Emerging trends in organotellurolate chemistry derived from platinoids

This perspective begins with the discussion of various basic synthetic approaches applied for the synthesis of several organotellurium ligands, their chemistry derived from platinum group metals, and the reactivity difference among them. It also gives an overview on the development of various bi, tri, and high nuclearity complexes syntheses. Investigations targeting the organotellurium ligand systems revealed a remarkable reactivity due to the dynamic nature of the lone pair available on the tellurium metal, which has led to a serendipitous isolation of the complexes [Cp*Ir(ppy)(h-Te2Ph2)] , [Cp*IrOs3(m-H)2 (m-Tetol)2(CO)7], [Pt{TeC5H3(3-R)N}2Te(PPh3)], [Pt{Ph2PCH(TeC5H3(3-R)NPPh2)}2] (R 1⁄4 H, Me), and various other high nuclearity heterometallic [Cp*Ir(CO)(m-TeC6H4)2MCp*Cl]Cl (M 1⁄4 Rh, Ir) complexes. Studies of the various complexes investigated the various binding modes of coordination and the facile cleavage of the Te–C and Te–Te bonds of tellurium-based ligand systems. Attempts have been made to present a comprehensive account of the subject matter. Various promising aspects of these complexes, such as their synthesis, reactivity, structures, and applications, are covered in this review.


Introduction
Organosulphur and selenium derived metal complexes have been well documented for more than several decades. 1-4 When one considers its homologues analog involving tellurium elements, it is clear that tellurium chemistry is still relatively uncharted. Due to their high reactivity, air sensitivity, and diffusive sets of orbitals, tellurium complexes are unstable and hence unexplored. Thus, the prompt reactivity and rich coordination of organotellurium ligand systems has drawn substantial interest over the last few years. 5- 10 The metalloid character of tellurium makes it amphoteric (acid as well as base) in nature depending upon the nature of the reactive substrate. 6,[11][12][13] Therefore, the reactions of organotellurium ligands with platinum group metal precursors represent an active area for further research. [14][15][16][17][18] The noteworthy reactivity of platinoids has been utilized in synthetic chemistry to isolate products in quite good yields with great selectivity under normal conditions. 19 These complexes are not only used as precursors but are also envisaged as an active species in various catalytic reactions. In particular, the superior stability of the platinum family complexes endow them with an opportunity to be utilized to comprehend the mechanistic details as well as the nature of complexes involved in particular catalytic cycles. 20 Renewed interest in the eld of coordination chemistry derived from organotellurium ligand systems has been stimulated by a number of recent publications 7,8,21-23 dealing with the oxidative addition reactions of platinum group metal precursors with various diarylditelluride ligand systems. The outcomes of these reactions depend upon the nature of the solvent, especially in chlorinated solvents, which yield multinuclear complexes together with several unidentied products; [21][22][23] whereas, the similar reaction in solvents like benzene and toluene affords different multinuclear complex. The formation of several products mainly results from the bond cleavage of Te-C bonds rather than Te-Te bonds. 22,23 The comparable bond energies of Te-Te and Te-C bonds and the increased metallic character of Te may be the reason for the unexpected reactivity of organotellurium ligands.
Thus, this perspective aims to cover the various synthetic approaches for organotellurium ligands, different aspects and versatility of platinum group metal complexes derived from these ligand systems, with a particular emphasis on hemilabile tellurolate ligand systems since the subject was recently explored.

Telluroethers
The synthetic approaches applied for the synthesis of various monodentate, bidentate, and hybrid telluroethers have been reviewed previously by various groups. 6,[24][25][26][27][28] However, there have been no signicant recent developments reported. The common synthetic methods applied for the synthesis of telluroethers are highlighted in the following.
