Syntheses, Structures and Spectroscopic Studies of Dimolybdenum Complexes Containing Bridging Butyrate and Phosphine Ligands

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1 Journal of the Chinese Chemical Society, 2003, 50, Syntheses, Structures and Spectroscopic Studies of Dimolybdenum Complexes Containing Bridging Butyrate and osphine Ligands Maw-Cherng Suen a ( ), Jhy-Der Chen b *( ), Tai-Chiun Keng c ( ) and Ju-Chun Wang c ( ) a Department of Textile Engineering, Nanya Institute of Technology, Chung-Li 32023, Taiwan, R.O.C. b Department of Chemistry, Chung-Yuan Christian University, Chung-Li, Taiwan, R.O.C. c Department of Chemistry, Soochow University, Taipei, Taiwan, R.O.C. By reactions of Mo 2 (O 2 CPr n ) 2 (P 3 ) 2 ( = Cl, 1;=Br,2) with 2 PCH 2 CH 2 P()CH 2 CH 2 P 2 (etp) in CH 2 2, the quadruply bonded complexes containing bridging butyrate and tridentate phosphine ligands of the type Mo 2 (O 2 CPr n ) 3 ( 3 -etp) ( = Cl, 3;=Br,4) were prepared. Their UV-vis and 31 P{ 1 H}-NMR spectra have been recorded and the structures of 1 and 2 have been determined by -ray crystallography. Crystal data for 1: space group P2 1 /c, a = (2) Å, b = (4) Å, c = (3) Å, = (3), V = 2130 Å 3,Z= 2, with final residuals R = and Rw = Crystal data for 2: space group P2 1 /c, a = (1) Å, b = (1) Å, c = (1) Å, = (1), V = (3) Å 3,Z=2,with final residuals R = and Rw = The * transition energies, 31 P{ 1 H}-NMR chemical shifts and the coupling constants are dependent on the natures of the halogen atoms and the carboxylate ligands. The through metal-metal couplings 3 J P-Mo-Mo-P for complexes of the type Mo 2 (O 2 CR) 3 ( 3 -P 3 ), which contain 3 -polydentate phosphine ligands are about 20 2 Hz. Keywords: Dimolybdenum complex; osphine ligand; -ray; NMR; Absorption spectrum. INTRODUCTION Transition metal complexes containing polydentate phosphine ligands have been subjected to many studies of their catalytic ability, structure-bonding relationship and application in solid-state material. 1 The coordination of the polydentate phosphines to the dimolybdenum unit exhibited detailed structural and bonding information due to metalphosphorus and phosphorus-phosphorus coupling constants. 2 In previous papers, we reported the syntheses, structures and 31 P{ 1 H}-NMR spectra of several dimolybdenum complexes of the types, Mo 2 (O 2 CCR 3 ) 3 ( 3 -etp) (R=HorF,=ClorBr and etp = 2 PCH 2 CH 2 P()CH 2 CH 2 P 2 ), 2a,2b Mo 2 (O 2 CCH 3 )- Cl 3 ( 3 -tetraphos-2) (tetraphos-2 = P(CH 2 CH 2 P 2 ) 3 ), 2a and Mo 2 Cl 4 ( 3 -etp). 