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The conformational properties of the diheteroaryl ketones of the furan and thiophene series have been studied both experimentally, in the solid state and in solution, and with a theoretical ab-initio approach. X-Ray analysis of the samples which are solid at room temperature was possible only for the compounds which gave suitable crystals, i,e. di(2-thienyl) ketone (2), di(3-thienyl) ketone (5), 2-fury1 3-fury1 ketone ( 7 ) , 2-thienyl 3-thienyl ketone ( 8 ) and 3-fury1 2-thienyl ketone (10). The crystal data are: (2j space group F dd2, a = 13.267(2), b = 21.853(4), c = 6.067(1) 8, Z = 8, R = 0.0637 for 388 observed reflections; ( 5 ) space group P2,/n, a = 6.1 74(1), b = 11.184(3), c = 13.203(2) 8, Z = 4, R = 0.0776 for 1 202 observed reflections; ( 7 ) space group P2,/c, a = 9.900(4), b = 10.934(6), c = 7.485(2) 8, Z = 4, R = 0.0461 for 984 observed reflections; ( 8 ) space group P2,/c, a = 6.1 83(1 ), b = 11 .I 09(1 ), c = 13.379(2) 8, Z = 4, R = 0.0732 for 1 186 observed reflections; (10) space group P i , a = 13.799(2), b = 10.039( 2), c = 6.084(1) 8, 2 = 4, R = 0.0639 for 1 973 observed reflections. The crystal structures of compounds (5) and ( 8 ) are disordered: for the former the disorder concerns the position of one ring, whereas both rings are involved in the latter. For compound (1 0 ) the structure of the two crystallographically independent molecules is the same. From the n.m.r. LIS (lanthanide-induced shifts) measurements (on 'H and I3C) indicative information is obtained on the type of conformer and its relative population present in the solution equilibrium mixture. M O ab-initio calculations, performed by employing the 3-21 G or 3-21 G" (with d-orbitals on the sulphur atom) basis sets for the ground-state conformers of all the molecules considered, show that the solid-state structures are very similar to those calculated for the conformers with the lowestenergy content and the calculated conformer populations are in qualitative agreement with measurements in solution. As a general trend, the solid-state structures, the preferred conformations found in solution and those calculated for the free molecules show that the preferred orientation of one heterocyclic ring with respect to the carbonyl group is of the type found in the corresponding formyl and acetyl derivatives and is maintained in all the diheteroaryl ketones. For the molecules having conformers of similar energy content and different dipole moment, i.e. di(2-furyl) ketone (1 ), it was demonstrated, on the basis of the classical theory of the solvent effect, that the equilibrium mixture of conformers in the vapour phase is different from that in solution and changes in solvents of different polarity. * For part 10 see ref. 25. Supplementary data: (see Instructions for Authors, J. Chem. SOC., Perkin Trans. 2, 1989, issue 1, pp. xvii-xix). Lists of H-atom co-ordinates, bond lengths and angles, and thermal paramters have been deposited at the Cambridge Crystallographic Data Centre.

The title compound has been synthesized and its X-ray structure determined. The structure was solved from diffractometer data by Patterson and Fourier methods and refined by least-squares techniques to R 4.7% for 1 947 independent reflections. Crystals are triclinic, space group PT, with unit-cell dimensions: a = 8.45(1), b = 8.26(1), c = 9.19(1) 8, a = 84.8(1), p = 105.2(1). y = 106.7(1)", Z = 2. Co-ordination araund palladium is square planar and involves two carbene carbons [Pd-C(1) 1,953(8), Pd-C(4) 1.972(10) 8 1 and two chlorine atoms [Pd-CI(1) 2.359(4), Pd-CI(2) 2.390(4) 81. C-N Distances in the carbene ligands are 1. 32(1) and 1.29(1) 8, and C-0 distances I .32(1) and I .34(1) 8. STUDIES on carbene complexes of transition metals have been critically evaluated in some recent reviews,1,2 and structures interpreted in terms of chemical bonds. However, neither in these reviews nor in subsequent reports 3-7 have non-chelate cis-biscarbene complexes been considered from the structural point of view. We have therefore determined the crystal structure of such a compound and now report the structure of the title compound. EXPERIMENTALPreparation of the Complex -A methanolic solution of K,PdCl, was allowed to react with a small excess of methyl isocyanide obtained by the method of ref. 8. A white product, identified (analysis, i.r.) as [Cl,(CH,NC),Pd], was precipitated, but shortly redissolved.The solution was allowed to evaporate very slowly and, after several weeks, pale yellow crystals were obtained. Analysis indicated the formula C,Hl,C12N20,Pd (C, 2 1.45 ; H, 4.2; N, 9.5. C,H1,Cl2N20,Pd requires C, 22.28; H, 4.36; N, 8.86%). Prismatic pale yellow crystals suitable for X-ray diffraction analysis were obtained by recrystallization from ethanol.Crystal Data.-C,H,,Cl,N,O,Pd, M = 323.5, a = 8.45(1), b = 8.26(1), c = 9.19(1) A, 106.7(1)O, = 84.8(1), p = 105.2(1), y = U = 592.8 A3, D, = 1.81, 2 = 2, D, = 1.78 g c ~n -~, F(000) = 320. Cu-K, radiation, 5, = 1.5418; p(Cu-K,) = 169.8 crn-'. Crystallographic Measurements.-Rotation and Weissenberg photographs, obtained by rotation around [OOl], were used to determine approximate unit-cell dimensions. These values and intensity data were obtained by use of an on-line Siemens AED single-crystal diffractometer, with Ni-filtered Cu radiation and the w-28 scan technique. Of 2 146 independent reflections, 1 947, having I > 2c(I), were considered observed and used in the refinement. A standard reflection was monitored every 20 reflections to t See Notice t o Authors No. 7 in J.C.S. Dalton, 1973, Index issue (items less than 10 pp. are supplied as full-size copies).

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