2.1.2. In situ generation of Na 2 Te. The second most efficient strategy for the formation of telluroethers is the in situ formation of Na 2 Te, followed by the addition of the corresponding alkyl halides to isolate the desired product. As an example, the class of cyclic ditelluroether [8]aneTe 2 was synthesized in a similar fashion by the addition of a half equivalent of 1,3 dibromomethane in a THF solution of Na 2 Te, followed by the addition of NaBH 4 and a further equivalent of 1,3 dibromomethane. 29,30 The syntheses of mixed donor thia/tellura [9]aneS 2 Te 2 , [11]aneS 2 Te 2 , [12] aneS 2 Te 2 , [14]aneS 3 Te macrocyclic ligands and tripodal telluroether were also carried out based on the same strategy. [31][32][33][34][35] 2.1. 3. Applying a Grignard reagent. Currently, Grignard reagent is also used as a promising methodology for the formation of telluroethers, especially for the isolation of heteroaromatic analogs and for unsymmetrical telluroether synthesis. In this case, the insertion of tellurium metal in to the Grignard reagent of the organic moiety corresponding to the expected telluroether is performed rst. Subsequently, the addition of halo aryl or alkyl is followed in freezing conditions. Recently, an unsymmetrical 1-naphthyl-based telluroether and symmetrical pyridyl telluroethers have been isolated with the same strategy. [36][37][38] In order to justify the mechanistic details, we performed the following process: (i) halopyridine underwent a complete metal-halogen exchange reaction with i-prMgCl to give pyridyl magnesium chloride, (ii) the latter compound was reacted with an equivalent of tellurium metal via an insertion mode, (iii) followed by the addition of alkyl/ aryl halide to yield the desired telluroether with the elimination of MgCl 2 .

Diorganotellurides
The majority of synthetic approaches have been applied to the synthesis of various ditelluride ligand systems. Among these, the insertion of elemental tellurium in to reactive M-C bonds is a quite common methodology. Various approaches are described below.
2.2.1. Insertion of chalcogen in to Li-Aryl bond. The insertion of elemental tellurium in to the Li-aryl bond followed by oxidation has been performed for the synthesis of various ditelluride ligand systems. The lithiation of bromoaryl compounds has been accomplished by the substitution of the bromo group through lithium metal with reagents like n-BuLi 39,40 or Bu t Li 41 in polar solvents like THF and petroleum ether at a temperature of À78 C. The resulting lithiated aryl group reacts with active elemental tellurium metal to yield a lithiated chalcogenolate ion, which on hydrolysis gives the corresponding diaryl ditelluride ligand systems.
2.2.2. Reaction of E 2À with haloaryl compounds. The reaction of heteroaromatic and aromatic halocompounds with ditellurido dianions in different solvents is another important synthetic pathway to synthesize a class of various ditelluride ligand systems. In most of the reactions, reducing reagents are generated through an in situ mode by a variety of reducing agents, like NaBH 4 , Li metal reaction, Na/NH 3 , and hydrazine. [42][43][44][45][46][47] Sodium borohydride reduction of elemental tellurium in ethanol, water, and ethoxy ethanol has been applied for the synthesis for Na 2 Te 2 . [45][46][47][48] The latter has been synthesized by an in situ and dropwise addition of haloaryl compounds, yielding the corresponding diaryl ditelluride; while hydrazine hydrate in the presence of NaOH in DMF has also been used to prepare Na 2 Te 2 , which on reaction with bromopyridine and chloropyrimidines, affords their corresponding ditellurides (Scheme 1). 49 2.2.3. Insertion of tellurium in to aryl magnesium halide. The lithiation of haloaryl compounds takes place at À78 C, but this cryogenic condition and the instability of the lithiated products results in a poor yield, which makes this method really an inconvenient route for the synthesis of ditellurides. In contrast, the stability and ease of handling of aryl magnesium bromide compounds makes this synthetic strategy quite trendy. Normally for all cases, a Grignard reagent of the corresponding alkyl or arylhalo compounds is rst synthesized, 36,37,50,51 followed by the insertion of elemental tellurium, which on acid hydrolysis yields the corresponding ditellurides in a moderate yield.

Rhodium and iridium
Very little work on rhodium and iridium complexes derived from organotellurium ligand systems, i.e., telluroethers, has been documented so far. Compared to ruthenium, the related references for rhodium and iridium complexes are less signicant in number. The reaction of TeR 2 (R ¼ Me, Et) with metallocyclic [(C 5 H 5 ) 2 Rh 2 (m-CO)(CF 3 CCF 3 )] derived an adduct in which TeR 2 was added to one of the rhodiums attached with bridging carbonyl, which was boned in a h 1 fashion. 69 Such a type of insertion followed by the addition of tellurium can be particularized due to diffusive sets of lone pairs and their ease of availability to coordination. While the reaction of RhCl 3   The uxional behavior of the latter complex was established by its 1 H NMR spectrum, which showed a single resonance for TeMe, CH 2 , and COD (Scheme 3). It was concluded, during a dynamic process, that a ip on and off movement takes place through the arm of the tripod around the metal center.
