- Email: [email protected]
Molecular structures of gaseous dichloro- and dibromo(dioxo)molybdenum(VI). Are the Mo O bond distances and the O Mo O bond angles similar?
197
Journal of Molecular Structure, 213 (1992) 197-206 Elsevier Science Publishers B.V., Amsterdam
Molecular structures of gaseous dichloro- and dibromo(dioxo)molybdenum(VI). Are the MO = 0 bond distances and the 0 = MO = 0 bond angles similar? Hanne Thomassen and Kenneth Hedberg Department of Chemistry, Oregon State University, Corvallis, OR 97331 (USA) (Received 30 March 1992)
Abstract The molecular structures of dichloro- and dibromo(dioxo)molybdenum(VI), MoO,Cl, and MoO,Br,, have been reinvestigated by electron diffraction at nozzle-tip temperatures of 100 and 170°C, respectively. The purpose was to check the results of two earlier studies in which significant differences in the values of the MO = 0 bond distance and the 0 = MO = 0 bond angle were reported. We find that the values of each of these parameters are similar in the two compounds. Our results (r,(A) and L,(deg) with estimated 20 uncertainties), based on assumed C,, molecular symmetry, are as follows. MoO,Cl,: r(Mo= 0) = l&36(4); r(MoC1) = 2.258(3); L 0 = MO = 0 = 1(X3(26); L Cl-Mo-Cl = 113.9(23). MoO,Br,: r(Mo = 0) = 1.683(6); r(Mo-Br) = 2.403(3); LO =Mo= 0 = 107.8(39); LBr-MoBr = 111.7(12).
INTRODUCTION
The structures of both MoO,Cl, [l] and MoO,Br, [2] in the gas phase have been investigated previously by electron diffraction. This work led to a value for the Mo=O distance (rg) 0.027 Hishorter, and one for the O=Mo=O angle ( L ,J 10’ larger in MoO,Br, than in MOO, Cl,. These results are quite surprising compared with measurements of the structures of other compounds, such as the chromyl halides, which show that the effect of halogen atoms on the type of bond and bond angle in question is similar. It seemed likely that the Mo=O bond length and the O=M=O bond angle in MoO,Br, measured in the previous investigation [2] were incorrect. For example, in CrO,F, [3], CrO,Cl, [4] and MoO,Cl, [l], the angle O=M=O, where the metal-oxygen link is formally a double bond, is less than the angle X-M-X by 4-7O. In the MoO,Br, study, however, the O=Mo=O angle Correspondence to: Professor K. Hedberg, Department of Chemistry, Oregon State Uni versity, Corvallis, OR 97331, USA.
0022-2860/92/$05.00 0 1992 Elsevier Science Publishers
B.V. All rights reserved.
198
H. Thomassen and K. HedberglJ. Mol. Struct., 273 (1992) 197-206
was found to be larger than L Br-Mo-Br (114(3)’ vs. 110.4(3)‘). This circumstance called for reinvestigation of the two molecules. EXPERIMENTAL
The samples of MoO,Cl, (Alfa; Cl 35.6%, Theo; Cl 35.65%) and MoO,Br, (Alfa; 99%) were used as received and transferred to sample ampoules fitted with teflon valves in a dry box. The diffraction experiments were carried out with the Oregon State apparatus fitted with an r3 sector at nozzle-tip temperatures of 100°C (MoO,Cl,) and 170°C (MoO,Br,). Nominal nozzle-toplate distances were 300 mm (shorter camera) and 750 mm (longer camera). The 8 x 10 inch Kodak projector slide plates were developed for 10 min in D-19 developer diluted 3 : 1. Other experimental conditions were as follows: beam current, 0.44-0.56pA; exposure time, 60-180s (longer camera) and 35 min (shorter camera); nominal accelerating potential, 62 kV; calibration against CS, in separate experiments, r,(C=S) = 1.557A and r,(S . . . S) = 3.109 A [5]. The number of plates selected for analysis and the data ranges were: for MoO,Cl,, four, 2.0 < s/A_’ < 15.0 (longer camera) and two, 7.0 < s/A-’ d 40.0 (shorter camera); for MoO,Br,, three, 2.0 < s/A-’ d 16.5 (longer camera) and two, 7.0 < s/A’ < 40.25 (shorter camera). The data intervals were As = 0.25Ai-l (s = 4&‘sin(8/2); 0 is the scattering angle). Total scattered intensities s4&(s) were obtained from the plates and the backgrounds removed by the usual procedures to generate molecular intensities of the form sl, (s) [6] .aComplex electron scattering amplitudes and the phases were taken from tables [7]. The final curves are shown in Figs. 1 and 2 for MoO,Cl, and MoO,Br,, respectively. STRUCTURE
ANALYSIS
Experimental radial distribution curves (r-D(r)) were calculated in the usual way by Fourier transformation of the functions I’(s) = sl,Z,,Zx (AMoA,)-’ exp( -0.0025s’) where X = Cl or Br. Data in the unobserved region s < 2A-’ were taken from the theoretical intensity functions. The final curves are shown in Fig. 3. The structures were defined in terms of the geometrically consistent r’a distances. The required r, type distances were generated from the usual expression, ra = r, + 6r + K - Z2/r = rg - Z2/r, through use of an appropriate vibrational force field that enabled calculation of the terms 6r (centrifugal distortion), K (the perpendicular amplitude correction) and I (the root-mean-square vibrational amplitude) with the progam ASYMBO [8]. Vibrational infrared and Raman spectra of MoO,Cl, and MoO,Br, have “Tabulated
values
26455 (15 pages).
are available
from B.L.L.D.
as Supplementary
Publication
number
SUP
199
H. Thomassen and K. HedbergjJ. Mol. Struct., 273 (1992) 197-206
EXPERIMENTRL
MIDDLE
CRMERA
IWERRGE
CURVES
THEORETICRL
DIFFERENCE -.+.a^_--I
10
-
_-+___--
-e*
I
I
20
30
J I
40 s
Fig. 1. Intensity curves for MoO,Cl,. The s41,experimental curves are shown magnified seven times with respect to the backgrounds on which they are superimposed. The average curves are s(s*l, - bkgd). The theoretical curve is calculated from parameter values given in Table 3.
