Graphical abstract
Introduction
Physical and chemical properties of both discrete molecules and extended solids depend critically on their frontier energy levels, i.e., the HOMO and LUMO of discrete systems, the valence and conduction bands of normal semiconductors, and the partially filled bands of metals and magnetic semiconductors. In qualitatively predicting the nature of these frontier energy levels, chemists commonly employ electron counting rules such as ionic and covalent electron counting rules in which the concept of oxidation state plays an important role. However, such a simple picture of bonding breaks down for compounds of late transition metal elements with electropositive ligands for which the transition metal d level lies in between the s- and p-levels of the ligands (e.g., PtSi3P2 and NiSi2P3) [1], [2]. For such compounds, a blind use of the conventional (i.e., ionic and covalent) electron counting rule incorrectly predicts that empty d-block levels of the transition metal atoms lie below the highest-lying occupied energy levels of the main group ligand atoms. To correct this deficiency of the conventional electron counting rules, a modified electron counting rule has been proposed some years ago [1].
For compounds with transition metal anions, e.g., the auride CsAu [3], [4], [4](a), [4](b), [4](c), [4](d), [4](e) and the platinide Cs2Pt [5], the 6s and 5d orbitals of the transition metal atom act as a reservoir for holding 12 electrons resulting in the 6s25d10 configuration for Au− and Pt2−. A similar situation is found for the recently discovered insulating fluorides and semiconducting oxides containing 18-valence-electron octahedral cations [PtIn6]10+ and [IrIn6]9+ [6], [7]. To a first approximation, the Pt atom in [PtIn6]10+ and the Ir atom in [IrIn6]9+ have also the 6s25d10 configuration with the oxidation states −2 and −3, respectively. Our search for further examples of transition metal anions led us to the compounds RE2M2In (RE = rare earth element, M = Ge, Cu, Au and Pt), Ca5Au4 (M = Au) and Ca3Hg2 (M = Hg) containing M–M dimers as well as the 18-electron Half-Heusler (18eHH) compounds AML (A = electropositive element, M = late transition metal, L = late main group element, e.g., ScAuSn), in which the elements M and L form a zinc blende lattice. Our electronic structure studies of these compounds on the basis of first principles electronic band structure calculations have shown that their transition metals exist as anions with the valence electron configuration (n + 1)s2nd10(n + 1)p1 or (n + 1)s2nd10(n + 1)p2, and the frontier orbitals of these compounds are described by the (n + 1)p orbitals of the transition metal rather than by the nd orbitals. In the following, we briefly summarize the essential qualitative picture that emerges from these electronic structure studies.
Section snippets
Transition metal anions with valence electron configuration (n + 1)s2nd10
Octahedral metallo-complexes MIn6 of the transition metal M (=Fe, Ni, Ru, Os, Ir and Pt) with electropositive element In are present in a series of metal-rich fluorides and oxides, e.g., PtIn7F13 [6], Pt2In14Ga3O8F15 [8], PtIn6(GaO4)2 [9], PtIn6(GeO4)2O [10] and IrIn7GeO8 [7]. These units have very short M–In distances of 253–257 pm and are embedded in a cage of up to 30 F and O atoms with In–O and In–F distances slightly shorter than those found in In2O3 and InF3, respectively. The [PtIn6]O30
Transition metal anions with valence electron configuration (n + 1)s2nd10(n + 1)p1
In intermetallic compounds of the type RE2M2In (RE = rare earth element, M = Pt, Cu and Au) [16], [16](a), [16](b), [16](c), [16](d), Ca5Au4 [17], Ca3Hg2 [18] and Ca5M3 (M = Au, Zn and Hg) [19], [19](a), [19](b), [19](c), there are M–M dimers with short distances of 255–300 pm. In these compounds, each M–M dimer is contained in a tetra-capped trigonal biprism made up of electropositive elements (i.e., RE8In4 in RE2M2In and Ca12 in Ca5Au4, Ca3Hg2 and Ca5M3) such that each metal M is located at the
Transition metal anions with valence electron configuration (n + 1)s2nd10(n + 1)p2
In the classical Zintl compound NaTl [22], [22](a), [22](b), [22](c), the Tl− anions with (ns)2(np)2 configuration form a diamond-like lattice and interact covalently as the isoelectronic elements of group 14. From the fact that numerous Zintl compounds are insulators, one might expect that NaTl has a band gap as well, but NaTl is a regular metal unlike diamond. In the 18-electron half-Heusler compounds CaAuL (L = Sb and Bi) and REAuL (RE = Sc and Y; L = Sn and Pb) the elements Au and L form a zinc
Transition metal vs. main group anions
For a large number of intermetallic compounds containing late 3d–5d transition metal atoms and electropositive elements, the nature of their frontier orbitals and hence their electronic properties can be easily understood once it is recognized that the transition metal atoms can be regarded as anions with bonding characteristics similar to those of the late main group elements. Fig. 10 compares the electron configurations for the probable anions of late main group and late transition metal
Concluding remarks
The concept of oxidation state and electron counting is a powerful tool with which the nature of frontier energy levels is predicted. In compounds of transition metal elements with electronegative ligands, the transition metal atoms are present most likely as cations, so that the frontier levels of these compounds are represented by the d-block energy levels whose major components are the nd orbitals of the transition metal atoms. In compounds with electropositive elements, however, transition
Acknowledgments
The work at NCSU was supported by the Office of Basic Energy Sciences, Division of Materials Sciences, U.S. Department of Energy, under Grant DE-FG02-86ER45259.
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