Similarly, when the reaction with RhCl 3 $3H 2 O is employed in various mole ratios with the ligand system Te(CH 2 SiMe 3 ), a variety of products has been isolated (Scheme 4) depending upon the mole ratio of the ligand (Fig. 2). 70 Most of these complexes have shown issues with their solubility in common organic solvents. The solubility issues have been overcome with the development of a new class of telluroether in which the backbone consists of a morpholine group. Rh(III) complexes derived from the same ligand N-{2aryltelluroethyl}morpholine (Scheme 4) 71 have shown ready solubility in all organic solvents other than diethyl ether and hexane, in which the complexes are sparingly soluble. Their high solubility makes them promising catalysts in the hydrogenation reactions of ketones.
Tellurium has also been ligated into a macrocyclic Schiff base, and the resulting ligand VII upon reaction with [PdCl 2 (PhCN) 2 ] yielded the complex [{PdCl 2 } 2 L]. In this complex,   palladium is coordinated with each tellurium as well as the nitrogen of the Schiff base. However, the same reaction in a 1 : 1 ratio of palladium precursor to the ligand gave the product [PdL] + VIII, 90 in which palladium is coordinated to the Te 2 N 2 core of the ligand, leaving the two nitrogen atoms uncoordinated (Scheme 6); 91 while in the case of [PtCl 2 (COD)], a symmetrical ring opening of the ligand took place to give the product IX, where platinum is coordinated to the TeN 2 C core. Surprisingly, the complexation mode of the tellurium ligand system was totally different from the similar selenium analogs.
In the case of the latter ligand system, palladium is directly coordinated to all four nitrogen atoms. 92,93 An interesting example of a halobridged complex, [M 2 X 2 (m- [93][94][95] These complexes were formed due to the chlorobridged cleavage reaction with the substitution of thio and seleoether through monotellurides. Usually, telluroether complexes are oriented in the cis form, which is slowly transformed to the trans isomer in solution. The conversion of the cis to the trans form was encountered in the 125 TeNMR spectrum of the latter complexes, which exhibited a single resonance rst, but in the longer acquisition results two prominent resonances were observed, attributed to the cis as well as trans forms. 94 An agostic interaction between telluroethers and metal atoms has also been documented in these complexes, where a toluene-methanol solution of [Pd 2 Cl 2 (m-Cl) 2 (TeMes 2 ) 2 ] or compound [PdCl 2 (-TeMes) 2 ] on reuxing for 30 min yielded a binuclear cyclopalladated complex [Pd 2 (m-Cl) 2 {CH 2 C 6 H 2 (4,6-Me 2 )TeMes 2 } 2 ] (Scheme 7). However, the latter binuclear compound was converted to the mononuclear compound [PdCl 2 {MesTeCH 2 C 6 -H 2 (4,6-Me 2 )TeMes} 2 ]. The formation of the mononuclear compound mainly arose due to nucleophilic attack of mesityl tellurolate at the Pd-C bond. 93 Recently, an intricate palladacycle was synthesized by the reaction between ditolyl telluride and palladium acetate in toluene solution, which yielded two complexes with the composition [(o-tolylTe) 2 OPd(OAc) 2 ] (Fig. 5) and trinuclear The former complex was a bidentate tellurinic acid anhydride, while the latter was coordinated to tolyl and telluride. 96

Ruthenium and osmium
