GENERAL DISCUSSION Dr. H. A. Skinner (University of Manchester) said The unexpectedly large in- creases in heat capacity of refractory metals at temperatures approaching their melting points as revealed by levitation “ drop” calorimetry and referred to by Margrave are also indicated by the pulse-heating studies of Cezairliyan at Washing- ton and of Lebedev’ at Moscow. These sharp increases may be due primarily to correspondingly sharp increases in the number of lattice vacancies in the metal structure; this possibility would require a concomitant expansion in volume of the specimen. The levitation technique might have some advantage for density/tempera- ture measurements in that it allows ample time for the heated specimen to achieve the equilibrium density at the observation temperature.The rapid pulse-heating method of Lebedev may be too fast to accommodate fully the volume expansion within the recording time ( s). Are density/temperature measurements con- templated-or feasible-at temperatures approaching the melting-points and beyond of refractory metals? Lebedev has communicated results of recent work at the Institute of High Temper- ature Studies in Moscow which he had intended to present as a paper to this discussion.* The Moscow studies resemble those reported by Cezairliya but the time-scale of the pulse heating used (10-5-10-6 s) was very short compared with the 1 s pulse-heating applied in experiments at the National Bureau of Standards. Refractory metals and alloys in the form of thin wires (diam.0.05-0.15 mm ; length 10-60 mm) were pulse-heated in air and in various insulating liquids by passage of a single near- square current pulse of density -5 x lo6 A cm-2. The wire resistance and the voltage drop across the wire were monitored during the time of melting from en- larged photographs of oscillograms taken from the oscilloscope screen. The start and end of melting was observable with many metals from discontinuities in the oscillograms ; coincidently the start and finish of melting were registered photo- electrically from the surface radiance of the wires. The high-speed pulse technique is understandably less accurate than the slow pulse method but it has proved very useful for the study of melting and for investigation of the liquid state.Because of the extremely short duration the wire specimens are not subject to deformation over the observation period even at temperatures above the melting point. Chemical reaction of the specimen with the ambient medium evaporation of the specimen and thermal losses are negligible over the lo-’ s pulse period. Measurements of the electrical resistivity of solid (ps)and liquid (pJ phases and of the latent heat of fusion (Afus) of the metals W Ta Mo Ir,Nb Rh Pt Fe Ni Au and of a number of alloys have been made by the high-speed pulse heating method. Some results are summarized on p. 30. For several metals (W Ta Fe and alloys W/Re Ni/Cr) the resistivity in the liquid state remained constant almost to the moment of explosion. A slow increase of pl with temperature was found for Mo Ir Nb Pt and Ni; for other liquid metals (Rh Cu Au Al) the increase in p1 with temperature was more rapid.Isthere satisfactory agreement in general between the results obtained by the rapid- ’I. Ya. Dikhter and s.V. Lebedev Teplofizika Vysokokh Temperatur 1970,8,155; 1971,9,929. * Pulse Heating of Metals AHand electrical conductivity of Nb and Rh fusion by S. V. Lebedev. 29 GENERAL DISCUSSION pulse technique with those from the slow-pulse measurements? There appears to be reasonable accord as regards ;Ifusof niobium. Is there more scope for comparison in respect of electrical resistivity? &"a/ metal J g-1 PllPs Au 70 13.1 29.1 2.22 Ni 318 60.8 82 1.35 Fe 244 130 134 1.03 W 299 117.5 127 1.08 Ta 202 114 126 1.10 Mo 430 79.2 93.5 1.18 Ir 200 70.3 92 1.31 Nb Rh 297 257 95.2 61 108.5 85.5 1.14 1.