200
H. Thomassen and K. HedberglJ. Mol. Strut.,
273 (1992) 197-206
EXPERIMENTQL
MIOOLE
CFlMERFl
FIVERFIGE CURVES
THEORETICFIL
DIFFERENCE v----I 10
- -b--"--w.dy,-_;-"~ I 20
I 30
1
I
40 s
Fig. 2. Intensity curves for MoO,Br,. See legend to Fig. 1.
been reported in the vapour and matrix-isolated phases [9], and the nine fundamental vibrational frequencies assigned to species 4A,(IR + Ra) + A,(Ra) + 2B,(IR + Ra) + 2B,(IR + Ra) of point group C,,. We used the assignment of Kovba and Mal’tsev [9(e)] to develop symmetry force fields
Mo=O
No-Cl
o-o
Cl** Cl
O.*Cl
DIFFERENCE
EXPERIMENTRL
no=0
No-Br
O-O
0-mBr
Br=.Br DIFFERENCE
I
I
I
I
I
1
2
3
4
5R
Fig. 3. Radial distribution curves for MoO,Cl, and MoO,Br,. The experimental curves a calculated in each case from composites of the two average intensity curves with use theoretical data for the region0 < s/A-’ d 1.75, and B/ii’ = 0.0025. The vertical lines indica the interatomic distances and have lengths proportional to the distance weights.
202
H. Thomassen and K. HedberglJ. Mol. Strut.,
273 (1992) 197-206
TABLE 1 Symmetry coordinates and vibrational wavenumbers for MOO, Cl, and MoO,Br,” Species
Coordinate
al
S, = r14 +
a2 b,
S, = % = S4 = S, = S, = S, = S, = S, =
b,
r15
r12+ r13 4as14- a315- az15- a314- az14 4a3,, - a315- az15- as14- az14 a316- az16- as14+ az14 r14- r15 - a315- az15+ a314+ az14 r12- r13 - aQ15 + az15- a314+ az14
Mode
MOO, s stretch MoX, s stretch MOO, s&s MoX, s&s MoX, twist MOO, a stretch MOO, rock MoX, a stretch MoX, rock
Wavenumber (cm-‘) MOO, Cl,
MoO,Br,
996 429 338 167 116 970 203 470 180
995 378 262 147 82 970 184 338 161
“The wavenumbers are calculated with the force field in Table 4 and differ from the observed [5(e)] by less than 1 cm-‘.
which gave exact fits to the observed wavenumbers. Valence force constants for Mo=O stretch and Mo=O stretch-stretch interactions were taken from ref. 9(b). The potential energy distribution (PED) for our force fields shows that, with two exceptions in the case of MoO,Br,, each normal mode vi is well described by one symmetry coordinate, the corresponding S, (Table 1). The MoO,Br, exceptions are v2 = 378cm-l and vg = 262cm-‘, for which the PED are respectively calculated to be S,(78), S,(17) and S,(24), S,(87); S, is the Mo-Br symmetric stretch and S, the MOO, scissor. According to our PED, Kovba and Mal’tser [9(e)] have reversed the assignments of vp and v,; otherwise our assignments and theirs are the same. Our symmetry coordinates and the corresponding force fields are given in Tables 1 and 2. The anharmonicity constants, IC= (1/6)aZ4, were introduced for bond distances using calculated I values and a = 2 Ap3. Possible anharmonicities of non-bonds were ignored. Four parameters, r,(Mo=O), r,(Mo-X), L,(O=M=O) and L.(X-Mo-X), were chosen to define geometries consistent with the assumed C,, molecular symmetry. Table 3 contains refined values for these and for the root-mean-square amplitudes of vibration determined by least-squares fitting of theoretical curves to average intensity curves for each camera distance. Although all parameters could be refined simultaneously, and the fitting procedure converged to give largely uncorrelated values for them, large uncertainties were attached to the amplitudes Z(O.. .O) for both molecules. Owing to the low correlations, test changes in these I values did not affect significantly either the other Zvalues or the structures. We chose
203
H. Thomassen and K. HedberglJ. Mol. Struct., 273 (1992) 197-206 TABLE 2 Force constants’ for MoO,Cl, and MoO,Br, Species
MoO,Br,
MOO, Cl,
S, 4 S, s* S, S, S, S, S!3
a.264 0.037 0.030 - 0.038 0.191 7.372 0.147 2.733 - 0.244
“Units are aJ k” interactions.