The coordination chemistry of ruthenium cluster complexes toward the highly reactive organotellurium ligand is very rich in the literature, but still various aspects of its reactivity pattern are uncultivated. With this prospect, the reuxing of Ru 3 (CO) 12 with Na 2 Te 2 and PPh 4 PBr at 80-130 C for 8 h yielded the cluster complex [Ru 4 (Te) 2 (Te 2 ) 2 (TeMe) 2 (CO) 8 ](PPh 4 ) 2 . 97 The crystal structure of the latter complex consisted of four rectangles with a center of inversion in the midst. While the oxidative addition of diorganoditelluride to [Ru 3 (CO) 12 ] yielded a variety of compounds. In particular, the reaction of diphenyl ditelluride resulted in the formation of a binuclear complex bridged by phenyl tellurolate along with the polymeric form of [Ru(CO) 2 (m-TePh)] n . 98,99 Performing the same reaction with the addition of halogen yielded the complex [Ru 2 X 2 (CO) 6 (m-TePh) 2 ] (X ¼ Br, I), which was isolated by breaking of the Ru-Ru bond. 99 Similarly, on reuxing a phosphine precursor of the ruthenium complex [Ru 3 (CO) 10 (m-dppm)] with tetrahydrofuran solution of Ar 2 Te 2 (Ar ¼ C 6 H 4 OEt-4) in a 1 : 2 mole ratio for 6 h yielded a mixture of the products [Ru 2 (CO) 4 (m-TeAr) 2 (m-dppm)], [Ru 2 (CO) 6 (m-TeAr) 2 ] and [Ru(CO) 4 (TeAr) 2 ] (Scheme 8). 100 However in toluene solution, other than [Ru 2 (CO) 4 (m-TeAr) 2 (m-dppm)], several products 101 have been afforded due to the competitive cleavage of Te-Te and Te-C bonds. At room temperature in CH 2 Cl 2 , reaction with a diphenyl ditelluride ligand system yielded the binuclear compound [Ru 2 (m-TePh) 2 (CO) 4 (m-dppm)] as well as the trinuclear unsaturated clusters [Ru 3 (m 3 -Te) 2 (m-TePh) 2 (CO) 6 (m-dppm)] and [Ru 2 (m 3 -Te)(m-TePh) 3 (CO) 6 (h 1 -COPh)(m-dppm)]. 102 Heating   (Fig. 7), which mainly differ from the phenyl orientation attached to the telluride metal center. 116 A very close output was obtained when performing the oxidative addition reaction of (TeTol) 2 with the mixed cluster [Cp*IrOs 3 (m-H) 2 (CO) 10 ]. 117 In this case, three cluster complexes were isolated with the composition [Cp*IrOs 3 (m-H) 2 (m-Tetol) 2 (CO) 7 ] (Fig. 8). These clusters had relatively different orientations of the tolyl group around the tellurium center, with two of them being stereoisomers having the tolyl group orientation away from the cluster core, i.e., exo, or inward toward the core, i.e., endo. These possibilities of obtaining various stereoisomers . 121 The reaction of iridium carbonyl clusters with diphenyl ditelluride is very selective with a very sluggish rate, requiring 20 h of continuous stirring to complete the reaction. The reuxing of PhTeTePh and [Ir 6 (CO) 15 ] 2À gave the anionic cluster [Ir 6 (CO) 14 (m-TePh)] À in tetrahydrofuran solution. 122 By applying the same experimental conditions with a 2 : 1.5 mole ratio of PhTeTePh with [Ir 6 (CO) 15 ] 2À yielded exclusively the neutral product [Ir 6 (CO) 13 (m-TePh) 2 ]; however, similar reactions with other chalcogenides yielded a mixture of products in which a similar neutral product was isolated in very poor yield via a solvent extraction methodology. The best strategy applied to isolate the above neutral complex was the reaction of [Ir 6 (CO) 16 ] with PhTeTePh in reuxing toluene. The same reaction with other chalcogenides was much less effective in terms of isolation of the cluster compound (Scheme 11). 122 In an attempt to synthesize hyper-valent iridium complexes, organotellurium compounds have played a key role to isolate such complexes. In this context, an oxidative addition of Ph 2 Te 2 with the Ir(I) compound [Cp*Ir(ppy)(solv)] + was applied. 123 Surprisingly the reaction led to the formation of the Ir(III) h 1 -ditelluride complex [Cp*Ir(ppy)(h 1 -Te 2 Ph 2 )](OTf). It is noteworthy that isolation of a complex with the coordination mode h 1 -REER is very much less common, e.g.
