High Temperature Microcalorimetric Studies of the Thermal Decomposition and Iodination of Polynuclear Carbonyls of Fe Co Ru Rh Re 0s and Ir BY J. A. CONNOR,*H. A. SKINNER AND Y. VIRMANI Chemistry Dept. University of Manchester Manchester M13 9PL Received 29th August 1973 From measurements of the enthalpies of thermal decomposition (and also in some cases of the enthalpies of reaction with iodine vapour) at elevated temperatures the standard enthalpies of forma- tion AH; at 298 K of the following crystalline metal carbonyls have been obtained (in kJ mol-l) Fe2(CO)9= -1410+ 12 ; Fe3(CO)12 = -1849k16; COZ(CO)* = -1209+8; cOq(c0)12 = -1845+ 16; RU3(C0)12 = -1920+20 ; Rh4(C0)12 = -(1820+ 12) ; Rha(C0)lG = -(2029) ; Rez(CO)lo = -1653 & 20 ; OS3(CO)12 = -(1749+ 20) ; Ir4(CO)12= -11820k 16 Estimates of the unknown values AHsub(298K) for the crystalline compounds have been used to derive the values AH;"() ; the latter were converted into AH values for the total enthalpies of dis- ruption of the gaseous carbonyls into metal atoms and CO gaseous molecules.These AH values have in turn been reduced to individual enthalpy contributions from the various metal-carbonyl and metal-metal bonds in the molecule. Terminal metal-carbonyl bond enthalpies increase both with atomic number and with enthalpy of atomization of the metal. The enthalpy contribution from a metal-carbon bond in a bridging M-CO-M is of the order one half of the corresponding terminal M-CO linkage. The bond enthalpy contribution from M-M in the polynuclear carbonyls of Fe and Co is found to be approximately 2/3 of that from the M-CO terminal bonds.The enthalpies of formation of several mononuclear metallic carbonyls have been rneas~red,l-~ but similar data are available only for two polynuclear metal carbonyls (of Mn and Co 6 ; nothing is known of the enthalpies of formation of the poly- nuclear carbonyls of Fe Ru Rh Re 0s and Ir. The conventional thermochemical approach (energies of combustion by bomb calorimetry) can be troublesome when applied to metal carbonyls due to incomplete oxidation of the metal. A different approach was used in the present work through the enthalpies of thermal decomposi- tion. The measurements were made by the "drop "calorimetric technique using the high-temperature Calvet twin microcalorimeter for this purpose.Two types of decomposition reaction were investigated calorimetrically (i) direct thermal de- composition in argon gas and (ii) reaction with iodine vapour to form the iodide(s) of the metal and liberate carbon monoxide. EXPERIMENTAL PREPARATION OF COMPOUNDS Commercial samples (Strem Chemicals Inc.) of RU~(CO)~~ and Re,(CO), were used assupplied. The remaining carbonyl samples were prepared and purified as described in the literature references listed below 18 J. A. CONNOR H. A. SKINNER AND Y. VIRMANI Fez(CO)9 from Braye and Hubel '; Fe3(CO)12 from McFarlane and Wilkinson ; CO~(CO)~, from Wender Sternberg Metlin and Orchin ; Co4(CO)12 from Chini Albano from and Martinengo lo; lU4(C0)12 and R~I~(CO)~~ Chini and Martinengo l1; OS~(CO)~~, from Johnson Lewis and Kilty l2 ; and 1r4(C0)12 from Chaston and Stone.13 CALORIMETRIC TECHNIQUE The Calvet twin-cell microcalorimeter (Setaram Lyon) was calibrated electrically over the