3.156 - 0.062 0.069
0.884 0.459
1.152 - 0.058
0.923
8.341 0.036 0.023 - 0.062 0.107 7.293 0.149 2.630 0.204
for stretches, aJradm2 for bends and aJ k’
4.137 0.120 0.348
0.791 - 0.188
1.524
0.869 0.355
rad-' for stretch-bend
fix l(0.e *0) at the calculated values in our final refinements of both molecules. The results are designated in Table 3 as model A. A second mode of refinement, wherein the individual distances were refined without imposition of any symmetry constraint, was also carried out. The five intramolecular distances determined by this refinement (the models B in Table 3) are not significantly different from those found for the corresponding models A. The correlation matrices for the A models are given in Table 4. The correlation matrices for model B are similar. to
DISCUSSION
As seen from Table 1, the agreement between our results and those from the earlier work on the two molecules is quite good for MOO, Cl,, but very poor for MoO,Br,. The differences in the case of MoO,Br, appear to be matters of both size and shape: we find both bonds to be larger and the O=Mo=O angle to be smaller than previously reported. The reason for the disagreement is not clear, since experimental details of the early work are unavailable. With allowance for the expected difference in the lengths of the bonds to the halogens, our structures for the two molecules are similar. The X-MO-X bond angle in the chloride is slightly larger than in the bromide, corresponding to greater electrostatic repulsion between the atoms of the more electronegative pair. The O=Mo=O bond angles are nearly the same, in agreement with the estimate L O=Mo=O = 109 + 3’ from the vibrational spectra [9(a)]. There are also similarities between the bond angles in the two molecules of this study and those in the chromyl halides: in CrO,F, [3], LO=Cr=O = 107.8(8)“, LF-Cr-F = 111.9(9)0, LF-Cr=O = 109.3(2)0; in CrO,Cl, [4], L O=Cr=O = 10&5(4)O, L Cl-Cr-Cl = 113.3(3)‘, L Cl-Cr=O =
H. Thomossen and K. HedbergiJ. Mol. Struck, 273 (1992) 197-206
204 TABLE 3
Structure results for MoO,Cl, and MoO,Br,“sb Parameter
Model A
Model B 1obsd
Refs. 1 and 2
1eskd rg
1obsd
0.036 1.687(4) 0.047 2.258(3)
0.041(7) 0.052(3)
r,, L,
rr
MOO, Cl, r(Mo=O) r(Mo-Cl) L OMoO L ClMoCl o...o Cl...0 Cl. . Cl L OMoCl Rd
1.676(4) 2.255(3) X16.3(26) 113.9(23) 2.683(46) 3.220(14) 3.780(50) 108.7(7) 0.12
1.686 0.041(7) 2.258 0.052(3)
1.698(6) 2.259(5) 104.0(20) 112.0(10) 2.698 [0.081] 0.081 2.689(68) [0.081] 2.679(46) 3.224 0.105(11) 0.133 3.221(15) 0.105(12) 3.255(12) 3.783 0.138(39) 0.101 3.771(58) 0.136(38) 3.747(29)
MoO,Br, r(Mo=O) r(MoBr) L OMoO L BrMoBr o...o Br...O Br . . Br L OMoBr Rd
1.661(6) 2.400(3) 107.8(39) 111.7(12) 2.685(68) 3.240(14) 3.972(28) 108.6(10) 0.10
1.683 0.046(S) 2.403 0.058(3)
1obsd
rg
0.045(7) 0.044(3)
0.090(34) 0.109(7) 0.117(18)
0.12 0.037 1.683(6) 0.047 2.402(3)
0.046(8) 0.058(3)
0.093 2.713(97) [0.093] 2.721 [0.093] 3.348 0.136(15) 0.172 3.347(22) 0.136(15) 3.973 0.140(18) 0.089 3.973(28) 0.140(18)
1.671(3) 2.367(3) 114(3) 110.4(3)
0.0439(75) 0.0578(35)
3.285(12) 0.125(13) 3.883(25) 0.143(22)
0.10
“Distances (r) and amplitudes (I) are in Angstroms, angles (L) are in degrees. bUncertainties in parentheses are estimated 20. Values for rg are as for r.. “Ref. 1, MoO,Cl,; ref. 2, MoO,Br,. dR = [~iw,A~/Iciw,(s,I,(obsd))2]1’2 where Ai = siZi(obsd) - s,l,(calcd).
108.3(1)“). These similarities suggest similar bonding properties of the Cr and MO atoms in this type of compound. An electron-diffraction study of the structure of MoOCl, [lo] gave the results r,(Mo=O) = 1.658(B) and r,(Mo-Cl) = 2.279(3)& i.e., the MoO bond is shorter and the Mo-Cl bond longer than in MOO, Cl,. Although the Mo-0 bond in MoOCl, is formally a double bond, its bond order is estimated to be nearly three from a bond order-bond distance correlation curve [ll]. According to this correlation curve, our measured values of the Mo=O bond length correspond to a bond order in MoO,X, of 2.5. Theoretical calculations also indicate that the Mo-0 bond in MoOCl, is stronger than that in MoO,Cl,: the calculated Mo=O bond dissociation energies are respectively 79 and 102 kcalmol-l [12]. The reason for the short bond in MoOCl, has been discussed in terms of dn bonding to the oxygens, in which
205
H. Thomassen and K. HedberglJ. Mol. Struct., 273 (1992) 197-206 TABLE 4 Correlation matrices (x 100) for refined parameters of MoO,Cl, and MoO,Br,”
r,(Mo=O) r,(M*X) L,O=Mo=O L, Cl-MoCl Z,(Mo=O) I, (MoX) I,(O...X) &(X.,.X)
0.14 0.07 91.2 81.0 0.23 0.09 0.38 1.37
0.20 q.05 136 42.0 0.28 0.05 0.51 0.62
100 -10 -1 13 -4 2 il - 3
4 100 2 5 -2
16 12 100 -34 -8 -5
2 -9 -2 100
-4 6 -5 -1 100 29 13 3
-3
3 4 6 -10 3 11 100 -17
-3 -2 -13 -5 4 11 -6 100
aCoefficients for MoO,Cl, and MoO,Br, are, respectively, below and above the diagonal. bStandard deviations (x 100) from least squares. Distances (r) and amplitudes (1) in Angstroms; angles (L) in degrees.