2c All these complexes contain a quadruply bonded unit and a tridentate phosphine ligand coordinated to the metal centers with a chelating/bridging bonding mode. The 31 P{ 1 H}-NMR spectra were applied to study the chemical shifts of the phosphorus atoms and 31 P- 31 P coupling constants. As an extension of this chemistry, we have synthesized and studied complexes of the types Mo 2 (O 2 CPr n ) 2 - (P) 2 ( = Cl or Br) and Mo 2 (O 2 CPr n ) 3 ( 3 -etp), containing butyrate and phosphine ligands. The influence of the bridging butyrate ligand and the halogen atoms on the chemical shifts and coupling constants of the phosphorus atoms will be investigated. Their influence on the absorption energy and intensity of xy xy * transitions will also be discussed. EPERIMENTAL SECTION General Procedures All manipulations were carried out under an atmosphere of dry, oxygen-free nitrogen by using Schlenk techniques or a nitrogen box, unless otherwise noted. Solvents were dried and deoxygenated by refluxing over the appropriate reagents before use. Methanol was purified by distillation from magnesium, n-hexane and THF from sodium/benzophenone, and dichloromethane from P 2 O 5. The visible absorption spectra in THF were recorded on a Hitachi U-2000 spectrophotometer. The 31 P{ 1 H}-NMR spectra were recorded on a Bruker 200 MHz NMR spectrometer. Starting Materials The reagents Mo(CO) 6 and Bis(2-diphenylphosphinoethyl)phenylphosphine (etp) were purchased from Strem Chemical Co. Preparations of Mo 2 (O 2 CPr n ) 4 Mo(CO) 6 (10 g, mmol) were placed in flask con-

2 234 J. Chin. Chem. Soc., Vol. 50, No. 2, 2003 Suen et al. taining 50 ml o-dichlorobenezene, 70 ml acetic anhydride and 4 ml n-hexane. The mixture was then allowed to stir at reflux temperature for 3 days. The yellow solid was filtered, washed with methanol and ether and then dried in vacuum. Yield: 5.32 g (52%). UV-vis (CH 2 Cl 2 ): 438 nm. Anal. Calcd. for C 16 H 28 O 8 Mo 2 (MW = ): C, 35.27; H, Found: C, 34.93; H, Preparations of Mo 2 (O 2 CPr n ) 2 Cl 2 (P 3 ) 2 Mo 2 (O 2 CPr n ) 4 (0.66 g, 1.2 mmol) and P 3 (0.63 g, 2.4 mmol) were placed in a flask containing 20 ml THF, followed by addition of (CH 3 ) 3 SiCl (0.91 ml, 7.2 mmol). The mixture was then allowed to stir at room temperature for 3 hr. The red-pink solid was filtered, washed with hexane and ether and then dried in vacuum. Yield: 0.56 g (48%). UV-vis (CH 2 Cl 2 ): 521 nm. Anal. Calcd. for C 44 H 44 Cl 2 O 4 P 2 Mo 2 (MW = ): C, 54.77; H, Found: C, 54.71; H, Preparation of Mo 2 (O 2 CPr n ) 2 Br 2 (P 3 ) 2 Mo 2 (O 2 CPr n ) 4 (0.66 g, 1.2 mmol) and P 3 (0.63 g, 2.4 mmol) were placed in a flask containing 20 ml THF, followed by addition of (CH 3 ) 3 SiBr (0.95 ml, 7.2 mmol). The mixture was then allowed to stir at room temperature for 3 hr. The red-pink solid was filtered, washed with hexane and ether and then dried in vacuum. Yield: 1.05 g (83%). UV-vis (CH 2 Cl 2 ): 527 nm. Anal. Calcd. for C 44 H 44 Br 2 O 4 P 2 Mo 2 (MW = ): C, 50.19; H, Found: C, 50.27; H, Preparation of Mo 2 (O 2 CPr n )Cl 3 ( 3 -etp) Mo 2 (O 2 CPr n ) 2 Cl 2 (P 3 ) 2 (0.096 g, 0.1 mmol) and etp (0.053 g, 0.1 mmol) were placed in a flask containing 20 ml CH 2 