Preparation, Properties, and Structure of the Polyphosphides VNi₄P₁₆, NbNi₄P₁₆, and WNi₄P₁₆

Abstract

The title compounds were prepared by reacting powders of the binary transition metal alloys with phosphorus in a tin melt. They crystallize with a monoclinic structure of space group C 2/c, which was determined from single-crystal X-ray data; VNi4P16: a = 1956.0(3), b = 565.9(1), c = 1106.6(2) pm, ß = 100.99(1)°, R -0.020 for 1904 structure factors and 97 variable parameters; NbNi4P16: a = 1973.0(3), b = 571.2(3), c = 1117.2(2) pm, ß = 100.74(1)°, R = 0.023 for 2431 F-values and 97 variables; WNi4P16: a = 1964.2(3), b = 568.3(2), c = 1109.3(2) pm,/? = 101.04(2)°, R = 0.031 for 371 F-values and 43 variables. The T atoms (T = V, Nb, W) have eight P neighbors, which form a slightly distorted square antiprism, and the Ni atoms have appro ximately octahedral P coordination. The P atoms form novel P8 units, which consist of 6 -membered rings with a P2 side chain. Chemical bonding is discussed on the basis of classical twoelectron bonds. A comparison of the interatomic distances suggests that the Fermi level cuts through a band formed by antibonding nickel-phosphorus states. A band structure is propos ed, which also rationalizes the metallic conductivity and the Pauli paramagnetism of the com pounds.The transition metals (T) form numerous poly phosphides with the composition TP4 [1]. Ternary compounds with a T : P ratio of 1:4 are also known, e.g. M oFe2P 12 and W Fe2P 12 [2], TiMn2P 12 [3], NbMn2P12, MoMn2P12, and WMn2P12 [4], VFe2P 12 [5], NbFe2P12 [6 ,7], MoNiP8 and WNiP8 [8 ]. In these polyphosphides the early transition metals have mostly square-antiprismatic or cubic (MoNiP8 and WNiP8) phosphorus coordination, while the late transition metals are surrounded octahedrally by phosphorus atoms. In VP4 • 4 NiP3 = VNi4P 16 and the isotypic compounds NbNi4P 16 and WNi4P16 report ed here the T : P ratio is 1:3.2 and thus these com pounds form an entirely new structure type. How ever, the near-neighbor coordinations of the tran sition metals are very similar to those of the struc tures with the composition T T '2P12. Again the chemical bonding in the TNi4P 16 polyphosphides can be rationalized on the basis of classical twoelectron bonds. A preliminary account of the struc ture of VNi4P 16 has been given earlier [5].* Reprint requests to W. Jeitschko. PreparationThe title compounds can be prepared in wellcrystallized form by reacting a T-nickel alloy (T = V, Nb, W) with red phosphorus in a tin melt. This preparation m ethod has been used with good results in the past for the preparation of Ti2NiP5 [9], Mo(W )NiP8 [8 ] and VFe2P 12 [5]. Starting materials were powders of vanadium, niobium, tungsten and nickel (all 99,9%), red phosphorus (Hoechst, "ultrapur"), and tin (Merck, "rein"). The T-nickel alloys were prepared by arc melting of cold-pressed pellets with the atomic ratio T : Ni = 1:1 or 1:2. The alloys were then ground to fine powders and react ed with red phosphorus and tin in the ratio (T /N i): P : Sn = 1:25:20 in evacuated sealed silica tubes (tube volume 5 cm3, P content 0.5 g). The tubes were annealed at 450 °C f...

The Metal Flux: A Preparative Tool for the Exploration of Intermetallic Compounds

“…And at 650 °C these react too slowly to produce the intended ternary phosphide. Similarly, powders of the binary alloys were used for the preparation of the ternary polyphosphides VNi 4 P 16 , NbNi 4 P 16 , and WNi 4 P 16 140. Three stacking variants of the binary manganese polyphosphide MnP 4 are known: 2‐MnP 4 ,56 6‐MnP 4 ,50 and 8‐MnP 4 49.…”

Metallische Schmelzen – Reaktionsmedien zur Präparation intermetallischer Verbindungen

“…Offensichtlich bilden sich beim Einsatz der elementaren Metalle (Ti und Ni) anstelle der Legierung TiNi zuerst die binären Phosphide der Übergangsmetalle, die dann bei 650 °C zu langsam reagieren, um die erwünschten ternären Phosphide zu erhalten. Ganz analog wurde zur Herstellung der ternären Polyphosphide VNi 4 P 16 , NbNi 4 P 16 und WNi 4 P 16 140 von Pulvern der Übergangsmetall‐Legierungen ausgegangen. Von dem binären Manganpolyphosphid MnP 4 gibt es drei Stapelvarianten: 2‐MnP 4 ,56 6‐MnP 4 50 und 8‐MnP 4 49.…”