DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, Pages 2123–2129 2123 Spectroscopic and structural studies on 1 : 2 adducts of silver(I) salts with tricyclohexylarsine Graham A. Bowmaker,a EVendy,b,c Brian W. Skelton b and Allan H. White b a Department of Chemistry, University of Auckland, Private Bag 92019, Auckland, New Zealand b Department of Chemistry, The University of Western Australia, Nedlands, W.A. 6907, Australia c Jurusan Pendidikan Kimia, FPMIPA IKIP, Malang, Jalan Surabaya 6, Malang, 65145, Indonesia A series of mononuclear complexes [Ag{As(C6H11)3}2X] (X = Cl, Br, I, CN, NCO, OClO3, O2NO or O2CCF3) have been synthesized and characterised by room temperature single crystal X-ray determinations and lowfrequency vibrational spectroscopy.For X = Cl, Br, I or OClO3 the crystal structures are isomorphous with a number of their previously recorded P(C6H11)3 counterparts, crystallising in the familiar monoclinic C2/c array with the Ag]X bond lying on a crystallographic 2 axis (the perchlorate is disordered) which also relates the pair of As(C6H11)3 ligands.The silver environment in all is planar three-co-ordinate XAgAs2. For X = CN, NCO or O2NO the derivative lower-symmetry triclinic P1� array, also common among the P(C6H11)3 analogues is found. The tri- fluoroacetate, resembling the nitrate in that the anion behaves as a small ‘bite’ bidentate ligand, [Ag{As(C6H11)3}2- (O2CCF3)], is monoclinic space group P21/n.The thiocyanate by contrast, utilising the ambidentate capacity of the SCN ligand, remarkably, is a linear polymer, ? ? ? {As(C6H11)3}2Ag(SCN)Ag(SCN) ? ? ?, triclinic, space group P1, in which the silver atom environment is four-co-ordinate, NSAgAs2. Far-IR spectra of the halide and pseudohalide complexes in the series exhibit single bands due to n(AgX) vibrational modes at 230, 165, 139, 262 and 311 cm21 for X = Cl, Br, I, NCO and CN respectively. These are all higher than the values for the corresponding P(C6H11)3 complexes, suggesting that the Ag]X bonding is stronger in the As(C6H11)3 complexes, this being the result of weakened Ag]L bonding in going from L = P(C6H11)3 to As(C6H11)3.In recent years single crystal X-ray determinations in concert with low frequency vibrational spectroscopic studies have defined isomeric forms and their bonding characteristics for a wide spectrum of adducts of coinage metal(I) salts, MX, with unidentate Group V bases. Whereas for donor atoms E = N a variety of stereochemical and electronic characteristics may be employed, linear RCN, trigonal planar pyridine derivatives, tertiary NR3, etc., extensive essays with heavier E = P, As, Sb tend to be largely restricted to tertiary ER3, adducts MX:ER3 (1 : n) for R = Ph being commonly defined for n = 4 as the ionic form [M(EPh3)4]1X2 for X = ClO4 or NO3,1 for n = 3 as mononuclear [M(EPh3)3X] (diverse X),2 for n = 2 as mononuclear [M(EPh3)2X] [small M, E (= Cu, P) only] or binuclear [(Ph3E)2M(m-X)2- M(EPh3)2],3 and for n = 1 as ‘step’ or ‘cubane’ tetramers [{(Ph3E)MX}4] (E = P or As only),4 all except [M(EPh3)2X] having tetrahedral metal atom environments.Nevertheless, studies with alternative R substituents of diVerent stereochemical or electronic characteristics have shown that it is possible to prepare extensive series of complexes of a particular structural type with a wide range of X, as shown recently in the arrays of adducts of silver(I) salts with tricyclohexylphosphine, nmax being 2 in mononuclear [Ag{P(C6H11)3}2X],5 while n = 1 forms may be frequently binuclear rather than tetranuclear.6 In that spirit we now extend our studies of these arrays to encompass adducts of silver(I) salts with tricyclohexylarsine, recording in the present report results obtained for the AgX:As(C6H11)3 (1 : 2) system, and, in the following,7 the 1 : 1 system.We report here