temperature range 423-773 K and its performance as a '' drop " calorimeter evaluated from measurements of the enthalpy of sublimation of crystalline iodine.Iodine samples (10-15mg) contained in thin capillary tubes were dropped from outside the calorimeter via the inlet chute into the argon-filled " live " cell of the calorimeter at the same time as identical but empty glass capillary tubes were let fall into the twin " reference " cell. The resulting thermogram area was measured and related to the known l4 enthalpy change for the live cell process I2(c G)+ 12k TZ) where T2= fixed calorimeter tempetature and Tl = arrival temperature of the I2 sample on reaching the reaction zone in the calorimeter.Thermal decomposition measurements were made similarly by dropping weighed carbonyl samples (a few mg) into the argon- filled live cell ; iodination measurements were made by dropping carbonyl samples in the live cell charged with excess I2 vapour. The extent of iodination of the metal was deter- mined from analysis of unused iodine and from analysis of the iodine content of the solid product (metal iodidelmetal) formed. AUXILIARY QUANTITIES Enthalpy changes (Ah,AH) and enthalpies of formation (AH:) are given in thermo- chemical calories defined by 1 cal = 4.1840 J.The following standard enthalpies of forma- tion ls were accepted (values in kcal mol-l) ; AH;(CO 9)= -26.416 ; AH"f12,g) = 14.92; AH"fFe12,c) = -27.0 ; AH:(CoI, C) = -21.2 ; AH?(Fe g) = 99.5 ; AHi(C0,g) = 101.5; AHOf(Ru,g)= 153.6; AH:(Rh,g) = 133.1 ; AHi(Re g) = 184.0; AH:(Os g) = 189.0 and AH?(Ir g) = 159.0. (HT-HZg8) values for the metals Fe Coy Ru Re. 0s and Ir were taken from the compilation by Hultgren Orr Anderson and Kelley.16 RESULTS DI -IRO N ENNEACARBoNY L Fe2(C0)9 Thermal decomposition measurements were made by dropping carbonyl samples (2-2.5 mg) into the calorimetric reaction vessel maintained at a fixed temperature T2 within the range 553-573 K. Decomposition took place rapidly within the reaction vessel with deposition of a bright metallic film on the walls of the Pyrex reaction vessel.Typical results are summarized in table la. The column Ah gives the measured enthalpy change in each experiment determined from the recorded thermo- gram area. The values AHubsrefer to the process Fe2(CO)9[c T11 -+ 2Fe[c T21 +9c0b T21 (1) where Tl is the temperature of the carbonyl sample on entering the reaction zone of the calorimeter. The values AH298 refer to the same process but carried out iso- thermally at 298 K; the reduction of AH,, to 298 K made use of tabulated (HT-H298) data 14* l6 for Fe(c) and CO(g). The reaction of Fe2(CO)9 with excess I2 vapour was studied in the range T2 = 524-573 K. The main product was a brown-red powder (FeI,) and there was also HIGH TEMPERATURE MICROCALORIMETRIC STUDIES some metal film deposit mainly within the capillary tube containers.Results are summarized in table lb. The values AHobsrefer to the calorimeter reaction F~,(CO),[C GI + n~,rg,GI + n~e~,[c, T,I+ (2-n)wc T,I+ 9corg GI. (2) The values AH* were obtained from AHobsby removing the exothermic contribution due to the iodination reaction n Fe[c T2] +n 12[g,T2] n FeI,[c T2]. (3) The adjustment made i.e. AH = AHobs+ 441.9 -0.0045(T2-298)] kcalmol-1 (3) was based