two such bonds to the single oxygen are formed in MoOCl, [10,12]. A detailed description of the bonding in CrOCl, and CrO,Cl, based on generalized valence-bond calculations has been given by Rappe and Goddard [12]: as far as the Cr=O bonds are concerned, two dn bonds are formed to the oxygen in the tetrachloride and only one to each oxygen in the dichloride, leading to a shorter Cr=O bond in the former. This description also accounts nicely for the Mo=O bond length differences in the corresponding molybdenum compounds. Thus, following Rappe and Goddard, it may be assumed that in MoO,Cl, two of the six valence electrons are involved in two bonds of considerable ionic character to Cl, and two are involved in each Mo=O bond. The Mo=O bonds are essentially covalent double bonds of which the d,(Mo) and p,(O) orbitals form a 0 component, and the d,,(Mo) and p,(O) a 71component; the pr electrons on 0 remain as a lone pair. In MoOCl,, four of the six valence electrons are in somewhat ionic bonds to Cl, leaving two d7celectrons on the MO atom for bonding to the 0x0 ligand. The Mo=O bond is essentially a triple bond comprising two 7cbonds made up of the dn orbitals on the MO atom, in combination with the p, and pr orbitals on oxygen and a donor-acceptor (T bond made up of the doubly occupied p, orbital on oxygen combined with the empty d, on molybdenum. It may be noted in passing that the bonding involving 32 valence-electron tetracoordinate species of some main group elements, such as SO,Cl,, appears to be quite different from that just discussed. In these compounds it is suggested [13] that d orbitals play little or no role, and that instead the structures reflect the operation of “negative hyperconjugation”. ACS reg. Nos: MoO,Cl, 13637-68-8, MoO,Br, 13595-98-7.
206
H. Thomassen and K. Hedberg/J. Mol. Struct., 273 (1992) 197-206
ACKNOWLEDGEMENT
Support of this work was provided by the National Science Foundation under grant CHEM-10070 to Oregon State University. REFERENCES 1 I.M. Zharskii, E.Z. Zasorin, V.P. Spiridonov, G.I. Novikov and V.N. Kupreev, Koord. Khim., 1 (1975) 574; Sov. J. Coord. Chem., 1 (1975) 473. 2 N.Ya. Shishkin and G.I. Novikov, Abstracts, 9th Austin Symp. on Gas-Phase Molecular Structure, Austin, TX, 1982, University of Texas, Austin, TX, 1982. 3 R.J. French, L. Hedberg, K. Hedberg, G.L. Gard and B.M. Johnson, Inorg. Chem., 22 (1983) 892. 4 C.J. Marsden, L. Hedberg and K. Hedberg, Inorg. Chem., 21 (1982) 1115. 5 Y. Morino and T. Iijima, Bull. Chem. Sot. Jpn., 35 (1962) 1661. 6 (a) G. Gundersen and K. Hedberg, J. Chem. Phys., 51 (1969) 2500. (b) L. Hedberg, Abstracts, 5th Austin Symp. on Gas-Phase Molecular Structure, Austin, TX, March 1974, No. T9, University of Texas, Austin, TX, 1974. 7 (a) A.W. Ross, M. Fink and R.L. Hilderbrant, International Tables for Crystallography, Vol. C, International Union of Crystallography, Kluwer Academic, Dordrecht/Boston/ London, 1992, p. 245. (b) D.T. Cromer and J.B. Mann, J. Chem. Phys., 47 (1967) 1892. (c) D.T. Cromer, J. Chem. Phys., 50 (1969) 4857. 8 L. Hedberg, Abstracts, 8th Austin Symp. on Gas-Phase Molecular Structure, Austin, TX, March 1978, No. TAlO, University of Texas, Austin, TX, 1978. 9 (a) W. Levason, R. Narayanaswamy, J.S. Ogden, A.J. Rest and J.W. Turff, J. Chem. Sot., Dalton Trans. (1982) 2009. (b) C.G. Barraclough and J. Stals, Aust. J. Chem., 19 (1966) 741. (c) T.V. Iorns and F.E. Stafford, J. Am. Chem. Sot., 88 (1966) 4819. (d) B.G. Ward and F.E. Stafford, Inorg. Chem., 7 (1968) 2569. (e) V.M. Kovba and A.A. Mal’tsev, Zh. Neorg. Khim., 20 (1975) 22; Russ. J. Inorg. Chem., 20 (1975) 11. (f) I.R. Beattie, K.M.S. Livingstone, D.J. Reynolds and G.A. Ozin, J. Chem. Sot. A (1970) 1210. 10 K. Iijima and S. Shibata, Bull. Chem. Sot. Jpn., 48 (1975) 666. 11 F.A. Cotton and R.M. Wing, Inorg. Chem., 4 (1965) 867. 12 A.K. Rappe and W.A. Goddard, J. Am. Chem. Sot., 104 (1982) 3287. 13 A.E. Read and P.V.R. Schleyer, J. Am. Chem. Sot., 112 (1990) 1434.
