Synthesis and Structural Characterization of ‘ Non-VSEPR ’ Structures of Oxo-Tungsten Complex

The crystal structure of [(CH3)4N]4 [WOCl4F][WO2Cl4] was determined by single crystal X-ray diffraction technique. The crystal is monoclinic, space group C 2/m, with a= 28.23(10) Å, b= 11.60(4) Å, c= 13.48(5) Å, β=118.43(7)o, V= 3886(2) Å, Z=4. The structure was solved by direct methods and refined by least-squares methods to a final R = 0.0512 for 3825 observed reflections with I>2σ(I). In crystal there are two crystallographic distinct anions, both with cis geometry; the O-W-F and OW-O angles are 97.5(3)° and 103.1(3)° respectively. All structures are cis configurations that confirm a preference for angles below 90° and 180° between cis and trans σ-donor ligands, respectively.


Introduction
Under certain circumstances, metal complexes with a formal d 0 electronic configuration may exhibit structures that violate the traditional structure models, such as the VSEPR concept or simple ionic pictures.Some examples of the behavior, such as the bent gas phase structures of some alkaline earth dihalides or the trigonal prismatic coordination of some early transition metal chalcogenides or pnictides, have been known for a long time.However, the number of molecular examples for 'non-VSEPR' structures has increased dramatically during the past decade, in particular in the realm of organometallic chemistry.At the same time, various theoretical models have been discussed, sometimes controversially, to explain the observed, unusual structures.Many d 0 systems are important in homogeneous and heterogeneous catalysis, biocatalysis (e.g.molybdenum or tungsten enzymes).
The development of improved models for chemical bonding has always benefited from specific cases in which the existing models fail.One well-known example is the failure of the widely used valence-shell electron-pair-repulsion (VSEPR) 1 model and also of simple ionic models to predict the bent structures of some of the molecular dihalides of the heavy alkaline earth metals (Ca, Sr and Ba) in the gas phase [2][3][4][5][6][7][8][9][10] .Many more complexes with a formal d 0 configuration of the central metal (and also with d 1 and d 2 configurations) have since been found, which do not conform to the simple structural models.
The increasing number of known non octahedral hexacoordinate d 0 , d 1 and d 2 complexes represents probably the most spectacular failure of the traditional structure models in this area of the periodic table.The octahedron, which is clearly favored by ligand repulsion 11 , has been and still is by far the predominant geometry for this coordination number, both in main group and in transition metal chemistry.
Depending on the actual combination of σ-donor and π-donor ligands, we may expect a considerable variety of coordination arrangements.Very recently, Seppelt et al 12 succeeded in carrying out initial solid-state structures determinations for four heteroleptic systems, [MoO(CH 3 ) 5 ], [WCl(CH 3 ) 5 ], [Mo(OCH 3 ) 2 (CH 3 ) 4 ] and [MoCl(OCH 3 ) 2 (CH 3 ) 3 ].While the first three complexes are close to trigonal-prismatic limit, the latter complex appears to prefer a distorted octahedral arrangement, which resembles strikingly a low-lying transition state computed for [WCl 3 (CH 3 ) 3 ].The results of these studies confirm a preference for angles below 90◌ ْ and 180◌ ْ between cis and trans σ-donor ligands, respectively.For studying the effect of various kinds of π-donor and d 0 system structures, we report here the synthesis and structures of oxo-tungsten.

Experimental
IR spectrum of the complex dispersed in KBr was recorded on a Shimadzu model 420 spectrophotometer.All reaction and air-sensitive product was manipulated in auto recirculating positive dry boxes, that purched by argon.Acetonitrile (Merck, PA) was twice distilled from phosphorus pentaoxide, to remove traces of moisture and stored over molecular sieves (type 4Å) activated at 390 °C.The (CH 3 ) 4 NF.2H 2 O was prepared from (CH 3 ) 4 NOH and HF as described in the literature 13 .

X-ray crystallography
The intensity data for light green prism crystal of complex was measured on a Bruker SMART 1000 CCD area-detector, temperature 153(2) K, Mo Ka radiation (k=0.71073A˚),graphite monochromator, phi and omega scans, Data collection and cell refinement with SMART 14 , data reduction with SAINT 15 , experimental absorption correction with SADABS 16 .Structure analysis and refinement: The structure was solved by direct methods (SHELXS-97) 17 ; refinement was done by full-matrix least-squares on F 2 using the SHELXL-97 program suite 17 .Crystal data and structure refinement are given in Table 1.

IR spectra
The assignments of IR bands are observed to be in good agreement with the X-ray structural data.The main stretching frequencies of the IR spectrum of the complex are tabulated in Table 2.The assignments of the IR spectrum in Table 2 refer to the cation and anion respectively.This type of hydrogen bonding in tetramethylammonium salts has been studied and reported by Harmon.The IR spectrum of this compound is similar to tetramethylammonium perchlorate salt.The IR spectrum shows the existence of hydrogen bonding.The comparison and studies of the IR spectrum are similar to in Harmon's study.

Description of structures
Tungsten is one of the important transition metals in biological systems 18 .It is an integral component of the multinuclear M center of nitrogenases and the active sites of a group of oxotransferases 19 .Its unique properties appear to be due to its ability to exist in a number of different oxidation states and to coordinate with a variety of ligands.W(V) and W(VI) are generally thought to be the most likely oxidation states involved.Mixed Valence tungsten (V, VI) Compounds have been known several years ago.The mixed valence compounds commonly formed in preparation process 20,21 .As seen crystal structure contains anionic Tungsten ions: a) tetrachlorofluorooxo ion b) tetrachlorodioxo.The coordination geometry about the molybdenum centre in each of these anions is essentially distorted octahedral.The X-ray data clearly demonstrate such inequality (Figures 1 and 2).(6)] is shorter than the sum of the single bond radi for W and the halides.The final atomic coordinates for the non-hydrogen atoms are given in Table 3 and selected bond lengths and angles are resumed in Table 4.
Table 3. Selected bond length [Å].However there are also several cases with strong π-donor ligands, for which π bonding has been considered to increase the deviations from the VSEPR-type arrangements.Depending on the actual combination of σ-donor and single or double-faced π-donor ligand, we may expected a considerably variety of coordination arrangements 22 .
The results of these studies confirm a preference for angles below 90º and below 180º between cis and trans δ-donor ligands, respectively.At the same time, the distortions allow the competing π-donor ligands to enhance their bonding to the metal.These conclusions are consistent with more general notions about the alleviation, by structural distortions, of conflicts between different π-donor ligands (or between δ-and π-donor ligands) competing for a given set of metal d orbitals 23 .The prototypical example for this type of competition is that discussed for distorted octahedral dioxotungsten complex, in which the two strong π-donor oxo ligand prefer a cisoid arrangement and the two weakest co-ligand is located trans to the oxo ligand, with typically small angles between them 24 .

Conclusion
The [(CH 3 ) 4 N] 4 [WOCl 4 F][WO 2 Cl 4 ].2CH 3 CN complex was synthesized, the hydrogen bonds forming between the electronegative atom of anion and methyl hydrogens of the cation.The structure of this compound belongs to the non-VSEPR structures.CCDC, Nos 191520 contains the supplementary crystallographic data for this paper.

Table 1 .
Crystal data and structure refinement for the complex

Table 2 .
The frequencies (cm -1 ) and assignment of cation and anion of compound (1)