the objects , features and advantages of the present invention will become more evident as the invention is more fully described herein . the inventive compounds are novel tungsten carboxylates having the formulas i , ii and iii : ## str2 ## because tungsten carboxylates of formula i like many tungsten compounds , appear to exist as mixed - valence species ( e . g . formula i envisions one w iii and one w iv per unit ; the formulas shown represent empirical formulas and not structural formulas . proposed structural representations of compounds in genus i and iii are shown below , ## str3 ## but applicants do not wish to be restricted to such structures ; compounds made by analogous methods and having the empirical formulas i and iii are intended to be encompassed within the invention . compounds of formula ii contain tungsten only in the + 2 oxidation state . by analogy to the known tungsten ( ii ) diacetate , they are believed more likely to exist - as dimers , or as straight - chain polymers of structure iib than as monomers as shown in structure iia . [ see holste , z . anorg . ally . chem . 398 , 249 - 256 ( 1973 ).] ## str4 ## nonetheless structure iia illustrates an important aspect of tungsten carboxylates : namely , that the tungsten - oxygen bonding cannot be represented as strictly a single bond between one tungsten and two oxygens . rather , as a result of delocalization of electrons through both oxygens of the carboxyl group , each tungsten is surrounded by four equivalent oxygens . as shown in formula iib , there are probably metal - metal bonds between adjacent tungstens , and these bonds are thought to be double - bond in nature . the structure iib allows one to rationalize the lipid solubility of the compounds of the invention : the &# 34 ; exterior &# 34 ; surface of the chains being comprised entirely of hydrocarbon residues would allow the solvent to interact extensively with the lipid r groups . tungsten carboxylates are exemplified wherein r is an aliphatic hydrocarbon , such as c 7 h 15 , but r can be any hydrocarbon chain , so long as the overall solubility and rheological properties of the tungsten carboxylate in aliphatic or aromatic hydrocarbon solvents are not significantly changed . the preferred subgenera in which r is c 6 to c 10 optimize the balance among solubility , rheology and reactivity of the starting acid for forming the tungsten carboxylates . the novel mixed valence tungsten ( iii ) and ( iv ) carboxylates take the form of blue glassy solids and are sensitive to air and moisture . the tungsten ( ii ) carboxylates of formula ii are dark green oils or glasses and are also moisture sensitive . the compounds of formula ii decompose in the presence of moisture according to the reaction : ## str5 ## the compounds of formulas i and iii are believed to react similarly . the solubility of the compounds in aliphatic and aromatic hydrocarbons makes them useful in solution casting techniques , such as spin - casting , dip - casting and spray - casting . the inventive tungsten carboxylates may be synthesized according to the following reactions : ## str6 ## the mechanism by which w vi is reduced to w iv and subsequently to w iii in equations 1 and 3 is speculated to involve oxidation of the ligands to produce an olefin and carbon dioxide . in the case where r is 1 - ethylpentyl , the corresponding olefin , 3 - heptene , has been identified among the gaseous products of the reaction . in equations 1 and 3 , r 1 and r 2 represent the appropriate alkyl , alkenyl or aralkyl : residues that would arise from the corresponding r group according to the following mechanism ## str7 ## the method of preparing the compounds of formula i comprises the steps of reacting an alkali metal with an excess of an organic acid to form a carboxylate salt solution ; reacting the carboxylate salt solution with a solution containing tungsten ( vi ) oxychloride in an aromatic solvent in an inert atmosphere to form a reaction mixture ; refluxing the reaction mixture to form a sodium tungsten ( iii & amp ; iv ) carboxylate , and extracting the sodium tungsten ( iii & amp ; iv ) carboxylate from the refluxed mixture . suitable aromatic solvents include toluene and benzene . suitable alkali metals include sodium , potassium and lithium . particularly suitable organic acids include 2 - ethylhexanoic acid and 4 - phenylbutyric acid or 3 - phenylbutyric acid . compounds of generic formula ii are prepared in an inert atmosphere by heating tungsten hexacarbonyl with a large excess of the appropriate carboxylic acid at reflux , for acids with boiling points below 200 ° c ., or at 200 ° c ., for those boiling higher . the heating is maintained until all the tungsten hexacarbonyl is consumed . the solution is filtered and the excess acid is distilled off under reduced pressure . compounds of generic formula iii are prepared by heating tungsten oxychloride with a large excess of the appropriate carboxylic acid in an inert atmosphere at about 160 ° c . the reaction is filtered and the excess acid is distilled off under reduced pressure . examples of sample preparations of the inventive compounds are set forth below . formula i , r = 1 - ethylpentyl working in a conventional dry box , 3 . 43 g ( 10 . 0 mmol ) of tungsten ( vi ) oxychloride was placed in a 200 ml schlenk flask . toluene ( 65 ml ) was syringed onto the sample . freshly cut sodium ( 0 . 949 g , 41 . 