Cl 2. The mixture was then stirred at room temperature for 3 h to yield a green solution. n-hexane was added to induce precipitation. The solid was filtered and washed with n- hexane and ether. The product was then dried under reduced pressure. Yield: 0.35 g (38%). UV-vis (CH 2 Cl 2 ): 643 nm. Anal. Calcd. for C 38 H 40 Cl 3 O 2 P 3 Mo 2 (MW = ): C, 49.25; H, Found: C, 48.74; H, Preparation of Mo 2 (O 2 CPr n )Br 3 ( 3 -etp) Mo 2 (O 2 CPr n ) 2 Br 2 (P 3 ) 2 (0.11 g, 0.1 mmol) and etp (0.053 g, 0.1 mmol) were placed in a flask containing 15 ml CH 2 Br 2. The mixture was then stirred at room temperature for 3 h to yield a green solution. n-hexane was added to induce precipitation. The solid was filtered and washed with n- hexane and ether. The product was then dried under reduced pressure. Yield: g (69%). UV-vis (CH 2 Cl 2 ): 644. Anal. Calcd. for C 38 H 40 Br 3 O 2 P 3 Mo 2 (MW = ): C, 43.27; H, Found: C, 43.29; H, ray Crystallography The diffraction data of Mo 2 (O 2 CPr n ) 2 Cl 2 (P 3 ) 2 was collected on a Siemens CCD diffractometer, which was equipped with graphite-monochromated Mo K ( = Å) radiation. Data reduction was carried by standard methods with use of well-established computational procedures. 3 Basic information pertaining to crystal parameters and structure refinement is summarized in Table 1. A red-pink crystal of Mo 2 (O 2 CPr n ) 2 Cl 2 (P 3 ) 2 was mounted on the top of a glass fiber with epoxy cement. Hemisphere data collection method was used to scan the data points at 2 < The structure factors were obtained after Lorentz and polarization corrections. The positions of the heavy atoms, including two molybdenum atoms, were located by the direct method. The remaining atoms were found in a series of alternating difference Fourier maps and leastsquare refinements. 4 The final residuals of the refinement were R = and Rw = The crystal data are listed in Table 1 and selected bond distances and angles are listed in Table 2. The crystallographic procedures for Mo 2 (O 2 CPr n ) 2- Br 2 (P 3 ) 2 were similar to those for Mo 2 (O 2 CPr n ) 2 Cl 2 (P 3 ) 2. The final residuals of the refinement were R = and Rw = The crystal data are listed in Table 1. Selected bond distances and angles are listed in Table 2. RESULTS AND DISCUSSION Syntheses The new starting complexes Mo 2 (O 2 CPr n ) 2 2 (P 3 ) 2 ( = Cl, 1; =Br,2) were prepared by reaction of Mo 2 - (O 2 CPr n ) 4 with (CH 3 ) 3 Si and two equivalents of P 3 in THF. The dimolybdenum complexes of the type Mo 2 - (O 2 CPr n ) 3 ( 3 -etp) (etp = 2 PCH 2 CH 2 P()CH 2 CH 2 P 2 ) ( = Cl, 3;=Br,4) containing bridging butyrate and 3 -etp ligands were prepared by the reactions of Mo 2 (O 2 CPr n ) (P 3 ) 2 with etp in CH 2 2. The reaction pathways for complexes 3 and 4 are shown in Scheme I. Clearly, the third halogen atoms of complexes 3 and 4 were obtained from the solvents. Structures Red-pink crystals of Mo 2 (O 2 CPr n ) 2 2 (P 3 ) 2 conform to the space group P2 1 /c with two molecules in a unit cell. Fig. 1 shows the representative ORTEP diagram for Mo 2 -

