The crystal structure of a stable form of benzamide has been determined by means of electrondensity projections down the two short crystallographic axes. The crystals are monoclinic, space group P2~/c with a ----5.59, b = 5.01, c ----21.93 A, fl = 90 ° 45'. Measurements of bond lengths and angles, with standard deviations of 0.017 A and 1-0 ° respectively, have enabled estimates to be made of bond orders in the amide group in which C-N ---1.31, C-O = 1.24 A. The molecules are non-planar with an angle of 26 ° between the amide group and benzene ring. The non-planarity is shown to be due to steric hindrance and is discussed in relation to similar molecules of known configuration. Hydrogen bonds connect pairs of molecules into centrosymmetrical dimers which are themselves linked by further hydrogen bonds into endless chains.

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The crystal structure of [Pd(dppm) Br], (dppm = Ph,PCH,PPh2) shows that the palladium(1) atoms are directly metal-metal bonded and adopt an idealised square-planar stereochemistry.THE preparation of the complexes [Pd(dam)X], (dam = Ph,AsCH,AsPh,; X = C1, Br) and their establishment as compounds of palladium(1) was recently rep0rted.l By the action of dppm (Ph,PCH,PPh,) and the halogenocarbonyls Pd(C0)X we have now prepared similar complexes of the type [Pd(dppm)X], and the crystal structure of one o f these, [Pd(dppm)Br],, is reported here.Crystal data : C,,HMBr2P,Pd,, M = 1141, monoclinic, space group P2,/c; a = 13-661(4), b = 16.726(4), c = 21.442(6) A, = 106-00(4)" (h = 0.7107 A for Mo-K,); U = 4710 A3, 2 = 4, D, = 1-61 g ~m -~, p = 27-18 cm-l for Mo-K,.Crystals were obtained from a dichloromethane-hexane solution and the structure was solved by conventional heavy atom methods. Least-squares refinement with

The electron density in crystals of a-pyridone has been measured with sufficient accuracy in two projections to establish the existence of the molecule in the pyridone form which differs from its tautomer, the hydroxy form, only in the position of one hydrogen atom. The molecular electron density is satisfactorily explained within the experimental error by an assemblage of isolated atoms, each with its own temperature factor, and possessing the electron distribution calculated by McWeeny using analytical approximations to atomic wave functions. Bond lengths have been measured with a standard deviation of 0.010/~ and can be explained in terms of a resonance struc= ture in which polar forms of the molecule play an important part. The hydrogen atom responsible for tautomerism is concerned in an intermolecular N-O hydrogen bond, which, repeated throughout the structure, links molecules into endless helices.

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