on recommended values for the enthalpy of formation of FeIz(c) and for ACp of reaction 14(3). [AHi[Fe12 c] = -27.0 kcal mol-l; ACD= 4.5 cal K-l mol-1 ; AH",[12 g] = 14.92 kcal mol-l]. The values AH* provide an indirect measurement of the enthalpy of the thermal decomposition process (I) and were corrected to AHzs8 as described earlier.The thermal decomposition and iodination studies gave virtually identical results (mean AH298 = 99 kcalmol-') corresponding to AH;[Fe,(CO), c] = -337 kcal mol-l. The overall uncertainty in the latter is estimated at &3 kcal mol-l. TABLETHERMAL DECOMPOSITION OF Fez(CO)9 AH,,./_ AHz9al expt . Fez(CO)glmg Ahlcal Tz/K kcalmol 1 kcal mol-1 1 2.025 0.659 553 118 99 2 2.450 0.796 553 118 99 3 2.035 0.669 553 120 100 4 1.975 0.661 573 122 101 5 1.835 0.602 573 119 98 mean AHzss = 99 kcal mol-I. TABLE OF Fez(CO)9 1~.-IODINATION apt. Fez(CO19I mg 12/w M/d TZ/K n AHobsl AH*l kcal mol-1 kcal mol-1 AH2981 kcalmol-1 1 4.380 15.005 0.522 524 1.so 43 117 100 2 6.415 14.315 0.817 544 1.74 46 117 99 3 3.985 14.765 0.459 553 1.84 42 117 98 4 3.965 14.440 0.448 553 1.90 41 119 100 5 4.510 12.770 0.517 573 1.86 42 118 97 mean AHzss = 99kcal mol-l.TRI-IRON DODECARBONYL Fe3(CO)12 Thermal decomposition and iodination measurements on Fe,(CO) were made over the range T2 = 494-544 K. The thermal decomposition results are summarized in table 2a AHobshere relating to the decomposition process Fe,(CO),,[c T11 + 3Fe[c T2l+ 12corg GI. (5) The iodination studies (table 2b) followed the pattern of similar studies on Fe2(CO)9 ; the values AH& refer to (3 -F)J?e[e T,]+ 12 CO[g T2] J. A. CONNOR H. A. SKINNER AND Y. VIRMANI and the values AH* (obtained from AH* = AHobs+3n -c41.9 -O.O045(T -298)] kcaf mol- (7) 2 by removing the exothermic contribution to AHobs from the formation of (3n/2)Fe12) refer to the thermal decomposition process (5).The agreement between the values AHzg8obtained directly (table 2a) and indirectly from iodination (table 2b) is fair ; we accept AH298 = 125f4 kcal mol-' as the final result corresponding to A\HOf[Fe3(C0)12,c] = -442+4 kcal mol-l. TABLEk-THERMAL DECOMPOSITION OF Fe,(CO) 2 Fe3(CO)i21 AHobsl-AH2981 expt. mg Ahlcal T21K kcal mol 1 kcal mol-1 1 2.920 0.814 494 140 120 2 3.215 0.973 544 152 127 3 2.780 0.825 544 149 124 4 3.060 0.881 544 145 120 5 3.095 0.922 544 150 125 mean AHzg8= 123 kcal mol-l. TABLE 26.-IODINATION OF Fe3(CO)12 1 4.415 13.760 0.319 494 1.72 36 142 122 2 3.230 13.950 0.233 518 1.81 36 147 124 3 3.595 12.670 0.242 518 1.88 34 149 126 4 3.945 13.680 0.307 518 1.81 39 153 131 5 3.945 13.910 0.258 518 1.94 33 152 129 6 5.040 14.670 0.399 544 1.75 40 147 122 mean AHzgs= 126 kcal mol-I.COBALT CARBONYLS Thermal decomposition studies were made on Co,(CO) and on CO,(CO)~ over the temperature range T2 = 453-514K. Freshly prepared octacarbonyl was used for each measurement and both carbonyls were handled in an atmosphere of pure N2 at all times. The dicobalt compound is less stable to heat than Co,(CO),, decomposing visibly at temperatures as low as 350 K. Both carbonyls were rapidly decomposed at the temperatures used in these studies yielding a bright clean filmof metal on the walls of the reaction vessel; metal was also formed within the capillary tube containers.DICOBALT OCTACARBONYL CO~(CO)~ Thermal decomposition results with Co,(CO) are summarized in table 3; the values AHobs refer to the process. and were converted to AH298 values using tabulated (HT-H298) data for Co(c)from c] Hultgren et aZ.l6 The mean AH298 leads to AH"~[CO,(CO)~ = -298 kcal mol-l. The error limits in this value are estimated at f2 kcal mol-l. HIGH TEMPERATURE MICROCALORIMETRIC STUDIES TABLE3.