syntheses, and structural and spectroscopic characterisation, of AgX :As(C6H11)3 (1 : 2) for X = Cl, Br, I, CN, NCO, NO3, ClO4 or O2CCF3 as mononuclear [Ag{As(C6H11)3}2X] arrays; by contrast, in keeping with the ambidentate nature of the SCN ligand, AgSCN:As(C6H11)3 (1 : 2) is found to be an infinite one-dimensional polymer with four-co-ordinate silver(I).Experimental Syntheses All compounds were obtained by dissolution of the appropriate silver(I) salt (1 mmol) with tricyclohexylarsine (2 mmol) with warming in acetonitrile (5 cm3), colourless crystals depositing on cooling and standing.X = Cl: m.p. > 109 8C (decomp.), no satisfactory analysis data obtained. X = Br: m.p. >125 8C (decomp.) (Found: C, 51.7; H, 7.8. Calc. for C36H66AgAs2Br: C, 51.69; H, 7.95%). X = I: m.p. >131 8C (decomp.) (Found: C, 49.1; H, 7.5. Calc. for C36H66AgAs2I: C, 48.94; H, 7.53%). X = CN: m.p. 148–150 8C (Found: C, 56.8; H, 8.7; N, 1.6. Calc. for C37H66AgAs2N: C, 56.78; H, 8.50; N, 1.7%).X = NCO: m.p. >158 8C (decomp.) (Found: C, 55.4; H, 8.4. Calc. for C37H66AgAs2NO: C, 55.64; H, 8.33%). X = NO3: m.p. >143 8C (decomp.) (Found: C, 52.7; H, 8.0. Calc. for C36H66AgAs2NO3: C, 52.82; H, 8.13%). X = ClO4: m.p. >189 8C (decomp.), no satisfactory analysis obtained. X = O2CCF3: m.p. 146–148 8C (Found: C, 52.2; H, 7.9. Calc. for C38H66AgAs2F3O2: C, 52.48; H, 7.65%). X = SCN: m.p. >171 8C (decomp.) (Found: C, 54.7; H, 7.9. Calc. for C37H66AgAs2NS: C, 54.55; H, 8.17%).Structure determinations Unique room-temperature diVractometer data sets were measured (2q–q scan mode, 2qmax as specified: monochromatic Mo-Ka radiation, l = 0.71073 Å, T ª 295 K) yielding N independent reflections, No with I > 3s(I) being considered ‘observed’ and used in the full matrix least squares refinements after gaussian absorption correction. Anisotropic thermal parameters were refined for the non-hydrogen atoms, (x, y, z, Uiso)H being included at estimated values.Conventional residuals R,R9 on |F| are quoted at convergence, statistical reflection weights being derivative of s2(I) = s2(Idiff) 1 0.0004s4(Idiff). Neutral2124 J. Chem. Soc., Dalton Trans., 1998, Pages 2123–2129 atom complex scattering factors were employed, computation using the XTAL 3.2 program system.8 Abnormal feature/idiosyncrasies/ variations in procedure are noted individually below (‘variata’). In all figures, 20% thermal envelopes are shown for the non-hydrogen atoms, hydrogen atoms (where shown) having arbitrary radii of 0.1 Å.Ring carbon atoms are labelled lmn (l = ligand number, m = ring number, n = atom number). A general comment is called for in respect of the treatment of cyanate and cyanide moieties in this and accompanying/ following papers. In all cases assignment of the atoms concerned was attempted on the basis of crystallographic and chemical considerations during refinement by consideration of the behaviour of residuals, thermal parameters, geometries, etc.In a few cases a ‘definitive’ assignment emerged, but in many cases the result was ambiguous, and that presented should be treated with appropriate circumspection. Thus, all cyanates are treated as N-bonded; in e.g. the example presented in the present paper apparent bond length distributions and the behaviour of residuals during refinement support this assignment, but the attachment of the ligand at the nitrogen is non-linear, and ‘thermal motion’ at the peripheral oxygen is abnormally high.A similar observation may be made in respect of cyanides: the weight of evidence supports the probability that these are C-bound in terminally attached species, or in isolated or bridging [NCAgCN]2 arrays, but are scrambled in situations where they behave as a bridging group between two similar atoms. Individual cases are described in detail, particularly for those few where departure from the