3 Dimolybdenum Complexes J. Chin. Chem. Soc., Vol. 50, No. 2, Table 1. Crystal Data for 1 and formula C 44 H 44 Mo 2 O 4 P 2 C 44 H 44 Br 2 Mo 2 O 4 P 2 fw space group P2 1 /c P2 1 /c a, Å 9.708(2) 9.737(1) b, Å (4) (1) c, Å (3) (1), (3) (1) V, Å (3) Z 2 2 d calc,g/cm cryst size, mm (Mo K ), mm data collcn instrum Siemens CCD Siemens CCD radiation monochromated in incident beam ( (Mo K ), Å) orientation reflcns no. all data > 10 (I) all data > 10 (I) temp. C scan method data collcn range(2 ), deg no. of unique data no. of observed data criterion for observed F 4.0 (F) F > 4.0 (F) no. of params refined R a Rw b quality-of-fit indicator c largest shift/esd, final cycle largest peak, e/å a R F o F c F o. b R w [ w( F o F c ) 2 w F o 2 ] 1/2.w 1/[ 2 (F o ) gf 2 o ]. c quality-of-fit [ w( F o F c ) 2 /N observed N parameters )] 1/2 Table 2. Selected Bond Distances (Å) and Angles for 1 and 2 Cl, 1 Br, 2 Mo-Mo(a) 2.102(1) 2.097(1) Mo-P 2.542(2) 2.548(1) Mo (2) 2.554(1) Mo-O(1) 2.117(3) 2.120(2) Mo-O(2a) 2.106(4) 2.095(3) -Mo-P 139.3(1) 137.8(1) P-Mo-O(1) 92.7(1) 92.6(1) P-Mo-Mo(a) 103.7(1) 105.6(1) -Mo-O(2a) 87.9(1) 87.6(1) O(1)-Mo-O(2a) 176.5(1) 176.6(1) -Mo-O(1) 91.5(1) 92.1(1) -Mo-Mo(a) 116.7(1) 116.3(1) O(1)-Mo-Mo(a) 91.0(1) 91.0(1) P-Mo-O(2a) 85.5(1) 85.3(1) Mo(a)-Mo-O(2a) 92.3(1) 92.2(1) (O 2 CPr n ) 2 Cl 2 (P 3 ) 2. It is seen from Fig. 1 that one halogen atom and one triphenylphosphine ligand coordinate each of Scheme I Reaction scheme for complexes 3 and 4 R C O O P 3 Mo Mo + etp 3 P O O C R CH 2 2 the molybdenum atoms. The two molybdenum atoms are also bridged by two butyrate ligands, which are trans to each other. The core structure is similar to those of Mo 2 (O 2 CC 6 H 5 ) 2 - Br 2 (P(n-Bu) 3 ) 2, 5,6 Mo 2 (O 2 CMe) 2 Cl 2 (P(n-Bu) 3 ) 2, 7 Mo 2 - (O 2 CCMe 3 ) 2 Cl 2 (PEt 3 ) 2, 8 and Mo 2 (O 2 CMe) 2 Cl 2 (P 3 ) 2. 9 Table 3 lists some of the important bond distances for these complexes and complexes 1 and 2. It is clearly seen that the Mo-Mo, Mo-P and Mo-O distances are all similar, regardless O Mo P(3) P(2) R C O Mo P(1) R=CH 2 CH 2 CH 3,=Cl,3 R=CH 2 CH 2 CH 3,=Br,4

4 236 J. Chin. Chem. Soc., Vol. 50, No. 2, 2003 Suen et al. Table 3. Bond Distances and Angles for Complexes of the Types Mo 2 (O 2 CR) 2 2 (PR 3 ) 2 Complexes Mo-Mo Mo- Mo-P Mo-O Reference Mo 2 (O 2 C) 2 Br 2 [P(n-Bu) 3 ] (3) 2.566(2) 2.526(5) 2.11(1) 5,6 Mo 2 (O 2 CMe) 2 Cl 2 [P(n-Bu) 3 ] (1) 2.427(2) 2.501(3) 2.105(5) 7 Mo 2 (O 2 CCMe 3 ) 2 Cl 2 (PEt 3 ) (4) 2.482(5) 2.513(4) 2.105(0) 8 Mo 2 (O 2 CMe) 2 Cl 2 (P 3 ) (1) 2.405(2) 2.566(2) 2.103(4) (1) 2.410(2) 2.542(2) 2.117(3) this work (1) 2.554(1) 2.548(1) 2.120(2) this work of the natures