Tungsten bidentate

Complexes of molybdenum and tungsten with bidentate sulfur ligands have been investigated extensively. In recent years, the work in this field has been escalated by the impetus of designing models of such bioinorganic enzymes as nitrogenase and xanthine oxidase (125). The early work reviewed by Coucouvanis (1) dealt exclusively with the isolation of oxomolybdenum(V) and -(VI) species. 

Preparation of Tungsten Tetracarbonyl Complexes with Bidentate P.O-Bound... 

The stability of complexes of the type [W(OR)6] is quite remarkable. It has been shown28 that short exposure of [W2(OPri)6py2] to approximately two molar equivalents of CO gives two products, one of which is (1). This structure can best be viewed as a substituted tungsten carbonyl in which two cis CO groups have been replaced by donor atoms of the bidentate neutral ligand [W(OPr )6], This complex is unusual in that it contains two atoms of the same element in oxidation states that differ by six units. 

This section describes the structural chemistry of 14 molecular structures, 3 of chromium, 8 of molybdenum, and 3 of tungsten. The only binary xanthate structures are those of chromium, all other structures are at least ternary in nature. For the chromium and tungsten structures, conventional monodent-ate and bidentate coordination modes of the xanthate ligand are found. By contrast, a rich, diverse, and fascinating coordination range is observed for the... 

Bis[dw-butyLphosphano] tellurium replaces norbornadiene, cycloheptatriene, acetonitrile, and dimethyl(methylene)oxosulfurane in chromium, molybdenum, and tungsten complexes. The P —Te compound acts as a bidentate ligand with the two phosphorus atoms coordinated to the metal atom1. The tetracarbonylchromium complex [R = CH(CH3)J can be recrystallized without loss of tellurium2. 

The very air-sensitive compounds [Mo(terpy)2] (purple) and [W(terpy)2] (green) have been described by a number of workers 44,174). The chromium compound may be prepared by the reaction of [Cr(CO)g] 44), [Cr(CN)g] 42,47), [Cr(CO)3(C6H6)] 47), or [Cr(bipy)3] 44) with two equivalents of terpy. The molybdenum 44,174) and tungsten 44) compounds have been prepared from [M(CO)g] in a similar manner. Electrochemical studies on [Mo(terpy)2] indicate three one-electron oxidations 8). Photosubstitution reactions of [M(CO)g] in the presence of limited amounts of terpy lead to the formation of [M(CO)4(terpy)] (M = Cr, Mo, or W) 205). It is unlikely that these compounds are seven-coordinate, and they may well provide examples of a bidentate terpy. [Mo(CO)3(terpy)] may be prepared by the reaction of terpy with [Mo(CO)3(mesitylene)], but attempts to prepare the other group VI complexes of this stoichiometry lead to the formation of [M(CO)4(terpy)] 205). 

Other types of tungsten-alkene complexes that merit mention include the first group six jj -benzene complex, TpW(NO)(PMe3) ((( -CeHe), that contains a tt-bound benzene ligand activated toward Diels-Alder cycloaddition.Also, the jj -fiillerene complexes, mgr-W(CO)3(L-L)(jj -C6o) (65), have been prepared from W(CO)4(L-L) (L-L = a bidentate N- or P-donor ligand) in the presence of In some... 

These complexes are excellent models for theoretical studies. The octacyano complexes of molybdenum and tungsten are stable and inert toward substitution reactions and therefore very suitable for theoretical studies of redox reactions and application of the Marcus theory. The photoreactivity of these systems is also proving to be important. The 0X0- and nitridocyano complexes of Mo(IV), W(IV), Tc(V), Re(V), and Os(VI) are very good candidates for kinetic studies of substitution reactions with both mono- and bidentate ligands and are of interest especially in view of the large variations in the observed reactivity. 

The fact that the substitution reactions of the tungsten complex with monodentate ligands (see Table VII) are about 1000 times faster than those with bidentate ligands (2-picolinic acid) (214), whereas the kinetic results point to a dissociative mechanism (see Section IIIDl), is... 

---