Journal of Molecular Structure, 213 (1992) 197-206 Elsevier Science Publishers B.V., Amsterdam
Molecular structures of gaseous dichloro- and dibromo(dioxo)molybdenum(VI). Are the MO = 0 bond distances and the 0 = MO = 0 bond angles similar? Hanne Thomassen and Kenneth Hedberg Department of Chemistry, Oregon State University, Corvallis, OR 97331 (USA) (Received 30 March 1992)
Abstract The molecular structures of dichloro- and dibromo(dioxo)molybdenum(VI), MoO,Cl, and MoO,Br,, have been reinvestigated by electron diffraction at nozzle-tip temperatures of 100 and 170°C, respectively. The purpose was to check the results of two earlier studies in which significant differences in the values of the MO = 0 bond distance and the 0 = MO = 0 bond angle were reported. We find that the values of each of these parameters are similar in the two compounds. Our results (r,(A) and L,(deg) with estimated 20 uncertainties), based on assumed C,, molecular symmetry, are as follows. MoO,Cl,: r(Mo= 0) = l&36(4); r(MoC1) = 2.258(3); L 0 = MO = 0 = 1(X3(26); L Cl-Mo-Cl = 113.9(23). MoO,Br,: r(Mo = 0) = 1.683(6); r(Mo-Br) = 2.403(3); LO =Mo= 0 = 107.8(39); LBr-MoBr = 111.7(12).
INTRODUCTION
The structures of both MoO,Cl, [l] and MoO,Br, [2] in the gas phase have been investigated previously by electron diffraction. This work led to a value for the Mo=O distance (rg) 0.027 Hishorter, and one for the O=Mo=O angle ( L ,J 10’ larger in MoO,Br, than in MOO, Cl,. These results are quite surprising compared with measurements of the structures of other compounds, such as the chromyl halides, which show that the effect of halogen atoms on the type of bond and bond angle in question is similar. It seemed likely that the Mo=O bond length and the O=M=O bond angle in MoO,Br, measured in the previous investigation [2] were incorrect. For example, in CrO,F, [3], CrO,Cl, [4] and MoO,Cl, [l], the angle O=M=O, where the metal-oxygen link is formally a double bond, is less than the angle X-M-X by 4-7O. In the MoO,Br, study, however, the O=Mo=O angle Correspondence to: Professor K. Hedberg, Department of Chemistry, Oregon State Uni versity, Corvallis, OR 97331, USA.
0022-2860/92/$05.00 0 1992 Elsevier Science Publishers
B.V. All rights reserved.
198
H. Thomassen and K. HedberglJ. Mol. Struct., 273 (1992) 197-206
was found to be larger than L Br-Mo-Br (114(3)’ vs. 110.4(3)‘). This circumstance called for reinvestigation of the two molecules. EXPERIMENTAL
The samples of MoO,Cl, (Alfa; Cl 35.6%, Theo; Cl 35.65%) and MoO,Br, (Alfa; 99%) were used as received and transferred to sample ampoules fitted with teflon valves in a dry box. The diffraction experiments were carried out with the Oregon State apparatus fitted with an r3 sector at nozzle-tip temperatures of 100°C (MoO,Cl,) and 170°C (MoO,Br,). Nominal nozzle-toplate distances were 300 mm (shorter camera) and 750 mm (longer camera). The 8 x 10 inch Kodak projector slide plates were developed for 10 min in D-19 developer diluted 3 : 1. Other experimental conditions were as follows: beam current, 0.44-0.56pA; exposure time, 60-180s (longer camera) and 35 min (shorter camera); nominal accelerating potential, 62 kV; calibration against CS, in separate experiments, r,(C=S) = 1.557A and r,(S . . . S) = 3.109 A [5]. The number of plates selected for analysis and the data ranges were: for MoO,Cl,, four, 2.0 < s/A_’ < 15.0 (longer camera) and two, 7.0 < s/A-’ d 40.0 (shorter camera); for MoO,Br,, three, 2.0 < s/A-’ d 16.5 (longer camera) and two, 7.0 < s/A’ < 40.25 (shorter camera). The data intervals were As = 0.25Ai-l (s = 4&‘sin(8/2); 0 is the scattering angle). Total scattered intensities s4&(s) were obtained from the plates and the backgrounds removed by the usual procedures to generate molecular intensities of the form sl, (s) [6] .aComplex electron scattering amplitudes and the phases were taken from tables [7]. The final curves are shown in Figs. 1 and 2 for MoO,Cl, and MoO,Br,, respectively. STRUCTURE
ANALYSIS
Experimental radial distribution curves (r-D(r)) were calculated in the usual way by Fourier transformation of the functions I’(s) = sl,Z,,Zx (AMoA,)-’ exp( -0.0025s’) where X = Cl or Br. Data in the unobserved region s < 2A-’ were taken from the theoretical intensity functions. The final curves are shown in Fig. 3. The structures were defined in terms of the geometrically consistent r’a distances. The required r, type distances were generated from the usual expression, ra = r, + 6r + K - Z2/r = rg - Z2/r, through use of an appropriate vibrational force field that enabled calculation of the terms 6r (centrifugal distortion), K (the perpendicular amplitude correction) and I (the root-mean-square vibrational amplitude) with the progam ASYMBO [8]. Vibrational infrared and Raman spectra of MoO,Cl, and MoO,Br, have “Tabulated
values
26455 (15 pages).
are available
from B.L.L.D.
as Supplementary
Publication
number
SUP
199
H. Thomassen and K. HedbergjJ. Mol. Struct., 273 (1992) 197-206
EXPERIMENTRL
MIDDLE
CRMERA
IWERRGE
CURVES
THEORETICRL
DIFFERENCE -.+.a^_--I
10
-
_-+___--
-e*
I
I
20
30
J I
40 s
Fig. 1. Intensity curves for MoO,Cl,. The s41,experimental curves are shown magnified seven times with respect to the backgrounds on which they are superimposed. The average curves are s(s*l, - bkgd). The theoretical curve is calculated from parameter values given in Table 3.