-THERMAL DECOMPOSITION OF cO,(co) AHod AH2981 expt. Coz(CO)a/mg Ahlcal T2IK kcal mol-1 kcal mol-1 1 3.000 0.870 470 99 88 2 2.995 0.850 470 98 87 3 3.235 0.891 475 98 86 4 3.355 0.965 475 98 86 mean AH298 = 87 kcal mol-l. TETRA COB A LT D OD E CA CA RBO NY L CO4(CO)12 Both thermal decomposition and iodination studies were made on CO~(CO),~.The thermal decomposition results are summarized in table 4a in which AHobsrefers to the process c04(co)12[c TI1 4c0[c T2] + l2c0[g T2]. (9) TABLETHERMAL DECOMPOSITION OF cOa(c0) 12 AHod AH2981 expt . Co4(CO)12Img Ah/cal TZIK kcal mol-1 kcal mol-1 1 3.505 0.851 458 139 122 2 3.050 0.803 514 151 127 3 3.020 0.793 514 150 126 4 2.805 0.702 514 143 120 mean AH298 = 124 kcal mol-'. TABLE46.-IODINATION OF CO,(CO), em. Co4(CO)lz/mg Idmg Ah/d T2/K n Noadkcal mol-1 AH*l kcal mol-1 AH29el kcal xn01-~ 1 3.480 10.170 0.565 456 0.64 93.0 138 121 2 3.440 11.960 0.445 458 1.02 74.0 146 129 3 2.950 13.700 0.340 514 1.18 65.4 148 125 4 3.355 13.235 0.407 514 1.06 69.4 144 120 5 3.025 10.430 0.328 518 1.19 62.0 146 122 mean AHzg8 = 124 kcal mol-'.In table 4b AHobs refers to the iodination reaction n COq(CO)12[C 2-11 +Z'z[g 2-21 -+ 4COI,[C GI + 12 cots TZI (10) where CoI = (n/2)C012 +(1 -(n/2))Co. The values AH*(obtained from AH,,,by removing the contribution from the iodination ofthe cobalt metal) refer to the thermal decomposition (9). The adjustment AH*= AHobs+ 2n[36.1- o.0045(T2 -29811 kcal mol-' (1 1) was based on the recommended value AhH"fCo12,c) = -21.2 kcal mol-' and used an assumed AC = 4.5 cal mol-l K-' for the reaction Co(c)+12(g)3 C012(c). The mean AH298= 124 kcal mol-l from both thermal decomposition and iodination studies (estimated uncertainty f4 kcal mol-l) leads to the value A.H"fCo,(CO),2 c]= -441 + 4 kcal mol-l .J. A. CONNOR H. A. SKINNER AND Y. VIRMANI TRI-RUTHEN I uM DO D E cA cA RB oN Y L Ru,(CO) Thermal decomposition and iodination studies were made on RU,(CO)~~. The carbonyl decomposes readily at temperatures >550 K ; the results of the thermal decomposition measurements are summarized in table 5. The iodination reaction studied over the range 550-570 K led to a product which was orange-red in colour and also to a metallic deposit. The red powder has not been identified but was thought to be the polymeric carbonyl iodide,17 [Ru(CO)~I~],. Further studies on this reaction are planned. TABLE 5.