[Cp*Mn(CO) 2 ] 2 (m-h 1 -REER), 124 but it is strongly believed that these coordination modes derived compounds take part as an intermediate in the oxidative addition of R 2 E 2 (E ¼ S, Se, Te) to give the Pd(0) and Pt(0) precursors. 125 The proven potency from the dynamic nature of tellurium metal can be encountered with

Palladium and platinum
Tellurium complexes with palladium and platinum phosphine precursors are comparatively more stable than any other precursors. The main stability factor is the p bonding involving d xy orbitals of the palladium and platinum metals with the available empty orbitals of phosphine, which results in the extent of s overlapping being stronger in the phosphine complexes. 127,128 Hence, these complexes play a crucial role in a reduction of the electron density around the metal center, which is enhanced due to the ease of donation of the lone pair available on the tellurium center. On the other hand, an alike reaction upon performing with [M(PPh 3 ) 4 ] (M ¼ Pd, Pt) with various ligand systems, such as CF 3 Te 2 Th 2 Te 2 , Ph 2 Te 2 , afforded mono-, bi-, tri-, and hexanuclear complexes [Pd 6 Te 6 Cl 2 (PEt 3 ) 6 ] (Fig. 10) depending on the nature of the solvent (Schemes 14 and 16). 21,[133][134][135][136][137][138] It has been well established that the bonding energy between C-Te and Te-Te is quite comparable compared to other analogs of the chalcogen family, and therefore the reaction with tellurium ligands afforded polynuclear compounds. Tanaka et al. found that upon performing an oxidative addition between Pd(0) or Pt(0) species with various telluroethers, cleavage of the C-Te bonds take place, leading to isolation of a compound with the composition [M(Ar)(TeAr)(PEt 3 ) 2 ] (M ¼ Pd, Pt) (Scheme 14). 22 Molecular orbital calculations also concluded that the activation energy barrier for the oxidative addition of dichlacogenides to [M(PH 3 ) 2 ] decreases in the order S > Se > Te (in terms of the addition of E-E bonds). 125 Hence, the exothermicity of the reaction is also decreased with respect to the M-E-R bond strength. This statement quanties that the oxidative addition of Te-Te bonds to low valent metal precursors is very simplistic compared to with the rest of the other dichalcogenides analogs and results in a complex (mononuclear) that is less stable compared to thiolato and selenolato complexes. However, the tendency for isolation of the dimerized product follows the reverse trend (S < Se < Te). Therefore, it can easily be concluded that binuclear complexes are higher in tellurium system compared to the other analogs.
Recently  in the presence of excess triphenyl phosphine. 139 4.3.2. Reactivity of various tellurolates with palladium and platinum chelated phosphine precursors. The reactivity of the chelating phosphine diphenylphosphinomethane (dppm) ligand is comparatively higher than that of other chelating phosphines. The main striking factor of the reactivity is strain, caused by the projection of the four-membered ring. The cone angle drawn on P-M-P ranges from 70-72 , which means it is highly acute. To overcome the acuteness, the complementary angle is widened in space to provide the space to react with the incoming ligand. In the case of the substitution reaction between [PtCl 2 (dppm)] and the sodium salt of aryl tellurolates (aryl ¼ Ph, Tol, Mes), cis congured mononuclear complexes were obtained. 133,140,141 However, a similar substitution and an oxidative addition with hemilabile ligand systems, like derivatives of 2 and 4-dipyridylditelluride, yielded an expected mononuclear complex as well as C-H activated [Pt{Ph 2 -PCH(TeC 5 H 3 (3-R)NPPh 2 } 2 )] (R ¼ H, Me) (Scheme 17). 8, 131 The latter complexes had a distorted square planer geometry around the metal, with the angle around the activated carbon center varying from 100-126 , which shows an allylic conguration of the carbon center (Fig. 12). It can be concluded that the reactivity of the same palladium phosphine precursors with 4-pyridyl tellurolates to lead to the tetranuclear compound [Pd 2 (m-Te)(m 2 -TeC 5 H 4 N)(4-TeC 5 H 4 N)(m-dppm)] 2 (ref. 131) can be rationalized by the high reactivity of the palladium phosphine precursor and tellurium-based ligand.
However, the chemistry related to diphenylphosphinoethane with tellurium systems seemed to be as expected. This may be due to the strainless ve-membered ring project by "dppe" ligand. On performing an oxidative addition/substitution reaction between [Pt(dppe) 2 ]/[PtCl 2 (dppe)] and the aryl ditellurides/ sodium salt of aryl tellurolate (aryl ¼ py, Ph, tol, Mes, Thienyl), a mononuclear product was isolated. 8, 140,141 Only in the case of the methyl-substituted telluropyridne ligand system was a complex with the composition [PtCl{TeC 5 H 3 (3-Me)N}(dppe)] isolated, which existed in equilibrium with the moiety [Pt{k 2 - Scheme 17 Reactions of [Pt 2 (dppm) 3 ]/[PtCl 2 (dppm)] with dipyridyl ditellurides/pyridyl tellurolates, respectively.  TeC 5 H 3 (3-Me)N}(dppe)] + . Conductometric measurements were performed to correlate the nature of the complex in solution and it was reported that in the case of highly polar solvents, like methanol and acetonitrile, the nature of the complex was a 1 : 1 electrolyte. Surprisingly, the palladium chemistry is totally different from their platinum analogs. On performing a similar reaction between palladium phosphine precursors [Pd(dppe) 2 ]/ [PdCl 2 (dppe)] with hemilabile ligand systems, like pyridyl (Fig. 13) and pyrimidyl ditelluride, 8,48,141 resulted the rapid conversion of mono-to trinuclear complex in a chlorinated solvent (Scheme 18). This result shows the high susceptibility of tellurium ligands toward the chlorinated solvents.