200
H. Thomassen and K. HedberglJ. Mol. Strut.,
273 (1992) 197-206
EXPERIMENTQL
MIOOLE
CFlMERFl
FIVERFIGE CURVES
THEORETICFIL
DIFFERENCE v----I 10
- -b--"--w.dy,-_;-"~ I 20
I 30
1
I
40 s
Fig. 2. Intensity curves for MoO,Br,. See legend to Fig. 1.
been reported in the vapour and matrix-isolated phases [9], and the nine fundamental vibrational frequencies assigned to species 4A,(IR + Ra) + A,(Ra) + 2B,(IR + Ra) + 2B,(IR + Ra) of point group C,,. We used the assignment of Kovba and Mal’tsev [9(e)] to develop symmetry force fields
Mo=O
No-Cl
o-o
Cl** Cl
O.*Cl
DIFFERENCE
EXPERIMENTRL
no=0
No-Br
O-O
0-mBr
Br=.Br DIFFERENCE
I
I
I
I
I
1
2
3
4
5R
Fig. 3. Radial distribution curves for MoO,Cl, and MoO,Br,. The experimental curves a calculated in each case from composites of the two average intensity curves with use theoretical data for the region0 < s/A-’ d 1.75, and B/ii’ = 0.0025. The vertical lines indica the interatomic distances and have lengths proportional to the distance weights.
202
H. Thomassen and K. HedberglJ. Mol. Strut.,
273 (1992) 197-206
TABLE 1 Symmetry coordinates and vibrational wavenumbers for MOO, Cl, and MoO,Br,” Species
Coordinate
al
S, = r14 +
a2 b,
S, = % = S4 = S, = S, = S, = S, = S, =
b,
r15
r12+ r13 4as14- a315- az15- a314- az14 4a3,, - a315- az15- as14- az14 a316- az16- as14+ az14 r14- r15 - a315- az15+ a314+ az14 r12- r13 - aQ15 + az15- a314+ az14
Mode
MOO, s stretch MoX, s stretch MOO, s&s MoX, s&s MoX, twist MOO, a stretch MOO, rock MoX, a stretch MoX, rock
Wavenumber (cm-‘) MOO, Cl,
MoO,Br,
996 429 338 167 116 970 203 470 180
995 378 262 147 82 970 184 338 161
“The wavenumbers are calculated with the force field in Table 4 and differ from the observed [5(e)] by less than 1 cm-‘.
which gave exact fits to the observed wavenumbers. Valence force constants for Mo=O stretch and Mo=O stretch-stretch interactions were taken from ref. 9(b). The potential energy distribution (PED) for our force fields shows that, with two exceptions in the case of MoO,Br,, each normal mode vi is well described by one symmetry coordinate, the corresponding S, (Table 1). The MoO,Br, exceptions are v2 = 378cm-l and vg = 262cm-‘, for which the PED are respectively calculated to be S,(78), S,(17) and S,(24), S,(87); S, is the Mo-Br symmetric stretch and S, the MOO, scissor. According to our PED, Kovba and Mal’tser [9(e)] have reversed the assignments of vp and v,; otherwise our assignments and theirs are the same. Our symmetry coordinates and the corresponding force fields are given in Tables 1 and 2. The anharmonicity constants, IC= (1/6)aZ4, were introduced for bond distances using calculated I values and a = 2 Ap3. Possible anharmonicities of non-bonds were ignored. Four parameters, r,(Mo=O), r,(Mo-X), L,(O=M=O) and L.(X-Mo-X), were chosen to define geometries consistent with the assumed C,, molecular symmetry. Table 3 contains refined values for these and for the root-mean-square amplitudes of vibration determined by least-squares fitting of theoretical curves to average intensity curves for each camera distance. Although all parameters could be refined simultaneously, and the fitting procedure converged to give largely uncorrelated values for them, large uncertainties were attached to the amplitudes Z(O.. .O) for both molecules. Owing to the low correlations, test changes in these I values did not affect significantly either the other Zvalues or the structures. We chose
203
H. Thomassen and K. HedberglJ. Mol. Struct., 273 (1992) 197-206 TABLE 2 Force constants’ for MoO,Cl, and MoO,Br, Species
MoO,Br,
MOO, Cl,
S, 4 S, s* S, S, S, S, S!3
a.264 0.037 0.030 - 0.038 0.191 7.372 0.147 2.733 - 0.244
“Units are aJ k” interactions.