-THERMAL DECOMPOSITION OF RUs(C0) 12 AHod AHzgsl expt. RuACO)1zlmg Ahlcal TzIK kcal mol-1 kcal mol-1 1 4.010 1.030 553 164 138 2 3.855 1.027 553 170 144 3 3.420 0.921 573 173 145 4 3.815 1.053 573 177 149 5 3.665 0.931 573 162 135 mean AHzsa= 142 kcal mol-I ; AH~[RU,(CO)~~, c] = -459k 5 kcal mol-'.RHODIUM CARBONYLS Rh4(C0)12 AND Rh6(C0)16 Thermal decomposition studies on Rh4(C0)12 were made over the range 518- 574 K. Freshly prepared material was used as this compound may deteriorate on standing. Preliminary results (six measurements) gave AHzg8 = 1 19 3 kcal mol-I corresponding to AHf(Rh4(C0)12 c] = -436 kcal mol-I. Thermal decomposition studies on Rh6(C0)16 were made over the range 458-574K. Preliminary results only are available ; from four measurements AHzss = 62 3 kcal mol-' corres-ponding to AH"f[Rh6(C0)16 c] = -485 kcal mol-'. Further work is needed to complete these studies and both the AH; values given above are tentative.DI-RHENIUM DECACARBONYL Re2(CO)lo Both thermal decomposition and iodination studies were made on Re2(CO),o. The thermal decomposition results obtained at 593 IS,are summarized in table 6a. The iodination reactions were studied at 558 and at 593 K. The product of iodination was a black powder (presumed to be ReI,) and traces of metal deposit were also formed on the walls of the reaction vessel and inside the capillary-tube containers. The overall product ReI, analysed with n = 2.5-2.9. The AHobsvalues (table 6b) relating to the calorimeter reaction Rez(CO)lo[c TI1+nI2[g T2l + 2ReIn[c T21+ locolg,T219 (1 1) were converted to AH* values by removing the exothermic contribution from the for- mation of ReI (2Re1 = (2n/3)Re13+(2-(2n/3))Re).In this case it was necessary to use an estimated value for AH"fReI, c]. Literature values exist for AH;[ReCl, c] and AHf"[ReBr, c] (-63 and -40 kcal mol-1 respectively) ; the increments in AH; for transition metal halides are normally numerically larger in passing from bromides + iodides than in passing from chlorides -+bromides and on this basis a value AHT[ReI, c] = -I0 (f5) kcal mol-I was adopted for present purposes. The adjustments made (table 6b) to AHobs to obtain AH* used AH -32 kcal mol-' for the iodination reaction Re[c T,]++I,[g T2]+ Re13[c T2]. HIGH TEMPERATURE MICROCALORIMETRIC STUDIES The agreement between the AH298values from thermal decomposition and from iodination is satisfactory but may be fortuitous bearing in mind the estimated value for AHi(Rel[, c).Accepting AH298-131 kcal mol-' AH"fRe2(CO)lo,c] is calcu- lated = -395 kcal rnol-l to which we attach an uncertainty of +5 kcal mol-l. TABLE&.-THERMAL DECOMPOSITION OF Rez(CO)lo AT 593 K expt. RedCO) 1olmg Ahlcal AHObs/kcal m01-l AHzg8lkcal mol-1 1 3.325 0.793 156 131 2 3.370 0.793 154 129 3 2.91 5 0.681 153 128 4 2.915 0.676 151 127 5 3.030 0.716 154 130 mean AHzg8= 129 kcal mol-l. TABLE 6b.-IODINATION OF Re2(CO)10 expt. Rez(C0)lolmg Ahlcal T2IK n AHotuI kcal mol-1 AH*/kcal mol-1 AH2981 kcal mol-1 1 3.930 12.080 0.550 558 2.85 91 152 131 2 4.000 13.350 0.526 558 2.85 86 147 126 3 3.735 13.500 0.533 558 2.9 93 155 134 4 3.965 12.385 0.568 558 2.75 94 152 131 5 5.390 14.300 0.875 593 2.6 106 161 137 6 4.220 13.100 0.669 593 2.5 103 157 133 mean AHzga= 132 kcal mol-l.TRI-OSMIUM DODECARBONYL OS~(CO)~~ Os,(CO), was studied by thermal decomposition at 593 K and by iodination at 573 and 593 K. Thermal decomposition was noticeable at 543 K but too slow for microcalorimetric study. At 593 K decomposition appeared to be rapid but was not totally confined to the reaction vessel; there was some escape of vapour from the reaction zone and the metallic deposit extended some way into the inlet tube. Iodination occurred readily at 573 K forming a bright yellow powder. This was thought to be the polymeric carbonyl iodide [Os(CO),I,] ; on strong heating in vacuo it decomposed to liberate iodine vapour and deposit a clean film of metal.Further studies of this reaction are planned. The thermal decomposition results at 593 K are summarized in table 7. TABLE 7.