of the phosphine and the carboxylate ligands. The Mo-Br distance is larger than that of Mo-Cl, which is due to the larger atomic radius in a Br atom. Although the structures of 3 and 4 were not characterized by -ray crystallography, their structures are definitely the same as those shown in Scheme I, based on the elemental analyses and spectroscopic studies. In the structures of 3 and 4, one of the molybdenum atoms is coordinated by one halogen atom and two phosphorus atoms while the other molybdenum atom is coordinated by two halogen atoms and one phosphorus atom. The two molybdenum atoms are also bridged by one butyrate ligand. Spectroscopic studies (1) Absorption spectrum The absorption spectra for complexes 1-4 are typical for complexes containing metal-metal quadruple bond. 10 Table 4 lists lowest energy bands as well as their intensities for Fig. 1. ORTEP drawing of 1 in its entirety. complexes 1-4 and similar complexes. The lowest energy bands can be assigned to * transitions. The red shifts 11,12 for xy xy * transitions as the halogen atoms become more reducing are clearly seen for complexes of the type Mo 2 (O 2 CR) 2 2 (P 3 ) 2, but not for complexes of the type Mo 2 (O 2 CR) 3 ( 3 -etp). The energies show larger red-shifts for the complexes with the CF 3 COO - ligands. However, elongation of the alkyl chain from acetate to butyrate ligands does not affect the xy xy * transition energies. The average red shifts from the more reducing CH 3 CH 2 CH 2 COO - to CF 3 COO - is 252 cm -1 for the complexes of the type Mo 2 (O 2 CR) 2 2 (P 3 ) 2, and 215 cm -1 for the complexes of the type Mo 2 (O 2 CR) 3 ( 3 -etp). It is also noted that the absorption coefficients of these complexes are significantly smaller than that of Mo 2 Cl 4 (PEt 3 )( 3 -tetraphos-2) (660 nm, 3590 M -1 cm -1 ). The results further verify the proposed intensityborrowing 13 mechanism that complexes with oxygen donor ligands should have higher energy LMCT states than Cl -,Br - or I - containing compounds, and thus a smaller probability for mixing between the LMCT character and the * transition. (2) NMR spectroscopy Fig. 2 shows the 31 P{ 1 H}-NMR spectra of complexes 3 (top) and 4 (bottom). Both of the spectra consist of three sets of peaks that are coupled to each other. It is clearly seen from Fig. 2 that the 31 P{ 1 H}-NMR chemical shifts of complexes 3 and 4 are dependent on the nature of the halogen atom. To assign the chemical shifts to their corresponding phosphorus atoms, we invoked the same procedures for the complex Mo 2 (O 2 CCH 3 )Cl 3 ( 3 -etp). 2a The chemical shifts and coupling constants are listed in Tables 5 and 6, respectively, which have been verified with their simulation spectra. Table 5 also shows the coordination shifts ( 31 P= 31 P complex - 31 P ligand ) for atoms P(1), P(2) and P(3). It is noted that atoms P(2) and P(3) which are coordinated to the same Mo atom have similar coordination shifts. The P(1) atoms that coordinated to the Mo centers with two halogen atoms and constrained in a six-membered ring have the largest coordination shifts. Other things being equal, the atoms P(3) in the chloro

