The electric field gradient (EFG) at the chlorine site is calculated for cubic compounds of the K2PtCl6-type (space group Fm3m), M2IMIVCl6, where M1 is an alkali metal and MeIV a tetravalent element. In the calculations the total EFG is subdivided into the contribution of the complex [MIVCl6]2-, EFGcomplex, and the contribution of the ions outside the complex, EFGlattice. EFGcomplex is calculated by the local electron density formalism using the MS-Xα-method, and EFGlattice is determined by the point charge model. It is found that EFGcomplex is positive whereas EFGlattice is negative. Including antishielding effects, the magnitude of EFGlattice is about one fourth of EFGcomplex. The trends in the EFG for the various compounds found theoretically are the same as the trends in the experimental nuclear quadrupole coupling constants e2Q q/h. However, the absolute values of EFGtheo are smaller than the values EFGexp deduced from e2Q q/h. For a fixed central atom MIV the (positive) EFGexp is increasing with increasing radii of the cations (and increasing lattice constant). This increase can be understood by an increase of EFGlattice. On the other hand, for fixed cations and different tetravalent metal atoms, the EFG is increasing with increasing electronegativity of the central atom. This can be understood by an increase of EFGcomplex. For distorted octahedra it is found that the change in the EFG due to the distortion is also due to a change in EFGcomplex.

The electric field gradient (EFG) at the chlorine site of the hexachlorometallates A2[MCl6] and A'[MCl6], M = Sn, Te, Pb, Pt, and A⊕, A'2⊕ = alkali metal ion, hexahydrated bivalent metal ion, or organic ammonium cation, is investigated and the dependence of the EFG on the size of the cation is discussed. — For the cubic alkali hexachlorometallates the observed trend in the EFG with increasing size of the cation is corroborated by quantum mechanical calculations using the Multiple‐Scattering‐Xα‐method (MS‐Xα) and the point charge model. A correlation v(35Cl) ∼ 1/r3 is found, where r is the distance between the ions in the lattice and the chlorine nucleus considered. For hexachlorometallates with organic cations, like pyridine, piperazine, 4‐picoline, ethylenediamine, 1.3‐propylenediamine, and other aliphatic or aromatic amines, this 1/r3 dependence is found for the averaged 35Cl NQR frequency, too. This is due to the distortion of the [MCl6]2⊕ octahedra, caused by hydrogen bonds N—H…Cl, and to the decrease of the lattice symmetry because of non‐sphericity of the cations. — A hydrogen bond has a large influence on the properties of one of the chlorine atoms of the hexachlorometallate complex. The M‐Cl distance is increased and therefore the EFG at the nucleus site is decreased. However, the average of the M‐Cl bond lengths or the 35Cl NQR frequencies, taken over all six chlorine atoms of a [MCl6]2⊕ complex, is not influenced by the chemical nature of the cation but only by its size or the size of the unit cell, respectively. The increase of 35Cl NQR frequency with increasing size of the cation originates from a decrease of the lattice energy.

A phase transition has been observed in bis(pyridinium) hexachlorometallates (C

Complex Compounds J Crystal Structure 1 Isotope Effects 1 Phase Transitions 1 Spectroscopy, Nuclear Quadrupole ResonanceThe crystal structure and chemical bond of ethylenediarnmonium hexachlorometallates, [H3N(CH2)2NH3] [xc16] 2Q, X = Sn, Pb, Te, Pt, was studied by X-ray diffraction and 35C1 NQR. [H3N(CH2)2NH3]2e [SnC16]2Q crystallizes with the space group Cf-Pi, Z = 1. The lattice constants are a = 704.4 pm, b = 711.2 pm, c = 743.7 pm, CI = 67.537-", p = 65.341", y = 60.407". The crystal structure was determined. It is described by a deformed CsC1-type lattice and the hydrogen bonds N -H . . . C1 cause a weak layer type solid. The compounds with X = Sn, Pb, Te, Pt are isotypic and the lattice constants are given. A three line 35Cl NQR spectrum is observed for all compounds in the temperature range 77 5 T/K 5 decomposition temperature, in agreement with the crystal structure. The hexachloroplatinate experiences a phase transition at T = 357.2 K, the high temperature phase I being of higher symmetry than P i . This shows up in the two line 35Cl NQR spectrum in phase I. Isotope exchange H + D at the ammonium groups influences T, and v(~'CI) slightly, an observation, which favours the assumption of rather weak hydrogen bonds N -H ... C1.

35Cl NQR spectra of eleven hexachlorometallates (IV) A1 [MCl6] and A'[MCl6], M = Sn, Te, Pb, and A+ =ethylammonium, 4-picolinium, anilinium, triethylammonium, chinoxalinium, and piperi-dinium ions, and A'2+ = 1.3-propylenediammonium ion, have been observed as a function of tem-perature. The ethylammonium hexachlorometallates(IV) (C2H5 NH3)2 [MCl6], M = Sn, Te, show a phase transition at 128.8 K and 204 K, respectively. Both compounds yield a single resonance line in their high temperature phases. In case of the stannate this single 35Cl resonance line splits up into two lines at Tc = 128.8 K, whereas for the tellurate no 35Cl NQR signals could be found in the low temperature phase. A phase transition was also found for the 1.3-propylenediammonium hexa-chlorostannate(IV) at 287 K where the six line NQR spectrum of the low temperature phase changes into a four line spectrum. In contrast, the corresponding plumbate shows no transition. All other compounds studied contain distorted [MCl6]2- octahedra, and therefore they yield more than one 35Cl resonance line. The complexes have been investigated in the temperature range temperature where the lines fade out. In case of the hexachlorostannates(IV) with the ethylammonium, the 1.3-propylenediammonium and the triethylammonium ions, the crystal structures of the compounds are known and compared with the results of the 35Cl NQR spectroscopy.