-THERMAL DECOMPOSMTON OF OSj(CO)12 AT 593 K expt. Os3(CO)lz/mg Ahlcal Affobs/kcal mol-1 AH29slkcal mol-I 1 3.565 0.520 132 102 2 3.290 0.479 132 102 3 2.380 0.346 131 101 4 3.365 0.482 130 100 5 4.650 0.659 129 99 mean AHzg8 = 101 kcal mol-I. In view of the deposition of some metal film outside the reaction vessel the mean AH,, = 101 kcal mol-l is regarded as a lower limit corresponding to AH",Os,(CO),, c] < -418 kcal rnol-l J. A. CONNOR H. A. SKINNER AND Y. VIRMANI TETRA -I R I D I UM D OD E cA cA R B oN Y L Ir4(CO)l Thermal decomposition studies on Ir4(CO)12 were made at T2 = 573 and 594 K.The decomposition produced a metallic mirror on the walls of the reaction vessel and a fine black powder within the capillary tube containers. Results are summarized in table 8. TABLE&-THERMAL DECOMPOSITION OF Ir4(CO)1 AHobsI AH2981 expt. 1r I 2/mo Ah/cal T2/K kcal mol-1 kcal mol-I 1 5.765 0.779 594 149 117 2 4.765 0.638 594 148 116 3 5.380 0.760 594 156 124 4 6.680 0.866 573 143 113 mean AHzg8= 118 kcal mo1-'. The mean AH298 corresponds to AH"fIr,(C0)12 c] = -435 kcal mol-' with uncertainty f4 kcal mol-l. DISCUSSION Available knowledge on the enthalpies of formation of metal carbonyls is collected together in table 9. Most of the carbonyls listed are solids at 298 K. The enthalpies of sublimation AHsub of only a few of these compounds have been measured experi- mentally.The bracketted values in table 9 are estimates which we consider acceptable within the broad limits (& 5 kcal mol-l) attached. The values AH:(g) carry both the experimental uncertainty in AH",c) and the uncertainty (experimental or estimated) in AHsub. TABLE~.-ENTHALPIES OF FORMATION OF METAL CARBONYLS AH;/kcal mol-1 AHssut,/kcal mol-1 AH;(g)/kcal mol-1 ref. MONONUCLEAR CARBONYLS -234f 2 17f0.5 -217f 2.5 --183.5f2 -183f 2 9.6+ 0.2* -173f 2 --134f 3 -l50f 1 6.6+0.3* -143.5f 1 -236.5rfI0.5 17.7f 0.3 -218.8f0.6 -229.0+ 0.8 18.1 +0.3 -210.9f 0.9 -(-165) POLYNUCLEAR CARBONYLS -401fl 15+ 1 -386f 1.5 (51 (3) -337f 3 (18+ 5) -319+ 6 this research -442k4 (23f5) -419f 7 this research -298f2 lS+ 1.5 -280f 3 (6); this research -441f4 (23f5) -418f7 this research -459+ 5 (24+ 5) -435f7 this research (-435+3) Wf5) (-4 1 1 +6) this research (485) (28*5) (-457 this research -395f 5 (20k5) -375+ 7 this research (-418+ 5) (25f5) (-393f7) this research (-435+4 (25+ 5) -41Of 7 this researGh * AHvap.HIGH TEMPERATURE MICROCALORIMETRIC STUDIES Table 10 gives the calculated enthalpies of disruption AHdisrupt, of gaseous metal carbonyls into gaseous metal atoms and CO molecules. For a mononuclear carbonyl M(CO), AHdisrupt refers to the process M(CO),[g 2981 -+ M[g 2981 +n CO[g 2981 calculated from AHdisrupt = AH",M SI +n AH",co gl-AH",M(CO)n 91. The metal-ligand bonds in M(CO) are all of the terminal type M-C=O (symbolized in table 10 by T).In a polynuclear carbonyl M,(CO), AHdlsrupt refers to M,(CO),[g 2981 + rnM[g 2981+n CO[g 2981 and includes the contribution from disruption of the metal-metal