5 Dimolybdenum Complexes J. Chin. Chem. Soc., Vol. 50, No. 2, Table 4. Absorption Energies and Extinction Coefficients of the * Transitions for Complexes of the Types Mo 2 (O 2 CR) 2 2 (P 3 ) 2 and Mo 2 (O 2 CR) 3 ( 3 -etp) (R CH 3,CF 3 or Pr n ; Cl or Br) Complexes Absorption nm Energy (cm -1 ) Extinction coefficient (19193) (18975) 1857 Mo 2 (O 2 CCH 3 ) 2 Cl 2 (P 3 ) (19267) 1041 Mo 2 (O 2 CCH 3 ) 2 Br 2 (P 3 ) (19011) 1941 Mo 2 (O 2 CCF 3 ) 2 Cl 2 (P 3 ) (18939) 1107 Mo 2 (O 2 CCF 3 ) 2 Br 2 (P 3 ) (18726) (15552) (15527) 1358 Mo 2 (O 2 CCH 3 )Cl 3 ( 3 -etp) 644 (15528) 1536 Mo 2 (O 2 CCH 3 )Br 3 ( 3 -etp) 643 (15552) 1204 Mo 2 (O 2 CCF 3 )Cl 3 ( 3 -etp) 652 (15337) 1062 Mo 2 (O 2 CCF 3 )Br 3 ( 3 -etp) 653 (15313) 1459 complexes have larger coordination shifts than those in the bromo complexes. As seen in Table 6, the coupling constants of the two phosphorus atoms that form the bridge, i.e. P(1) and P(2), are zero for complexes 3, 4, Mo 2 (O 2 CCH 3 )Cl 3 ( 3 -etp) and Mo 2- (O 2 CCH 3 )Br 3 ( 3 -etp), while those for the Mo 2 (O 2 CCF 3 )Cl 3 - ( 3 -etp) and Mo 2 (O 2 CCF 3 )Br 3 ( 3 -etp) are 5.6 and 4.4 Hz, respectively. The butyrate and acetate ligands which have higher trans influence (better coordination ability to the Mo atoms) than CF 3 COO - ligand weaken the trans Mo-P bonds by causing rehybridization of the Mo-P orbitals. This increases the Mo-P bond distance and decreases the magnitude of 3 J P-Mo-Mo-P to a value similar to that of 3 J P-C-C-p. Since the phases of these two coupling constants are of the opposite signs, zero values are observed for the complexes 3, 4, Mo 2 (O 2 CCH 3 )Cl 3 ( 3 -etp) and Mo 2 (O 2 CCH 3 )Br 3 ( 3 -etp). The 2 J P(2)-P(3) values are 55.3 and 53.9 Hz for complexes 3 and 4, respectively, which are similar to those for the acetate complexes, but significantly larger than those for the trifluoroacetate complexes. If the observed couplings, 2 J P(2)-P(3) and 31 Fig. 2. P{ 1 H}-NMR spectra of 3 (top) and 4 (bottom) in CDCl3.

6 238 J. Chin. Chem. Soc., Vol. 50, No. 2, 2003 Suen et al. Table P{ 1 H}-NMR Chemical Shifts and Coordination Shifts ( ) formo 2 (O 2 CR) 3 ( 3 - etp)(r = CH 3,CF 3 or Pr n ; Cl or Br) Compound P(1) P(2) P(3) Mo 2 (O 2 CCH 3 )Cl 3 ( 3 -etp) 46.61(60.27) 21.80(37.06) 30.46(42.12) Mo 2 (O 2 CCH 3 )Br 3 ( 3 -etp) 46.79(58.45) 23.92(38.18) 25.91(37.58) Mo 2 (O 2 CCF 3 )Cl 3 ( 3 -etp) 46.19(57.85) 22.17(37.43) 30.43(42.09) Mo 2 (O 2 CCF 3 )Br 3 ( 3 -etp) 45.92(57.58) 25.87(41.43) 27.49(39.15) (59.55) 20.89(36.15) 28.97(40.63) (58.37) 23.77(39.03) 25.78(37.42) Table P{ 1 H}NMR