The cis-isomer of di-cL-carbonyl-dicarbonyldi-rr-cyclopentadienyldi-iron has been isolated by low-temperature crystallisation and its structure determined by a single-crystal X-ray analysis. The crystals are monoclinic, a = 8.880(2), b = 12.301 (3). c = 13.1 40(3) 8, p = 108.64(1)". The space group is P2Jc with Z = 4. After leastsquares refinement with over 1500 observations R is 0.051. The i.r. spectrum of the complex has been measured in the ranges 200-1 000 and 1600-2000 cm.-l and assignments have been suggested. Study of the l H n.m.r. spectra of the cis-and trans-isomers as a function of temperature confirms that solutions of the complex contain both isomers. Mossbauer and mass-spectral data are also presented.WE describe here the preparation, spectroscopic properties, and characterisation by X-ray analysis, of the cisisomer of the dimer (I). The previous paper in this Series described an accurate redetermination of the crystal structure of the trans-isomer of the same compound (11)' originally studied by M i l k 2 The behaviour 1

Figure 4.-The coordination polyhedron about the Pr(II1) ion showing the monocapped square-antiprismatic configuration.is bent down toward plane B as can be seen in Figure 3. Since this complex contains constraints of chelating as well as three different kinds of atoms in the coordination sphere, it does not lend itself to detailed analysis of the coordination polyhedron. Day and Hoard2; have noted that the quasi-Cd axis [N(1)-Pr] corresponding to the Cdv symmetry of the idealized monocapped square antiprism must generate four axes [Pr-N(l), Pr-N(2), Pr-N(2)', and Pr-Cl(1)] normal to which are observed one-five-three layering of the ligated atoms. Examination of the model does show such layering.A reasonable pattern of three-dimensional hydrogen bonding (Table 111) can be proposed as O(1) + Cl(1)"' and O(5) O ( 2 ) + Cl(2) and O(4) O ( 3 ) + Cl(2) and C1(l)vr O(4) + C1(2)11 and C1(2)Ir1 O(5) + C1(2)Iv and C1(2)v This scheme thus links each complex ion to all six of its neighboring ions via hydrogen bonding. This threedimensional hydrogen bonding is manifested in the excellent crystals corresponding to the octahydrated complex. The crystal and molecular structure of tris( glycinato)chrornium( 111) monohydrate, Cr( C S H~N O~)~ HzO, has been determined by single-crystal X-ray analysis. The cell constants are a = 6.256 ( l ) , b = 14.649 (l), c = 12.267 (1) A, and fl = 100.39 (1)'.The space group is P&/c and with 2 = 4 the calculated density is 1.755 g/cin3 compared to the observed 1.76 (1) g/c1n3. Scintillation counter diffractonietry was used to measure the intensities of 2631 independent reflections significantly above background. The phase problem was solved by the application of direct methods and the structural parameters refined by a block-diagonal least-squares procedure to a final R of 0.0266. All hydrogen atoms in the structure were located and their positional parameters were refined. Anisotropic thermal parameters were used for all atoms except hydrogen. The chromium ion is octahedrally coordinated by three glycinato ligands so that the three nitrogen atoms are mutually cis.

The crystal and molecular structure of dichloro(dipheny1)tin has been determined by single-crystal X-ray diffraction methods using the intensities of about 3800 reflections measured by counter diffractometry. The crystals belong to the triclinic system, a = 15.905(2), b = 9.367(1), c = 9.034(1) A, 0: = 76.89(1), p = 93.21 (1). y = 94.78(1)", space group P i , and Z = 4. The structure was solved by Patterson and Fourier methods and refined by blockdiagonal least-squares techniques, using individual anisotropic thermal parameters, to R 0.040. All hydrogen atom positions were located but were not refined.There are two discrete molecules of Ph,SnCI, in the asymmetric unit with identical geometry. The mean bond distances are: Sn-CI 2.346(2), Sn-C 2-1 14(3), and C-C 1.387(13) A. Mean values for the valence angles at the tin atom are : CI-Sn-CI 100, CI-Sn-C 107, and C-Sn-C, 125.5".There is no molecular association in the crystal, the shortest Sn . . . CI contact being 3.77 A. The interpretation of Mossbauer spectroscopic results in terms of a polymeric network in the crystal structure involving six-co-ordinate tin is thus shown to be incorrect.WE are studying the nature of the Sn-Fe bond in certain organomet allic complexes, and have carried out X-ray diffraction studies on the series of compounds (x-C,H5)Fe(CO),SnPh,C13-.n (n = 0-3) and the analogous tribromo-compound.5 Since there is no apparent restriction on the mutual orientation of the two parts of the molecule about the metal-metal bond we concluded that d,,+, contributions to that linkage were unimportant and that the bond was predominantly of o-character. However, it is less obvious that there should be any necessary steric restriction in the case of d,,-d, bonding between the metal atoms, though a definite stereochemical arrangement is certainly required for 6-bonding.6 In seeking to test for the possible presence of d,,--d, contributions to the metal-metal bond we noted that replacement of a phenyl group by chlorine in the series of compounds examined gave rise to a regular change in the valence angles at the tin atom. Similar changes are observed in the molecular geometry of compounds of the type [(x-C,H,)Fe(CO),],-SnRZ77* and we have observed that replacement of a phenyl group in the first series by an iron moiety gives a compound of the second type with very similar geometry at the tin atom. Since carbon has no available d-orbitals this apparent equivalence of the two groups seems good evidence that it is not necessary to invoke d,,-d, bonding in order to account for the observed

Szlmmary The existence of the cis-isomer of [w-C,H,Fe (C0),l2 in the solid state has been established by i.r. and X-ray methods.THE i.r. spectrum of bis-(w-cyclopentadienyldicarbonyliron) 1

The c rysta I st ru ct u re of the trans-isomer of d ip-car b o n y Id i c a r b o n y Id i -7 ~-c y cl o pe n t a d i e n y I d iiron has been redetermined by three-dimensional methods. The space group is P2Jc with a = 7.046(1), b = 12.453(2), c = 7.990(1) A, p = 108.44(1)", and Z = 2. The 904 reflections measured by counter diffractometry were used in the refinement by block-diagonal least-squares methods. With individual anisotropic thermal parameters and no allowance for hydrogen atom contributions, R = 0.044, WR = 0.056. Bond lengths and angles are : Fe-Fe 2.534(2) ; Fe-C(bridge) 1.91 0, 1.91 8(5) ; Fe-C(termina1) 1.748(6) ; Fe-C(ring) 2.082-2.1 21 (8) ; C-O(bridge) 1 .I 88(6) ; C-O(termina1) 1 .I 57(7) ; and C-C 1~346-1~405(11) A; and Fe-C(bridge)-Fe 82-9(2) ; C(bridge)-Fe-C(bridge) 97.1 (4) ; C(bridge)-Fe-C(termina1) 93.7(2) ; Fe-C-(bridge)-0 138.4-1 39.1 (3) ; and Fe-C(termina1)-0, 178-4(8)".The identity of the Fe-Fe distance in both cisand trans-isomers makes it unlikely that the former is stabilised preferentially by a 7 ~-or S-contribution to the metal-metal bond as has been suggested.