Polyhedra are prevalent in crystallography; we seek to rationalise structures in terms of tetrahedra, octahedra and other regular shapes. Indeed, many structure-property relationships can be described in terms of interactions between rigid coordination spheres, such as perovskite phase transitions and negative thermal expansion. Often, however, polyhedra are not rigid instead displaying some degree of distortion; analysing this can be difficult, particularly if the polyhedron shows deviations both in terms of both bond lengths and angles. Often, these distortions can give rise to complex electronic effects, resulting in useful magnetic materials. While local symmetry mode analysis can give a comprehensive structural picture in many cases, it is often difficult to interpret if multiple distortion modes act cooperatively. Alternatively, continuous shape measures [1] can be used to quantify deviations from a reference polyhedron, but this reference is not always trivial to define. Here, we present a much simpler measure of polyhedron distortion, based on the fitting of ellipsoids to coordination environments. We have applied this new general method to a range of metal oxide materials, revealing new insights into their properties; from a switching bistability in symmetry-disallowed perovskite phase transitions to a weak off-centre 'd5-effect' for Fe3+ ions, that could potentially be exploited in multiferroic materials.[2] We have implemented the method in the python package PIEFACE. While the method has currently been applied to metal oxides, it could easily be generalised to other materials that obey a polyhedral description, including coordination complexes, framework materials and organic molecules.

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niversita di Rlilano. $ Morgan and Hurstall obtained a brown compound, supposedly a foini of iPt(bipy),][Pt<:l,] (required: C, 28.4; H, 1.9; N, 6.6; C1, 1 6 . 8 " ~~) by boiling a:i aqueous solution of Ii,[PtCl,] with 2,2'-bipyridyl. .llthough an attempt to repeat their preparation gave 3 brown product (Found: C, 25.2; H, 2.2; N , 5.9; C1, 1 7 . 0 " 6 ) , this had an i.r. absorption a t 3500 cm-l and \vas, froin its X-ray powder photograph, a mixture of yellow jI't(L)ipy)s][PtCI,j and yellow [Pt(bipy)CI,] (A) ; other lines were unidentified. However, we Once obtained a nicely crystalline I)rnwn form o f [I't(bipv),][CIC),1,, discussed later as (H). compounds ; thus, the so-called ' y-[Pt (NH,),Cl,] ,' despite 3 H.

Solvent effects on the electronic spectra of the complexes [Pt(bipy)X,] (bipy = 2,2'-bipyridyl; X = CI, Br, or I),[Pd(bipy)CI,], and [Pt(bipy)(py),I2+ (py = pyridine) are reported and correlated with the ,ET parameters of the solvents. The assignments of the electronic spectra are discussed.

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The final product from controlled-potential reduction of aqueous solutions of trans-[Rh(en),CI,]+ (where en = ethylenediamine), under an argon atmosphere, has been shown to be trans-[Rh(en),H (OH)] +. The pseudo-acidbase equilibrium (i) is shGwn to operate in solutions which are strongly alkaline (pH > 12). In the absence of bury, Kent RhITLH + OH-RhI + H, C> (0 oxygen, aqueous solutions of trans-[Rh(en),H(OH)]+ are stable toward both acid and base (0 < pH < 12) a t room temperature : in the presence of oxygen, no reaction is observed in acidic sclutions whereas rapid reactions are observed in basic solutions, giving initially the hydroperoxorliodium(rr1) complex [Rh(en),(OH) (OOH)] +, which on concentration yields the blue superoxo-bridged rhodium(ll1) dimer [(H,O) (en),Rh0,Rh(en),(H,0)]5+.Mechanisms of these reactions are discussed.THE rates of halide exchange in tmm-[Rh(py),X,]+ reduction of tvni?~-[Rh(py),~X,]~ has been shown to (X = C1 or Br) have been found to be markedly in-give unstable RhIII-H species2 which in this case hibited by traces of 0xygen.l It thus seems probable Attempts t o that these halogen exchanges involve catalysis by some reduced species (containing either RhIII-H or RhI) Vaughan, Chem* which also reacts rapidly with oxygen. Chemical 2 I<. D. Gillard atid G. Willtinson,