3 mmol ) was placed into a 250 ml 2 - neck flask . the 2 - neck flask was connected to the schlenk flask by a bent elbow . the second neck of the 2 - neck flask was stoppered using a rubber septum . outside of the dry box , 45 ml of 2 - ethylhexanoic acid was syringed onto the sodium and the mixture was heated below the boiling temperature of 113 ° c . until the sodium had completely reacted . the adapter to the schlenk flask was purged with nitrogen gas before opening the system to a connected bubbler . the 2 - ethylhexanoic acid - salt solution was added to the tungsten ( iv ) oxychloride solution while stirring at room temperature . under a purge of nitrogen gas , a condenser was connected to the schlenk flask . the reaction mixture was refluxed using an oil bath heated at 125 ° c . after 16 hours , the solution was cooled to room temperature under a purge of nitrogen . the toluene was removed by vacuum distillation . to remove the excess 2 - ethylhexanoic acid , a dynamic vacuum was used while heating at 110 ° c . with an oil bath . the glassy blue product was extracted from the sodium chloride in the refluxed mixture with pentane . formula ii r = 1 - ethylpentyl working outside of the dry box , 4 . 04 g ( 11 . 4 mmol ) of w ( co ) 6 was weighed and placed into a 240 ml schlenk flask fabricated for refluxing reaction mixtures . using a syringe , 80 ml ( 0 . 5 mol ) of 2 - ethylhexanoic acid ( eha ) was added to the flask . after connecting a condenser that was attached to a nitrogen line , the reaction flask was heated using an oil bath . the temperature of the oil bath reached 195 °- 200 ° c . maintaining the temperature the solution was heated until w ( co ) 6 no longer sublimed up on the walls of the flask ( 4 days ). a dark green solution was observed . periodically the w ( co ) 6 was washed down from the walls by agitating the solution . before the heat was removed , the valve on the schlenk flask was closed to prevent air from going into the flask . working in a dry box , the solution was filtered through a 0 . 45 micron cellulose acetate filter . the filtered solution was transferred to a 200 ml round bottom single - neck flask which was connected to a 250 ml schlenk flask using a bent elbow . the excess eha was removed by heating the dark green solution with a 130 ° c . oil bath under vacuum . a dark green material with a thick oil consistency was obtained . the infrared spectrum of the material was consistent with product of formula ii where r is 1 - ethylpentyl ( carbonyl at 1680 cm - 1 ). formula iii , r = 1 - ethylpentyl working in the dry box 4 . 13 g ( 12 . 1 mmol ) of ow cl 4 was weighed into a schlenk flask that was fabricated for refluxing reaction mixtures . then , 60 ml ( 0 . 375 mol ) of 2 - ethylhexanoic acid was added to the reaction flask . working outside of the dry box , the nitrogen purged condenser was attached to the reaction flask ( the valve was still closed at this point ). after heating the reaction flask in a 100 ° c . bath for five minutes , the valve was opened . the condenser was connected to a bubbler and the oil bath was heated to 160 ° c . the reaction mixture turned deep blue / purple . it was maintained at 160 ° c . for 24 hours . before the heat was removed , the valve on the reaction flask was closed to prevent exposure to air . the reaction flask was taken into the dry box and the solution was filtered through a 0 . 45μ cellulose nitrate filter . using pentane , the remaining material was rinsed out of the flask . a dark blue solid was collected on the filter medium . the filtered solution was transferred to a 250 ml one neck flask . the pentane was removed in vacuo and the excess acid was distilled off at 120 ° c . under vacuum . the material was placed in a 250 ml one neck flask . the flask was connected to a fine porosity frit and a 250 ml schlenk flask . about 100 ml of diethyl ether was distilled onto the product in vacuo . the ether - soluble portion of the material was extracted into the 250 ml schlenk flask . the dark blue solid product ( formula iii r = ethylpentyl ) was isolated by removal of the ether in vacuo . the tungsten carboxylates are ideal precursors for providing a tungsten oxide thin film over electrodes used in hydrogen sulfide gas sensors . it is well known that tungsten oxide is an ideal film for coating of electrodes in hydrogen sulfide gas sensors , because tungsten oxide films have shown good selectivity and sensitivity to hydrogen sulfide gas . the resistance to a current passed through the chemiresistor comprised of a tungsten oxide film coating on electrodes decreases when hydrogen sulfide is in the ambient gas . the decrease in resistance is believed to be caused by an exchange / reduction between o - 2 and s - 2 with the production of ws 2 , which has a greater conductivity than wo 3 . the resulting exchange between o - 2 and s - 2 can be measured by an increase in voltage at a detector device . this is accomplished by having the sensor connected to a standard operational amplifier circuit incorporating the detector device . the decrease in resistance translates into an increase in voltage which is relative to the concentration of the hydrogen sulfide gas . fig3 shows the relationship between the concentration of the hydrogen sulfide gas and the increase in voltage of the sensing device caused by the decreased resistance of the chemiresistor . the present invention provides an improved film coating of tungsten oxide on the electrodes used in hydrogen sulfide gas sensors . the film of the novel tungsten carboxylate precursor of the present invention is applied or deposited on the electrodes , preferably arranged in an interdigitated configuration , by a known solution casting technique . while it is not possible to solution cast tungsten oxide ( because it is insoluble in solvents typically used in solution casting processes ), the inventive tungsten carboxylate precursors can be applied to electrodes , including interdigitated electrodes supported on an inert substrate by solution casting