Coupling Constants for Mo 2 (O 2 CCR 3 ) 3 ( 3 -etp) (R CH 3,CF 3 or Pr n ; Cl or Br) Compound P(1)-P(2) P(1)-P(3) P(2)-P(3) Mo 2 (O 2 CCH 3 )Cl 3 ( 3 -etp) Mo 2 (O 2 CCH 3 )Br 3 ( 3 -etp) Mo 2 (O 2 CCF 3 )Cl 3 ( 3 -etp) Mo 2 (O 2 CCF 3 )Br 3 ( 3 -etp) J P(1)-P(2) can be divided into metal, 2 J P-Mo-P or 3 J P-Mo-Mo-P, and backbone, 3 J P-C-C-P, contributions, where the latter is assumed to be similar to the coupling in the free ligand, 14 the magnitudes of the observed coupling constants would require the contributions from 2 J P-Mo-P and backbone coupling 3 J P-C-C-P to be of the same sign, while the contributions from 3 J P-Mo-Mo-P, and 3 J P-C-C-P in the same six-membered ring to be of the opposite sign, assuming the Karplus-type angular dependence of the coupling constants in the ring system to be minimal. Comparison of the 3 J P-C-C-P values of the free etp ligand which is 19.7 Hz with the observed coupling constants of 3 and 4 clarify this aspect. It is also noted that the coupling constants of P(1)-P(3), which couple only through metal-metal bonds, are similar and are 20.6 and 18.9 Hz, respectively. We thus conclude that the through metal-metal couplings 3 J P-Mo-Mo-P for complexes 3 and 4 are about 20 2 Hz. This result is similar to those expected for the complexes Mo 2 (O 2 CCH 3 )Cl 3 - ( 3 -etp), 2a Mo 2 (O 2 CCR 3 ) 3 ( 3 -etp) (R =HorF;=Clor Br) 2b and Mo 2 (O 2 CCH 3 )Cl 3 ( 3 -tetraphos-2). 2a ACKNOWLEDGEMENT We thank the National Science Council of the Republic of China for support. Received July 24, REFERENCES 1. Mayer, H. A.; Kaska, W. C. Chem. Rev. 1994, 94, (a) Lee, C.-T.; Chiang, S.-F.; Chen, C.-T.; Chen, J.-D.; Hsiao, C.-D. Inorg. Chem. 1996, 35, (b) Suen, M.-C.; Chiang, S.-F.; Chen, J.-D.; Chern, S.-S.; Hsiao, C.-D. J. Chin. Chem. Soc. 1998, 45, 263. (c) Lee, C.-T.; Yang, W.-K.; Chen, J.-D.; Liou, L.-S.; Wang, J.-C. Inorg. Chim. Acta 1998, 274,7. 3. SMAR/SAINT/ASTRO, Release 4.03, Siemens Energy and Automation Inc., Madison, WI, Sheldrick, G. M. SHELTL, Release 5.03, Siemens Analytical -ray Instruments Inc., Karlsruche, Germany, Potenza, J. A.; Johnson, R. J.; Filippo, J. S. Inorg. Chem. 1976, 15, Filippo, J. S.; Sniadoch, R. J. Inorg. Chem. 1975, 15, Prout, K.; Parkin, G. J. Soc. Dalton Trans. 1982, Arenivar, J. D.; Mainz, V. V.; Andersen, R. A.; Zalkin, A. Inorg.Chem. 1982, 21, Cotton, F. A.; Powell, G. L. Polyhedron 1985, 4, Cotton, F. A.; Walton, R. A. Multiple Bonds between Metal Atoms; 2nd ed.; Oxford University Press: London, Hopkins, M. D.; Schaefer, W. P.; Bronikowski, M. J.; Woodruff, V. M.; Miskowski, V. M.; Dallinger, R. F.; Gray, H. B. J. Am. Chem Soc. 1987, 109, Chen, J.-D.; Cotton, F. A.; DeCaino, E. C. Inorg. Chim. Acta 1990, 176, Hopkins, M. D.; Gray, H. B.; Miskowski, V. M. Polyhedron 1987, 6, Grim, S. O.; Barth, R. C.; Mitchell, J. D.; Gaudio, J. D. Inorg. Chem. 1977, 16, 17.