above norm is suggested.Crystal/refinement data for [AgX{As(C6H11)3}2] º C36H66Ag- As2X. (a) Monoclinic, space group C 2/c (C6 2h, no. 15), Z = 4. (i) X 5 Cl. M = 792.1, a = 17.016(3), b = 9.209(3), c = 24.821(5) Å, b = 108.07(2)8, U = 3698 Å3, Dc = 1.423 g cm23, F(000) = 1648, mMo = 24.2 cm21, specimen 0.23 × 0.40 × 0.11 mm, A* min,max = 16, 1.72, 2qmax = 608, N = 5375, No = 3466, R = 0.050, R9 = 0.062. (ii) X 5 Br. M = 836.6, a = 16.997(7), b = 9.239(2), c = 24.826(8) Å, b = 108.20(3)8, U = 3704 Å3, Dc = 1.500 g cm23, F(000) = 1720, mMo = 34.2 cm21, specimen 0.19 × 0.34 × 0.17 mm, A* min,max = 1.61, 1.82 (analytical correction), 2qmax = 558, N = 4056, No = 2642, R = 0.035, R9 = 0.035.(iii) X 5 I. M = 883.6, a = 17.094(2), b = 9.344(6), c = 24.99(1) Å, b = 108.67(2)8, U = 3781 Å3, Dc = 1.552 g cm23, F(000) = 1792, mMo = 31.1 cm21, specimen 0.58 × 0.44 × 0.10 mm, A* min,max = 1.36, 3.37 (analytical correction), 2qmax = 608, N = 5490, No = 3543, R = 0.039, R9 = 0.040.(iv) X 5 ClO4. M = 856.1, a = 17.03(1), b = 9.568(4), c = 25.185(8) Å, b = 109.57(4)8, U = 3866 Å3, Dc = 1.471 g cm23, F(000) = 1776, mMo = 23.2 cm21, specimen 0.15 × 0.21 × 0.09 mm, A* min,max = 1.18, 1.31 (analytical correction), 2qmax = 508, N = 3471, No = 1898, R = 0.057, R9 = 0.058. Variata. For the above compounds the cell and coordinate setting previously defined5 for certain P(C6H11)3 counterparts was adopted. In the perchlorate the chlorine atom is located on the 2 axis, while the unco-ordinated oxygen atoms are necessarily disordered about the twofold axis in the C2/c model; the coordinated oxygen atom was also modelled as disordered, lying somewhat oV the Ag ? ? ? Cl axis.(b) Triclinic, space group P1� (Ci 1, no. 2), Z = 2. (i) X 5 CN. M = 782.7, a = 9.205(3), b = 9.759(4), c = 23.673(7) Å, a = 95.13(3), b = 97.38(2), g = 117.29(3)8, U = 1847 Å3, Dc = 1.407 g cm23, F(000) = 816, mMo = 23.5 cm21, specimen 0.22 × 0.10 × 0.15 mm, A* min,max = 1.23, 1.41, 2qmax = 508, N = 6527, No = 2861, R = 0.056, R9 = 0.051.(ii) X 5 NCO. M = 798.7, a = 9.327(7), b = 9.816(2), c = 23.692(4) Å, a = 95.54(2), b = 96.52(3), g = 117.41(4)8, U = 1886 Å3, Dc = 1.406 g cm23, F(000) = 832, mMo = 23.0 cm21, specimen 0.46 × 0.20 × 0.06 mm, A* min,max = 1.14, 1.57, 2qmax = 508, N = 6447, No = 4083, R = 0.060, R9 = 0.068. (iii) X 5 NO3. M = 818.7, a = 9.199(2), b = 9.813(3), c = 23.844(5) Å, a = 95.08(2), b = 96.13(2), g = 115.16(2)8, U = 1915 Å3, Dc = 1.419 g cm23, F(000) 852, mMo = 22.7 cm21, specimen 0.42 × 0.55 × 0.32 mm, A* min,max = 1.27, 1.83, 2qmax = 508, N = 6717, No = 5285, R = 0.043, R9 = 0.050. Variata.For the above compounds the cell and coordinate setting previously defined for certain P(C6H11)3 counterparts 5 was adopted. In the cyanate and nitrate various components of various cyclohexyl rings were modelled as disordered over pairs of sites, occupancies set at 0.5 after trial refinement.In the cyanate the thermal parameter refinement of the peripheral component was ill behaved and the isotropic form was adopted, possibly encompassing disorder or isomerism of the potentially ambidentate ligand. Although modelled as C-bound, a C/N, disordered model for the cyanide is indistinguishable on the basis of refinement, a possibility supported by rather aberrant thermal parameters. (c) X = O2CCF3. M = 869.7, monoclinic, space group P21/n [C5 2h, no. 14 (variant)], a = 10.085(5), b = 24.36(1), c = 16.462(8) Å, b = 93.91(4)8, U = 4034 Å3, Dc (Z= 4) = 1.432 g cm23, F(000) = 1800, mMo = 21.7 cm21, specimen 0.29 × 0.34 × 0.47 mm, A* min,max = 1.72, 2.00, 2qmax = 508, N = 6838, No = 4579, R = 0.046, R9 = 0.052. Variata. One of the ligand rings was modelled as disordered over two sets of sites, disordered atom occupancy set at 0.5 after trial refinement. (d ) X = SCN. M = 814.8, triclinic, space group P1 (C1 1, no. 1), a = 13.767(5), b = 13.589(3), c = 11.952(6) Å, a = 100.23(3), b = 90.98(4), g = 118.47(2)8, U = 1920 Å3, Dc (Z = 2) = 1.408 g cm23, F(000) = 848, mMo = 23.1 cm21, specimen 0.09 × 0.10 × 0.46 mm, A* min,max = 1.17, 1.32, 2qmax = 508, N = 6749, No = 5040, R = 0.042, R9 = 0.041 (preferred hand).CCDC reference number 186/983. See http://www.rsc.org/suppdata/dt/1998/2123/ for crystallographic files in .cif format. Spectroscopy Far-infrared spectra were recorded at 4 cm21 resolution at room temperature as Polythene discs on a Digilab FTS-60 Fourier transform infrared spectrometer employing an FTS-60V vacuum optical bench with a 6.25 mm mylar film beam splitter, a mercury lamp source and a pyroelectric triglycine sulfate detector.Discussion Crystal structures Depending on the nature of the anionic moiety X, species of the type [M(ER3)n]X or [{(R3E)nMX}m] may be isolated for M = univalent coinage metal, E = P, As or Sb, n rising to a maximum of 4 in ionic compounds or 3 in those where X is covalently bound, the ligand substituent being variously alkyl or aryl.With R = cyclohexyl, structurally characterised examples are more limited and characterised by diminished n. For simple molecular or ionic adducts, the maximum n recorded in structurally characterised examples is 2, and those for E = P, i.e. P(C6H11)3, as ligand, only. For M = Cu or Ag, ‘simple’ X, X interacts with one metal alone in MX :P(C6H11)3 (1 : 2) complexes, all being mononuclear, regardless of whether X is a (pseudo-)halide or oxyanion, [M{P(C6H11)3}2X] arrays being structurally defined for (a) Cu, X = I,9 N3,10 OClO3,11 FBF3 12 (X unidentate) or O2NO13 [X (small bite) bidentate], (b) Ag, X = Cl, Br, I, CN, SCN, NCO,5 OClO3 5,14 (unidentate) or O2NO5,14 (small bite bidentate).By contrast, for M = Au (which we shall not consider further), all AuX:P(C6H11)3 (1 : 2) adducts take the form of ionic [Au{P(C6H11)3}2]X (X = Cl,15 SCN16 or PF6 17), with a linear two-co-ordinate metal atom in the cation.The results of the present room-temperature single crystal X-ray studies are consistent in stoichiometry, connectivity and stereochemistry with the complexes being of 1 : 2 AgX:As(C6-J. Chem. Soc., Dalton Trans., 1998, Pages 2123–2129 2125 H11)3 formulation [the first coinage metal As(C6H11)3 complexes so characterised]. The structures of a selection of the compounds studied are shown in Figs. 1 and 2. For X = Cl, Br, I, CN, NCO, OClO3 or O2NO, all anionic moieties except the nitrate (which is a small bite bidentate) behave as unidentate ligands, so that the metal is (pseudo-)planar three-co-ordinate As2MX.Like their P(C6H11)3 counterparts,5 these complexes form a limited number of isomorphous structural systems, arising from a monoclinic C2/c cell, Z = 4, in which the M]X bond is disposed on a crystallographic 2 axis which relates the two E(C6H11)3 ligands, only one of which is crystallographically independent, or a derivative triclinic P1� array, Z = 2, in which one independent [M{E(C6H11)3}2X] moiety comprises the asymmetric unit of the structure.In the P(C6H11)3 array the C2/ Fig. 1 Molecular projections of (a) the perchlorate, (b) the nitrate, (c) the trifluoroacetate, normal to the As2Ag plane c form was found for the X = Cl, Br, I, CN or SCN adducts (the anionic moiety in the last being disordered); for the present As(C6H11)3 array that form is found for the X = Cl, Br or I adducts, inclusive also of the O-bonded perchlorate complex.In the P(C6H11)3 array the position of the latter was somewhat ambiguous, an earlier determination having described it as in the triclinic P1� for, but further studies suggested the possibility of the achievement of the higher C2/c symmetry in admixture or as an alternative to it, the diVerence between the two forms possibly being very slight (dependent on crystallisation