3.156 - 0.062 0.069
0.884 0.459
1.152 - 0.058
0.923
8.341 0.036 0.023 - 0.062 0.107 7.293 0.149 2.630 0.204
for stretches, aJradm2 for bends and aJ k’
4.137 0.120 0.348
0.791 - 0.188
1.524
0.869 0.355
rad-' for stretch-bend
fix l(0.e *0) at the calculated values in our final refinements of both molecules. The results are designated in Table 3 as model A. A second mode of refinement, wherein the individual distances were refined without imposition of any symmetry constraint, was also carried out. The five intramolecular distances determined by this refinement (the models B in Table 3) are not significantly different from those found for the corresponding models A. The correlation matrices for the A models are given in Table 4. The correlation matrices for model B are similar. to
DISCUSSION
As seen from Table 1, the agreement between our results and those from the earlier work on the two molecules is quite good for MOO, Cl,, but very poor for MoO,Br,. The differences in the case of MoO,Br, appear to be matters of both size and shape: we find both bonds to be larger and the O=Mo=O angle to be smaller than previously reported. The reason for the disagreement is not clear, since experimental details of the early work are unavailable. With allowance for the expected difference in the lengths of the bonds to the halogens, our structures for the two molecules are similar. The X-MO-X bond angle in the chloride is slightly larger than in the bromide, corresponding to greater electrostatic repulsion between the atoms of the more electronegative pair. The O=Mo=O bond angles are nearly the same, in agreement with the estimate L O=Mo=O = 109 + 3’ from the vibrational spectra [9(a)]. There are also similarities between the bond angles in the two molecules of this study and those in the chromyl halides: in CrO,F, [3], LO=Cr=O = 107.8(8)“, LF-Cr-F = 111.9(9)0, LF-Cr=O = 109.3(2)0; in CrO,Cl, [4], L O=Cr=O = 10&5(4)O, L Cl-Cr-Cl = 113.3(3)‘, L Cl-Cr=O =
H. Thomossen and K. HedbergiJ. Mol. Struck, 273 (1992) 197-206
204 TABLE 3
Structure results for MoO,Cl, and MoO,Br,“sb Parameter
Model A
Model B 1obsd
Refs. 1 and 2
1eskd rg
1obsd
0.036 1.687(4) 0.047 2.258(3)
0.041(7) 0.052(3)
r,, L,
rr
MOO, Cl, r(Mo=O) r(Mo-Cl) L OMoO L ClMoCl o...o Cl...0 Cl. . Cl L OMoCl Rd
1.676(4) 2.255(3) X16.3(26) 113.9(23) 2.683(46) 3.220(14) 3.780(50) 108.7(7) 0.12
1.686 0.041(7) 2.258 0.052(3)
1.698(6) 2.259(5) 104.0(20) 112.0(10) 2.698 [0.081] 0.081 2.689(68) [0.081] 2.679(46) 3.224 0.105(11) 0.133 3.221(15) 0.105(12) 3.255(12) 3.783 0.138(39) 0.101 3.771(58) 0.136(38) 3.747(29)
MoO,Br, r(Mo=O) r(MoBr) L OMoO L BrMoBr o...o Br...O Br . . Br L OMoBr Rd
1.661(6) 2.400(3) 107.8(39) 111.7(12) 2.685(68) 3.240(14) 3.972(28) 108.6(10) 0.10
1.683 0.046(S) 2.403 0.058(3)
1obsd
rg
0.045(7) 0.044(3)
0.090(34) 0.109(7) 0.117(18)
0.12 0.037 1.683(6) 0.047 2.402(3)
0.046(8) 0.058(3)
0.093 2.713(97) [0.093] 2.721 [0.093] 3.348 0.136(15) 0.172 3.347(22) 0.136(15) 3.973 0.140(18) 0.089 3.973(28) 0.140(18)
1.671(3) 2.367(3) 114(3) 110.4(3)
0.0439(75) 0.0578(35)
3.285(12) 0.125(13) 3.883(25) 0.143(22)
0.10
“Distances (r) and amplitudes (I) are in Angstroms, angles (L) are in degrees. bUncertainties in parentheses are estimated 20. Values for rg are as for r.. “Ref. 1, MoO,Cl,; ref. 2, MoO,Br,. dR = [~iw,A~/Iciw,(s,I,(obsd))2]1’2 where Ai = siZi(obsd) - s,l,(calcd).
108.3(1)“). These similarities suggest similar bonding properties of the Cr and MO atoms in this type of compound. An electron-diffraction study of the structure of MoOCl, [lo] gave the results r,(Mo=O) = 1.658(B) and r,(Mo-Cl) = 2.279(3)& i.e., the MoO bond is shorter and the Mo-Cl bond longer than in MOO, Cl,. Although the Mo-0 bond in MoOCl, is formally a double bond, its bond order is estimated to be nearly three from a bond order-bond distance correlation curve [ll]. According to this correlation curve, our measured values of the Mo=O bond length correspond to a bond order in MoO,X, of 2.5. Theoretical calculations also indicate that the Mo-0 bond in MoOCl, is stronger than that in MoO,Cl,: the calculated Mo=O bond dissociation energies are respectively 79 and 102 kcalmol-l [12]. The reason for the short bond in MoOCl, has been discussed in terms of dn bonding to the oxygens, in which
205
H. Thomassen and K. HedberglJ. Mol. Struct., 273 (1992) 197-206 TABLE 4 Correlation matrices (x 100) for refined parameters of MoO,Cl, and MoO,Br,”
r,(Mo=O) r,(M*X) L,O=Mo=O L, Cl-MoCl Z,(Mo=O) I, (MoX) I,(O...X) &(X.,.X)
0.14 0.07 91.2 81.0 0.23 0.09 0.38 1.37
0.20 q.05 136 42.0 0.28 0.05 0.51 0.62
100 -10 -1 13 -4 2 il - 3
4 100 2 5 -2
16 12 100 -34 -8 -5
2 -9 -2 100
-4 6 -5 -1 100 29 13 3
-3
3 4 6 -10 3 11 100 -17
-3 -2 -13 -5 4 11 -6 100
aCoefficients for MoO,Cl, and MoO,Br, are, respectively, below and above the diagonal. bStandard deviations (x 100) from least squares. Distances (r) and amplitudes (1) in Angstroms; angles (L) in degrees.