In the case of platinum precursors of diphenylphosphinopropane (dppp), like [Pt(dppp) 2 ]/[PtCl 2 (dppp)], performing a similar reaction as mentioned above with various hemilabile ligand systems afforded mono-and trinuclear products. 8, 48 However, the reaction with palladium phosphine precursors yielded only trinuclear products in a moderately good yield. A mechanistic study was performed to isolate the trinuclear product by the mild reaction between [PdCl 2 (dppp)] and [Pb(TeC 5 H 4 N) 2 ] (Scheme 19). 8 Upon recording the 31 P{ 1 H} NMR within 2 h stirring of the reaction, there was a single resonance corresponding to mononuclear complexes, which again on further stirring for up to 4-5 h at the same temperature showed two quite shielded resonances, corresponding to bi-and trinuclear products. By the extraction of the product in dichloromethane, the trinuclear product was exclusively isolated. The formation of bi-and trinuclear products was also characterized with single crystal X-ray analysis.
Weigand et al. had applied a new class of Pt(0) precursors [Pt(dppn)(nb)] derived from the chelating phosphine diphenylphosphineno naphthalene (Ph 2 P-napthyl-PPh 2 ). 136 These precursors showed a remarkable reactivity toward ditelluride systems. Upon reaction with various cyclic/saturated, acyclic/saturated, and cyclic/unsaturated ditellurides, the momonuclear [Pt(Te 2 C 5 H 8 O)(dppn)] and binuclear [Pt(Te 2 C 5 -H 8 O)(dppn)] products were isolated, which on keeping for a prolonged time period in the solvent decomposed into several products (Scheme 20). It was noticed that alkyl tellurides result in the products by the cleavage of Te-Te bonds, however aryl telluride derives the products not only by the Te-Te bond cleavage but also by Te-C bond cleavage. 136 Notable attention 7,8 has been drawn by the outcome of the above-mentioned document conclusion on hemilabile ligands. The discussion focused on complexes derived by hemilabile organochalcogens, which have been the subject of considerable interest due to their rich coordination chemistry. In fact, as a concluding remark, the coordination potential of hemilabile ligand systems, especially those that possess a heteroatoms as well as their corresponding anions, referred to as chalcogenolates, is immense. 1, 9,10 There is indeed a considerable versatility in the various coordination modes of these molecules, which may include monodentate binding through E or through heteroatoms, bridging through a single E, bridging through both E and N, or chelating via the E to N backbone. 48,134 All these coordination modes, either alone or in combination, have been observed or assigned on the basis of spectroscopic and/or crystallographic evidence of both homo-and heteroleptic metal complexes (Scheme 21).

Metal tellurides
The binuclear complexes [M 2 (m-Te) 2 (dppe) 2 ] (M ¼ Pd, Pt) ( Fig. 14) act as a powerful metallophilic ligand to provide a binding site for various transition metals, d 10 systems, and lanthanide metal centers. The former complexes can be isolated by the reaction between M 2+ , dppe, and NaTeH in a N,N-dimethylformamide and acetonitrile mixture. 142 The reaction of [Pt(CH 3 CN) 4 (NO 3 ) 2 ] with a similar composition to the former complexes led to the formation of pentanuclear complexes [Pt {Pd 2 (m-Te) 2 (dppe) 2 } 2 ] 2+ (ref. 142) (Fig. 15) in which the platinum metal center was coordinated with all four bridging tellurido linkages. Similarly, various trinuclear products have been isolated by the reaction between M 2+ , PXP, and NaTeH in N,Ndimethylformamide for more than 20 h stirring (Scheme 22). 143 Undoubtedly, strong coordinating solvents, like DMF, CH 3 CN, may enable the coordination of tellurium to further platinum moieties with ease, resulting in tri-to pentanuclear products. [143][144][145][146] Morley et al. also isolated the trinuclear product [Pt 3 {(m-Te) 2 (dppe) 2 } 2 ] 2+ by the oxidative addition of [Pt(dppe) 2 ] and vitreous tellurium under reuxing for 5 h. 147 The isolation of the trinuclear product via an oxidative addition mode is unprecedented and could be rationalized by the fact that the higher nucleophilicity of tellurium makes it highly susceptible to the nucleophilic attack of chlorinated solvents.