conditions?). Here, with As(C6H11)3 as ligand, we find the array unproblematically C2/c, although, as modelled in that space group, the perchlorate is necessarily disordered about the twofold axis; in detail, the silver atom lies on that axis, as does the chlorine, the latter albeit with a thermal parameter slightly higher than e.g., the anionic ligands, but not so high as to suggest gross disorder.By contrast, the co-ordinated oxygen atom is resolvable into a pair of symmetry-related oV-axis components 1.31(4) Å apart; the associated geometry should be treated appropriately circumspectly.The ligand components match those of the P(C6H11)3 analogues, hich coordination geometries about the metal are compared in Table 1. The alternative form recorded for the remaining P(C6H11)3 complexes of ref. 5 is a triclinic P1� array derivative of the C2/c form, found for X = NCO, O2NO (and OClO3), the two P(C6H11)3 ligands now having diVerent conformations with disorder resolvable in ring 21n.This form is also found among the As(C6H11)3 complexes, again for the X = NCO or O2NO arrays, but also for X = CN, the CN/P(C6H11)3 complex having been found to be C2/c. The disorder characteristics among the ligand rings are diverse, aVecting rings 12n and 21n in varying degree [ring 21n having been modelled as disordered in the previous P(C6H11)3/nitrate complex], manifested either as exalted thermal motion and/or resolvable disorder in either or both of these rings. In the cyanide and the cyanate (particularly the latter) the behaviour of the anionic moiety is not conducive to definitive assignment of co-ordination of one particular end of that ligand; the ‘bent’ co-ordination of the cyanate in particular is at variance with the NC assignment supported by the bond length. The Ag]N]C angle [133(1)8] in the present complex is significantly less than the value [151.2(4)8] in the P(C6H11)3 analogue, also modelled as N-bound.5 Given that in both P(C6H11)3 and As(C6H11)3 adducts the nitrate can co-ordinate as O,O9-bidentate, albeit of small ‘bite’, it may be that the coordination site has a more appropriately spacious ambience (vis-à-vis the C2/c form), permitting high ‘thermal motion’ and/or alternative co-ordination modes more readily. The geometries of these complexes are also compared with those of their P(C6H11)3 counterparts in Table 1.The trifluoroacetate complex, also mononuclear, crystallizes in a form diVerent to the above, being monoclinic, P21/n, Z = 4, with one [Ag{As(C6H11)3}2(O2CCF3)] molecule, devoid of crystallographic symmetry, comprising the asymmetric unit of the structure.It resembles the nitrate in that the anion behaves as a bidentate ligand of small ‘bite’, raising the co-ordination number of the silver to four. The ligand conformation, exhibiting disorder in one of the substituent rings, more nearly resembles that of the triclinic P1� examples than the monoclinic C2/c (which have molecular 2 symmetry). Given the seriously diVerent base strengths of O2CCF3 vs.O2NO, it might be expected that this would be reflected in the parameters of the metal atom environment; changes are relatively minor, perhaps the opposite of expectations which would suggest that O2CCF3 being a stronger base than O2NO should co-ordinate more strongly. The Ag]O distances in the O2CCF3 are appreciably larger, with Ag]As shorter and As]Ag]As enlarged.A comparison of the relationship between the d(Ag]E) bond lengths and the E]Ag]E bond angles for [Ag{E(C6H11)3}2X] (E = P or As) is shown in Fig. 3. This shows that the longer Ag]E bonds are generally accompanied by smaller E]Ag]E angles, but the2126 J. Chem. Soc., Dalton Trans., 1998, Pages 2123–2129 Table 1 Molecular core geometries for mononuclear [Ag{E(C6H11)3}2X] complexes. Distances in Å, angles in 8. Values for E = As are given below those for