bonds (symbolized by M)as well as from the metal-to-carbon bonds in the molecule. The latter may include bridging carbonyl linkages M-C-M as well as terminal carbonyl bonds II 0 (each bridging metal-carbon bond M-CO is symbolized by B). The structures I of the polynuclear carbonyls have been determined in the crystalline state 23-31 and the bond descriptions given in table 10 accept that the crystal molecular structures are retained in the gaseous state. The values Tgive the average contribution of a terminal metal-carbonyl bond in a mononuclear carbonyl to the total enthalpy of disruption T = AHdistuptfn.The values given for Band Min thepolynuclear carbonyls of iron were calculated assuming TABLE ~~.-ENTHALPIES OF DISRUPTION OF METAL CARBONYLS carbonyl AHdismpt/kcalmol-1 bonds bond enthalpy/kcal mol-1 MONONUCLEAR 154 22.5 ll8.&2 140.4+2 6T5TST ?" = 25.7k0.4 = 23.7k0.4 T=28.1k0.4 130 +3 4T = 32.5k0.8 l4o.jrfr1 (217) 217.6k 1 255.4+ 1 4T 5T 6T 6'ii F= 35.1k0.3T=36.310.2 T=42.6k0.2 T=(43.4) POLYNUCLEAR 256+ 2 lOT+ M 280+ 6 6T+6fi+R B = 15.4 400+7 lOT+4B+3m = 19.2 272rfr 3 6T+4B+m = 13.8 5071 7 9T+ 6B+ 6m M= 22 579+ 7 (626+6)(833 479+ 7 12T+3M 9T+ 68+ 6M lOT+ El (643_+ 7)729+ 7 12T+ 3m 12"+6m J. A. CONNOR H. A. SKINNER AND Y.VIRMANI that Tin Fe(CO) is transferable unchanged to Fe2(CO)9 and Fe,(CO),, and that the bond enthalpies B and M are likewise transferable from Fe,(CQ) to Fe3(CO)12. Simi- lar assumptions were made in calculating B and M in the cobalt carbonyls CO,(CO)~ and CO~(CO),~. From the T,M and B values so calculated we may note that in the Fe and Co carbonyls M -0.68 T,and that B N +T. If we now assume that these approximate relationships between T M and B apply to other polynuclear carbonyls it is possible to extract the Tand M values for other metals than Co and Fe despite the limited AHdisrupt data available. The values obtained are listed below. The T values for the transition metals of the first row range from 24-35 kcal mol-' for the second row from 36-41 kcal mol-' and for the third row from 43-46 kcal mol-' ; the values are plotted against AH",M g) in fig.1. 46 44 42 7-40 2 38 36 -z 8 34 4 32 R 30 28 26 24 70 90 I00 120 140 160 180 200 dH;(M g)/kcal mol-' FIG.1. Rh6(CO)16 has not been included in the analysis of T M values as thermo- chemical studies are incomplete and also because the unusual structural features 29 raise difficulties of description of the bonding in this compound. Suffice to remark that transference of Tfrom Rh4(CO), to FUI~(CO),~ would appear to lead to signi- ficantly weaker M values in the hexarhodium compound that found in Rh4(CO),2. HIGH TEMPERATURE MICROCALORIMETRIC STUDIES H. A. Skinner Adv. Organometal. Chem.1964,2,49 (Academic Press New York). F. A. Cotton A. K. Fischer and G. Wilkinson J. Amer. Chem. SOC. 1956,78 5168. J. A. Connor H. A. Skinner and Y.Virmani J.C.S. Faraday I 1972 68 1754. D. S. Barnes G. Pilcher D. A. Pittam H. A. Skinner D. Todd and Y. Virmani Int. ConJ Chem. & Uses of Molybdenum (University of Reading Sept. 1973). W. D. Good D. M. Fairbrother and G. Waddington J. Phys. Chem. 1958 62 853. A. Cartner B. Robinson and P. J. Gardner J.C.S. Chem. Comm. 1973 317. E. H. Braye and W. Hubel Inorg. 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