Redox studies of these complexes were studied by cyclic voltammetry. 142,144,145 In the case of telluride bridged cluster complexes, it all depends upon the nature of the phosphine and metal center. The cyclic voltammograms of dppe-derived complexes showed a reversible nature, whereas it was irreversible in other cases. The order of chemical reversibility followed the trend: dppe > dppp > dppm.

Catalysis
The efficient and selective transformation of various functional groups have taken place with sodium hydrogen telluride (NaHTe) and Na 2 Te, which have been reported in further applications in recent years. [160][161][162][163][164] In particular, the reduction of aromatic aldehydes to alcohols is the most important transformation by Na 2 Te in NMP (N-methyl-2-pyrorolidone). 165,166 Similarly, an attempted reduction of aromatic nitriles serendipitously led to the pharmacologically important product 7diaza-9H purines in low yields (Scheme 24). [166][167][168] One more striking example of the conversion by debromination of vic-dibromides to alkenes is that catalyzed by pmethoxyphenyltelluride. Again on reinvestigation, it was found that the more electron-rich diorganoditellurides associated with reducing agents, like glutathione (GSH) or sodium ascorbate, are better debrominating agents than the previous reaction (Scheme 24). [169][170][171] Recently, it was found that ruthenium complexes derived by various telluroethers, like [(h 6 -C 6 H 6 )Ru(L)](PF 6 ) (L ¼ 2-    63 have proven potential as an excellent catalyst in the oxidation of primary and secondary alcohols. The TON values for the oxidation of various alcohols, like cyclic, acyclic, and benzylic, ranged from 7.8-9.6 Â 10 4 . Strikingly, the complex (h 6 -C 6 H 6 )[Ru(2-MeSC 6 H 4 CH 2 -NHCH 2 CH 2 TeC 6 H 4 -OMe)](PF 6 ) appeared to be the most efficient in this process with the highest % conversion (98%). A comparative study of all the ruthenium complexes derived by various chalcogen analogs showed that the catalytic efficiency varies in the order of Te > Se > S. This fact can be rationalized by the soer ligand making it easier to form the intermediate oxy species Ru]O, which plays a pivotal role in any oxidation reaction (Scheme 25).
The second important class of reaction, i.e., transfer of hydrogen reaction, is also catalyzed by the complexes [(h 6 -C 6 H 6 ) Ru(L)](PF 6 ) (L ¼ 2-MeSC 6 H 4 CH]NCH 2 CH 2 EC 6 H 4 -R; R ¼ H, E ¼ S, Se; 2-MeSC 6 H 4 CH 2 NHCH 2 CH 2 EC 6 H 4 -R; R ¼ OMe, E ¼ Te) (Scheme 26). In the case of acetophenone, the conversion efficiency was up to 98%, while in various other aliphatic secondary ketones, it ranged up to 90%. Similarly as an oxidation reaction, the catalytic efficiency of these reactions also varies in the same order. An earlier well-established catalyst for this reaction was Ru(II) complexes of 2-(aminomethyl)pyridine phosphine with a TOF of 10 À5 h À1 and up to 97% conversion in 2-propanol using NaOH as a base. Comparatively, a half sandwich ruthenium complex of the above-mentioned composition derived with various telluroethers of pyrrolidine, morpholine, and benzotriazole moieties 62,172,173 also showed a similar efficiency rate with a short reaction time.

Suzuki and Heck reaction
Due to the air and moisture sensitivity of phosphorus-derived complexes, there is increasing interest in phosphine-free ligands [174][175][176][177] for the Suzuki-Miyaura reaction. With this prospect, palladium chalcogenolate 178 complexes are considered as existing substituents for C-C coupling reactions. Various coupling reactions are primarily catalyzed by palladium selenated and tellurated compounds. An early study performed by Prof. A. K. Singh et al. showed that palladium tellurolate complexes are as efficient as their selenium analogs. 179 The conversion was found to be up to 85%, particularly for activated 1-bromo-4-nitrobenzene (Scheme 27).