E = P. Values for the triclinic P1� phase (rather than the monoclinic C2/c phase) are characterized by the presence of two distance/angle entries X Cl Br I CN OClO3 NCO O2NO O2CCF3 Ag]E 2.4712(8) 2.5628(7) 2.4734(9) 2.5559(7) 2.478(1) 2.5587(8) 2.4803(9) 2.558(2), 2.568(2) 2.429(1), 2.4323(9) 2.517(1) 2.457(1), 2.454(1) 2.541(2), 2.540(2) 2.451(2), 2.440(2) 2.5243(8), 2.5185(8) 2.432(2), 2.437(2) 2.506(1), 2.511(1) Ag]X 2.489(1) 2.481(2) 2.618(1) 2.556(1) 2.778(1) 2.715(2) 2.153(6) 2.14(2) 2.720(7), 3.014(9) 2.61(2) 2.205(6) 2.38(1) 2.47(1), 2.79(1) 2.452(8), 2.553(6) 2.531(6), 2.666(6) 2.515(7), 2.585(7) E]Ag]E 128.29(3) 122.21(4) 129.42(3) 123.39(3) 130.75(4) 125.48(4) 124.86(4) 120.86(8) 147.34(3) 152.40(7) 133.65(4) 127.31(7) 139.14(9) 134.39(3) 141.19(6) 137.41(4) E]Ag]X 115.85(2) 118.90(2) 115.29(2) 118.31(2) 114.62(3) 117.26(2) 117.57(2) 114.7(6), 124.2(6) 98.5(1), 113.5(1) 102.8(5), 103.9(5) 111.6(1), 114.5(1) 115.5(2), 116.7(2) 110.5(2), 109.0(2) 106.5(2)–116.6(2) 103.5(1)–109.2(1) 105.5(2)–113.2(2) Data for the E = P array are taken from ref. 5, inclusive of data for the perchlorate from ref. 14. For the present As(C6H11)3 arrays, for the cyanide, ‘C]N’ is 0.90(3) Å and ‘Ag]C]N’ 175(2)8; for the cyanate ‘C]N,O’ are 0.75(2), 1.34(3) Å, and Ag]N]C, N]C]O 133(1), 172(2)8 [cf. 1.06(1), 1.25(2) Å; 151.2(4), 174.5(6)8 for the E = P analogue]; 5 for the nitrate Ag]O]N are 101.6(7), 95.2(4)8, and for the perchlorate Ag]O]Cl are 137(2)8. E = P data for the O2CCF3 adduct are as yet unpublished; the E = P structure is isomorphous, albeit devoid of disorder.It should be noted that the present monoclinic C2/c array, characteristic of silver(I) complexes, and with b ª 1088, is diVerent to that found in some copper(I) analogues where b ª 968, the cyclohexyl ring dispositions diVering in the two ‘polymorphs’; see ref. 18. Fig. 2 A single strand of the polymer of the thiocyanate correlation shows a considerable degree of scatter, particularly in the case of the E = As complexes.It has previously been noted that the P]Ag]P angle in some three- and four-coordinate AgX/diphosphine complexes is sensitive to the nature of the Ag]X bonding,19 and a correlation between increasing d(Hg]P) and decreasing q(P]Hg]P) has been observed for some four-co-ordinate triphenylphosphine mercury(II) complexes [HgX2(PPh3)2].20 This correlation has been attributed to an eVect of the anionic ligand X on both of these structural parameters, as a result of competition for electron density between the M]P and the M]X bonds.5 The straight lines in Fig. 3 are approximate correlations for [Ag{E(C6H11)3}2X] with unidentate anionic ligands X, and these show that d(Ag]E) is approximately equally sensitive to changes in X for both E = P and As, but that q(E]Ag]E) is more sensitive to such changes for E = As. This is consistent with the view that As(C6H11)3 is a weaker electron donor than P(C6H11)3, so that the eVects of changes in the Ag]X bonding on the As]Ag]As angle are significantly greater. The points for [Ag{As(C6H11)3}2X] (X = NO3 or O2CCF3) in Fig. 3 are displaced to smaller d(Ag]E) and q(E]Ag]E) relative to the line corresponding to the correlation for the unidentate ligands X = ClO4, CN, Cl, Br or I. The mostJ. Chem. Soc., Dalton Trans., 1998, Pages 2123–2129 2127 Table 2 Silver environments in AgSCN:As(C6H11)3 (1 : 2). Distances in Å, angles in 8. The two values in each entry are for silver (associated ligands) with n = 1 or 2 respectively. Primed atoms belong to the adjoining moiety Ag(n)]As(n1) Ag(n)]As(n2) Ag(n)]S(n) Ag(n)]N(n9) S(n)]C(n) N(n)]C(n) Ag(1) ? ? ? Ag(2,29) 2.573(1), 2.574(3) 2.580(2), 2.581(2) 2.602(5), 2.602(5) 2.33(1), 2.36(2) 1.64(2), 1.62(2) 1.12(2), 1.21(2) 6.433(3), 6.435(3) As(n1)]Ag(n)]As(n2) As(n1)]Ag(n)]S(n) As(n1)]Ag(n)]N(n) As(n2)]Ag(n)]S(n) As(n2)]Ag(n)]N(n9) S(n)]Ag(n)]N(n9) Ag(n)]S(n)]C(n) S(n)]C(n)]N(n) C(n)]N(n)]Ag(n9) 