techniques . this is possible because the novel corboxylates are soluble in the solvents used in solution casting techniques and have the necessary rheology and surface wetting properties . the resulting thin films from ii and iii decompose to tungsten oxide when heated to above approximately 350 ° c . by conventional curing methods ; the films from i decompose to sodium tungsten oxide . the mechanism of the decomposition is not clearly established but appears to involve oxidation of tungsten by atmospheric oxygen , perhaps mediated through the ligand . a general procedure for coating a substrate is provided by the following example : working in the dry box , a toluene solution containing the precursor compound was syringed onto a quartz glass plate . the thin film was laid down using a photoresist spinner at 2000 rpm for 20 seconds . the quartz glass was either placed on a hot plate or in an oven . after 30 minutes , the glass was removed and a transparent thin film was observed . the results from x - ray diffraction are shown in table 1 . table 1______________________________________com - poundof ex - ample film deposit heating # conditions conditions (° c .) nature of film______________________________________1 17 mg / 50 μl 500 °/ hot plate sodium tungstate2 17 mg / 50 μl 520 °/ hot plate hexagonal phase wo . sub . 32 40 mg / 100 μl 300 °/ oven amorphous wo . sub . 32 40 mg / 100 μl 500 °/ oven cubic phase wo . sub . 3 containing small amount of triclinic2 40 mg / 100 μl 300 °/ hot plate cubic phase wo . sub . 3 containing small amount of triclinic2 40 mg / 100 μl 500 °/ hot plate cubic phase wo . sub . 3 containing small amount of triclinic3 9 mg / 100 μl 300 °/ oven amorphous wo . sub . 33 9 mg / 100 μl 500 °/ oven partially crystalline cubic phase wo . sub . 33 9 mg / 100 μl 300 °/ hot plate amorphous wo . sub . 33 9 mg / 100 μl 500 °/ hot plate partially crystalline triclinic phase wo . sub . 3______________________________________ the results demonstrate that not only can a coated tungsten oxide electrode be produced by thin film casting techniques , but the microstructure of the film can be modulated by changing the precursor and the heating conditions . fig1 shows a sensor 22 of the present invention having a substrate support layer 10 made from inert materials , such as quartz and containing or having mounted thereon conductors of a conductor layer 11 , 12 , 13 , 14 . the conductors 11 , 12 , 13 and 14 are made from conducting material , such as gold or palladium . electrical current can be passed from a standard power supply via a conducting wire or other means through the conducting layer 11 . the conducting layer 11 is in contact with an adjacent resistor or heater layer 15 . the resistor layer 15 generates heat from the conducted current . the current is then passed from the resistor layer 15 to the conductor 14 and via a standard conducting means back to a power supply . on the upper side of the resistor layer 15 , there is a silicon - oxide based dielectric layer 16 , upon which there is mounted an electrode layer comprising electrodes 17 and 17 &# 39 ;. a sensing film 18 according to the instant invention is deposited over the electrodes 17 and 17 &# 39 ;. the dielectric layer 16 functions in the sensor 22 to shield the resistor layer 15 from reacting directly with the sensing film 18 . the resistor layer 15 , heats the coated electrodes 17 and 17 &# 39 ; to improve sensitivity and selectivity of the sensing film 18 , as is commonly done in gas sensor technology . electrical current is also passed from a power supply through a conductor 12 , to the electrode 17 . the current transfers to electrode 17 &# 39 ; and is passed through the conductor layer 13 which is connected to a standard operational amplifier circuit with a detector means , of the type known in the art . the electrode layer 17 and 17 &# 39 ; is preferably arranged as interdigitated electrodes which have been coated with the sensing film 18 , using a solution casting technique . the sensing film 18 , is a tungsten oxide or sodium tungsten oxide thin film formed from thermally decomposing the novel tungsten carboxy : late compounds . the sensing film 18 , selectively reacts with hydrogen sulfide gas in the ambient atmosphere to cause an increase in the conductance of a current passed through the electrodes 17 and 17 &# 39 ;. fig2 shows a schematic representation of the thin sensing film 18 , deposited on an interdigitated array of conducting electrodes 17 and 17 &# 39 ;. fig3 is a graphical representation of a typical relationship between hydrogen sulfide concentration and output voltage of a hydrogen sulfide gas sensor 22 of the type shown in fig1 when coated according to the general procedure with the compound of example 1 and heated at 500 ° c . the trend shows that there is an increase in output voltage with an increase in hydrogen sulfide gas concentration . it is believed that tungsten oxide reacts with hydrogen sulfide gas to form tungsten sulfide . it is also believed that the introduction of oxygen gas in the absence of h 2 s promotes the resulting tungsten sulfide ( ws 2 ) to reform the tungsten oxide film . sensors fabricated according to the present invention have improved substrate conformity , a more uniform doping ability , less potential stress in the films and are more conveniently fabricated than those made by known methods . accordingly , the preferred embodiments of the invention have been illustrated and described in detail . it is to be understood that numerous changes and variations can be made in the composition and manufacture of the invention without departing from the spirit of the invention , the scope of which is defined by the appended claims .