two such bonds to the single oxygen are formed in MoOCl, [10,12]. A detailed description of the bonding in CrOCl, and CrO,Cl, based on generalized valence-bond calculations has been given by Rappe and Goddard [12]: as far as the Cr=O bonds are concerned, two dn bonds are formed to the oxygen in the tetrachloride and only one to each oxygen in the dichloride, leading to a shorter Cr=O bond in the former. This description also accounts nicely for the Mo=O bond length differences in the corresponding molybdenum compounds. Thus, following Rappe and Goddard, it may be assumed that in MoO,Cl, two of the six valence electrons are involved in two bonds of considerable ionic character to Cl, and two are involved in each Mo=O bond. The Mo=O bonds are essentially covalent double bonds of which the d,(Mo) and p,(O) orbitals form a 0 component, and the d,,(Mo) and p,(O) a 71component; the pr electrons on 0 remain as a lone pair. In MoOCl,, four of the six valence electrons are in somewhat ionic bonds to Cl, leaving two d7celectrons on the MO atom for bonding to the 0x0 ligand. The Mo=O bond is essentially a triple bond comprising two 7cbonds made up of the dn orbitals on the MO atom, in combination with the p, and pr orbitals on oxygen and a donor-acceptor (T bond made up of the doubly occupied p, orbital on oxygen combined with the empty d, on molybdenum. It may be noted in passing that the bonding involving 32 valence-electron tetracoordinate species of some main group elements, such as SO,Cl,, appears to be quite different from that just discussed. In these compounds it is suggested [13] that d orbitals play little or no role, and that instead the structures reflect the operation of “negative hyperconjugation”. ACS reg. Nos: MoO,Cl, 13637-68-8, MoO,Br, 13595-98-7.
206
H. Thomassen and K. Hedberg/J. Mol. Struct., 273 (1992) 197-206
ACKNOWLEDGEMENT
Support of this work was provided by the National Science Foundation under grant CHEM-10070 to Oregon State University. REFERENCES 1 I.M. Zharskii, E.Z. Zasorin, V.P. Spiridonov, G.I. Novikov and V.N. Kupreev, Koord. Khim., 1 (1975) 574; Sov. J. Coord. Chem., 1 (1975) 473. 2 N.Ya. Shishkin and G.I. Novikov, Abstracts, 9th Austin Symp. on Gas-Phase Molecular Structure, Austin, TX, 1982, University of Texas, Austin, TX, 1982. 3 R.J. French, L. Hedberg, K. Hedberg, G.L. Gard and B.M. Johnson, Inorg. Chem., 22 (1983) 892. 4 C.J. Marsden, L. Hedberg and K. Hedberg, Inorg. Chem., 21 (1982) 1115. 5 Y. Morino and T. Iijima, Bull. Chem. Sot. Jpn., 35 (1962) 1661. 6 (a) G. Gundersen and K. Hedberg, J. Chem. Phys., 51 (1969) 2500. (b) L. Hedberg, Abstracts, 5th Austin Symp. on Gas-Phase Molecular Structure, Austin, TX, March 1974, No. T9, University of Texas, Austin, TX, 1974. 7 (a) A.W. Ross, M. Fink and R.L. Hilderbrant, International Tables for Crystallography, Vol. C, International Union of Crystallography, Kluwer Academic, Dordrecht/Boston/ London, 1992, p. 245. (b) D.T. Cromer and J.B. Mann, J. Chem. Phys., 47 (1967) 1892. (c) D.T. Cromer, J. Chem. Phys., 50 (1969) 4857. 8 L. Hedberg, Abstracts, 8th Austin Symp. on Gas-Phase Molecular Structure, Austin, TX, March 1978, No. TAlO, University of Texas, Austin, TX, 1978. 9 (a) W. Levason, R. Narayanaswamy, J.S. Ogden, A.J. Rest and J.W. Turff, J. Chem. Sot., Dalton Trans. (1982) 2009. (b) C.G. Barraclough and J. Stals, Aust. J. Chem., 19 (1966) 741. (c) T.V. Iorns and F.E. Stafford, J. Am. Chem. Sot., 88 (1966) 4819. (d) B.G. Ward and F.E. Stafford, Inorg. Chem., 7 (1968) 2569. (e) V.M. Kovba and A.A. Mal’tsev, Zh. Neorg. Khim., 20 (1975) 22; Russ. J. Inorg. Chem., 20 (1975) 11. (f) I.R. Beattie, K.M.S. Livingstone, D.J. Reynolds and G.A. Ozin, J. Chem. Sot. A (1970) 1210. 10 K. Iijima and S. Shibata, Bull. Chem. Sot. Jpn., 48 (1975) 666. 11 F.A. Cotton and R.M. Wing, Inorg. Chem., 4 (1965) 867. 12 A.K. Rappe and W.A. Goddard, J. Am. Chem. Sot., 104 (1982) 3287. 13 A.E. Read and P.V.R. Schleyer, J. Am. Chem. Sot., 112 (1990) 1434.