It is well documented that Pd-Se bonded compounds have promise for the Heck reaction. 178 Within this continuation, a Schiff base-derived telluroether palladium complex showed good selectivity for the isolation of a trans-alkene product. The catalytic activity depended on the nature of the halide, while the electron-withdrawing groups on the aryl ring increased the reaction rate. For aryl bromides, a very small amount of complex (0.001 mmole) was sufficient to catalyze the Heck reaction (Scheme 27). 179

Material science
Binary systems of Pt and Te (as a combination of metal and semiconductor) are important precursors for their application in the eld of sensors and magnetic memories. 180 particular, the composites of PtTe 2 are known to enhance the Seebeck coefficient of PbTe bulk particles. 183 Similarly, multicomponent rod-shaped mixed composites FePt-PtTe 2 have exhibited high coercivity (Hc > 500). 184 Pt 3 Te 4 also has shown a catalytic ability in the transformation of nitrophenol to aminophenol. 185 Platinum group metal chalcogen materials are also prominently used for low resistance ohmic contacts. These thermodynamically stable contacts are very crucial for device durability. At the interface of Pt/CdTe diffusion couples, a non planner reaction layer of intermetallic CdPt and Pt-Te is formed. 186 Platinum group metal chalcogenides comprise various binary and ternary chalcogenide materials. Undoubtedly, the preparatory methods dealing with platinum group metal sulde and selenide materials applying single source molecular precursor's methods are greater in number with respect to their tellurium precursors. 187 The complex [PdCl{Te(3-MeC 5 H 3 -N)}(PR 3 )] (R ¼ propyl, Ph) upon heating in a furnace at 340 C under an argon atmosphere yielded a molecular precursor of the binary composition PdTe. 130 The latter compositions supported on carbon have relevance in various catalytic reactions used at an industrial scale. 188 However, complexes like [PdCl 2 (4-MeOC 6 H 4 TeCH 2 CH 2 N(CH 2 CH 2 ) 2 O)] also result in the formation of Pd-Te nanoparticles of 5 nm diameter. 189 A similar composition has also been documented by heating the hexanuclear complex at >250 C. 138 Thiolate-and selenolate-derived palladium complexes with the composition [Pd 2 (m-ER) 2 (h 3 -C 4 H 7 )] (E ¼ S, Se) upon reuxing in xylene solution afforded Pd 4 E (E ¼ S, Se); however, the tellurolate analogs resulted in the formation of Pd 3 Te 2 composites at room temperature. 190 Recently, a composition of Pd 7 Te 3 has been isolated upon reuxing the complex [PdCl 2 (TeMes 2 ) 2 ] in xylene solution. 93

Biological importance
Upon evaluating the biological activity of tellurium-derived ruthenium complexes, it has been found that the complexes are quite potent as anticancer agents. Recently, a biological study of the complexes [(h 6 -p-Me C 6 H 4 Pr i ) 2 Ru 2 (m-TeC 6 H 5 ) 3 ]PF 6 and [(h 5 -C 5 H 5 ) 2 Ru 2 (m-TeC 6 H 5 ) 3 ]PF 6 (ref. 96) showed their good biological activity on normal and human cancer cell lines. The former complexes are even cyto-toxic in nature, which revealed their strong selectivity to cancer cells compared to normal ones. These promising outcomes of ruthenium complexes as anticancer agents deserves more exploration toward various other biological activities.

Conclusions
The versatile reactivity of organotellurolate ligands is quite evident from the ongoing discussions and reports in the literature. Their remarkable reactivity is derived due to presence of so (tellurium) and hard donor atoms (N, O, S) in various ligands derived from organic moieties, like thinyl, pyridyl, furan, and pyrimidyl. In the meantime, one in particular cannot be declined: that the notable reactivity may also arise due to the comparable bond energies of Te-Te and Te-C bonds, which facilitates a competitive cleavage among them. Furthermore, among all the other chalcogens, the easy availability of lone pairs and high polarizability makes them great building block synthons for various heterometallic and high nuclearity metal complexes. It is hoped that this perspective will support further enthusiasm in this eld and provide momentum for further research in this eld to help establish their potential application in various elds.

Conflicts of interest
There are no conicts to declare.