121.84(7), 122.11(9) 112.1(1), 111.8(1) 103.3(3), 107.6(4) 111.8(1), 112.6(1) 109.5(4), 102.0(3) 94.2(5), 96.8(4) 98.7(7), 99.6(6) 178(1), 174(2) 144(2), 139(1) likely reason for this is that NO3 and O2CCF3 act as bidentate ligands in these complexes.This is supported by the observation that the corresponding points for [Ag{P(C6H11)3}2X] (X = NO3 or O2CCF3) are much closer to the correlation line for unidentate X; NO3 acts as a unidentate ligand in this case while O2CCF3, although formally bidentate in both cases, shows a greater tendency towards unidentate co-ordination (i.e.a greater diVerence between the two Ag]O bond lengths) in the X = P case. As discussed previously for the [Ag{P(C6H11)3}2X] complexes, the E]Ag]E angle shows an overall decrease as the donor strength of X increases, but the behaviour of the halide complexes is anomalous in this respect, as this angle shows a small increase along the series X = Cl, Br, I, althoe donor strength of X2 is expected to increase along this series.The exact reason for this behaviour is not known at present, but we note that the same trend is observed in the present [Ag- {As(C6H11)3}2X] series. The present work has also defined the nature of the AgSCN:As(C6H11)3 (1 :2) complex. Whereas the P(C6H11)3 counterpart is relatively unexceptional, being mononuclear and of the monoclinic C2/c family,5 here we find a one-dimensional polymer, ? ? ? {As(C6H11)3}2Ag(SCN){As(C6H11)3}2Ag- (SCN) ? ? ?, with four-co-ordinate As2AgSN silver (Fig. 2); comparison is invited with the structure of AgCN:As(C6H11)3 (1 : 1) in the following paper in the context of potential or actual ambidentate co-ordination by these ligands.7 The complex crystallises in the rare triclinic space group P1 with two formula units comprising the asymmetric unit. The structural parameters that define the silver atom environments are given in Table 2. Whereas the distances within the two silver environments are very similar, there are significant and substantial differences in their angular geometries.The structure overall, however, is of higher pseudo-symmetry, the quasi-screw axis relating the two independent components of the asymmetric unit being clearly evident in Fig. 2. It is of interest that the As]Ag]As angles in the present complexes are not greatly Fig. 3 The d(Ag]E) vs. q(E]Ag]E) correlation for [Ag{E(C6H11)3}2X] [E = P (d) or As (j); data for E = P from ref. 5]. The straight lines are drawn in as ad hoc unidentate data diVerent to the counterpart angles in the mononuclear arrays, suggesting that the possibility of polymer or dimer formation in those arrays may not be energetically very diVerent from that in the monomer. Infrared spectroscopy The far-IR spectra of [Ag{As(C6H11)3}2X] (X = Cl, Br, I, NCO or CN) are shown in Figs. 4 and 5. Strong bands due to As(C6H11)3 ligands are evident down to about 280 cm21; ligand bands also occur in the region below this, but these are generally quite weak.Strong bands, the wavenumbers of which are dependent on the nature of X, are also observed in the spectra. Thus, for X = Cl, Br or I, strong, halogen-sensitive bands are observed at 230, 165 and 139 cm21 respectively (Fig. 4), and these are assigned as the n(AgX) modes for these complexes. The wavenumbers of these modes correlate well with those of other AgX complexes. This can be verified by comparison with data for the corresponding P(C6H11)3 complexes,5 as shown in Table 3, and for other AgX complexes with PPh3 and AsPh3 ligands.20 For a range of complexes of the latter type it has been shown that the dependence of n(AgX) on r(AgX) can be represented by the empirical formula (1) where b = 22 340, 22 490, n/cm21 = b(r/Å)2m (1) 41 300 and m = 5.09, 5.18, 5.68 for X = Cl, Br, I respectively.21 These correlations are shown as the continuous lines in Fig. 6, and the data for [Ag{E(C6H11)3}2X] (E = P or As; X = Cl, Br or Fig. 4 Far-IR spectra of [Ag{As(C6H11)3}2X]: X = Cl (a), Br (b) or I (c). The bands assigned to the n(AgX) modes are labelled with their wavenumbers2128 J. Chem. Soc., Dalton Trans., 1998, Pages 2123–2129 I) are represented as points on this graph. From this it is evident that the data for the E(C6H11)3 compounds lie very close to the correlation lines that were established for the EPh3 complexes, nearly all of which have structures that are diVerent from those of the E(C6H11)3 complexes.As is evident from the data in Fig. 6, the As(C6H11)3 com- Fig. 5 Far-IR spectra of [Ag{As(C6H11)3}2X]: X = NCO (a) or CN (b). The bands assigned to the n(AgX) modes are labelled with their wavenumbers Fig. 6 Plots of n(AgX) wavenumber against bond length r(AgX) from equation (1). Data points are for [Ag{E(C6H11)3}2X] [X = Cl (d), Br (j) or I (m); data for E = P from ref. 5 are the points with greater r(AgX) and smaller n(AgX) for each X] Table 3 Wavenumbers (cm21) of the n(AgX) modes in the IR spectra of [Ag{E(C6H11)3}2X] X Cl Br I NCO CN E = P* 207 149 121 241 288 E = As 230 165 139 262 311 * Ref. 5 pounds show shorter r(AgX) and greater n(AgX) than for the corresponding P(C6H11)3 complexes. A similar observation has been made recently in connection with the data for [Ag(EPh3)X] (E = P or As; X = Cl, Br or I), and has been attributed to weakened Ag]L bonding in going from L = PPh3 to AsPh3.22 Possible reasons for this have been given in terms of contributions from steric and electronic eVects.22 The data for [Ag{E(C6H11)3}2X] are consistent with those for [Ag(EPh3)3X], but do not seem to oVer any further means of distinguishing between the two possible causes of the observed trends.The assignment of the n(AgX) modes for [Ag{As(C6H11)3}2X] (X = NCO or CN) is complicated by the presence of ligand bands in the same region. However, a comparison of the spectra of these two compounds (Fig. 5) suggests n(AgX) 262 and 311 cm21 for X = NCO and CN respectively.Comparison of these results with those for the corresponding P(C6H11)3 complexes (Table 3) provides strong support for these assignments. In particular, there is an increase in n(AgX) of about 20 cm21 from the P(C6H11)3 to the corresponding As(C6H11)3 complex, as is also found for the X = Cl, Br or I compounds. The increase in n(AgX) for the X = NCO complex is somewhat puzzling, however, as the X-ray data suggest an increase in the Ag]X bond length in this case (Table 1), suggesting that the Ag]X bond is weaker in the As(C6H11)3 complex.This is not consistent with the IR result given above, and is contrary to the trend observed in all of the other complexes in Table 1. In this connection, it is relevant that the Ag]As/As]Ag]As data for this complex, as presented in Fig. 3, are also somewhat anomalous, the point for this compound lying well to the left of the correlation line for the other [Ag{As(C6H11)3}2X] compounds involving unidentate X.The reason for these anomalies is not known at present, although we note that the crystal structure determination for [Ag{As(C6H11)3}2(NCO)] presented some diYculties (see Experimental section). It would appear that a structure in which Ag]P is greater and Ag]X is smaller than the values given in Table 1 would be more consistent with the spectroscopic data for this compound, and the structural data for the compounds in Table 1.Acknowledgements We acknowledge support of this work by grants from the Australian Research Grants Scheme and the University of Auckland Research Committee. We thank Ms. Catherine Hobbis for recording the far-IR spectra. References 1 G. A. Bowmaker, EVendy, R. D. Hart, J. D. Kildea, E. 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