# Pt 128 63.5 94 1.48 Dr.A. Cezairliyan (Nat. Bureau Stand. Washington D.C.) said The transient method described by Lebedev et al. for the measurement of heat of fusion and elec- trical resistivity of metals and alloys during melting is interesting and shows great promise. It is commendable that they could obtain meaningful results in experi- ments of such short duration (10 p). Their results are in reasonably good agreement with those of other investigators. Our recent preliminary work (pulse-millisecond resolution) has yielded a value of 290 J g-l for the heat of fusion of niobium. This value is approximately 2.4 % lower than the value (297 J g-I) reported by Lebedev et al.; however this difference is well within the estimated uncertainties of the re- ported values.Dr. S. V. Lebedev (Inst. High Temperatures Moscow) (communicated) The measured values ps p1 refer to the sample size at room temperature. The estimated limits of error are -2 % for ps p1 and -5 % for &usion. The values of ps p1 and Afusion reported for Au Ni Fe and the values psfor W Ta and Mo are in fair agree- ment with literature values from other types of measurement. The reason for the difference of 5 % in psfor Nb from that obtained by Cezairliyan is not yet clear ; the value obtained for Afusion(Nb) from the same measurement agrees quite well with results reported from use of the magnetic levitation technique.Further details are given in TeploJizika Vysokokh Temperatur 1971,9 635 ; 1973 11 1182 ; and Zhur. Tekh. Fiz. 1972 29 1752. Dr. J. A. Connor (University of Manchester) said Further work has been carried out on the thermal decomposition and iodination of Rh4(C0)12 and of Rh6(C0)16. Thermal decomposition studies on a freshly prepared sample of Rh4(C0)12 were made over the range 485-574 K giving the results summarized in table 1. Measurements on the iodination of Rh4(C0)12 at 516K gave a mean value AH = 30.3 kcal mol-' ,for a reaction approximated by Rh,(CO),,(c 298)+$12(g 516) -+ $Rh13(c 516)+$Rh(c 516)+ 12CO(g 516). No experimental value is available for the enthalpy of formation of RhI,(c); we have estimated a value AH -45 kcal mol-l for the reaction Rh(c)+$I,(g) + Rh13(c) on the basis of comparison with known values for other transition metal halides.Using this the measured reaction heat corresponds to AH -150.3kcal mol-' GENERAL DISCUSSION for thermal decomposition at 516 K,and to AH298 = 126.3 kcal mol-'-in fair agreement with the mean AH298 of table 1. The directly measured AH298 = 124.6(+4) kcal mol-l replaces the preliminary value given in the paper and leads to the revised values AH;[Rh4(CO)12 c] = -442 kcal mol-l AHj![Rh4(CO)12,g] --418 kcal m01-l~ and AHdisrupt633 kcal mol-l. Further studies on Rh6(C0)16 -were made using the original sample of material. Because of the difficulties of accurate analysis of the purity of this compound we continue to regard the results as "preliminary " pending studies on different samples of prepared material.TABLE DECOMPOSITION OF Rh4(C0)12 1.-THERMAL expt Rh.i(COhlmg Ah/cal T2IK i\Hobs/kcal mol-1 AH29dkcal mol-1 1 4.040 0.823 574 152.3 122.0 2 3.805 0.749 574 147.2 116.9 3 3.690 0.756 518 153.2 129.2 4 2.795 0.553 485 148.7 128.3 5 2.690 0.524 524 145.8 120.4 6 3.320 0.682 533 153.6 127.9 7 2.140 0.437 528 152.9 127.3 mean AH2s8= 124.6kcal mol-I The thermal decomposition results on Rh6(C0)16 are summarized in table 2. The mean AH298 = 65.0(&4) kcal mo1-l corresponds to AH;[Rh6(CO)16 c] = -488 kcal mo1-' AHj![Rh6(CO)16tg] -460 kcal mol-l and to AHdlsrupt-836 kcal mol-'. Iodination studies indicated that AHzg8 is -70 5 kcal mol-l some-what higher than measured by direct thermal decomposition (this is in respect of the assumed value for the enthalpy of formation of Rh13).TABLE2.-THERMAL DECOMPOSITION OF Rh,(CO) 1 6 exPt RhdCO)is/mg Ahlcal Tz/K AHobs/kcal mol-1 A H29a/kcal mol-1 1 6.025 0.559 529 98.8 64.1 2 5.800 0.528 518 97.0 64.0 3 7.265 0.704 521 103.3 69.9 4 6.370 0.544 518 92.7 59.7 5 5.280 0.478 513 96.5 64.3 6 5.790 0.500 458 92.0 68.2 mean AH29a= 65.0 kcal mol-1 The enthalpy of disruption of Rh4(C0)12 measures the sum (9T+6B+6M) in the notation used earlier. In this instance in solution at least there is rapid scrambl- ing even at -65°C between bridging and terminal carbonyl groups,l so that the assumption that B -0.5 T receives additional support. If we accept that M -0.68T then T = 39.4 kcal mol-' and M = 26.8 kcal mol-l.The bond analysis of the disruption enthalpy is more difficult for Rh6(C0)16 because the solid-state structure contains four triply bridging carbonyl ligands and also because the molecule is formally an 86-electron system. If the assumptions are made that all bridging carbonyl ligands are bound simultaneously to only two rhodium atoms and that the Rh6 cage obeys the effective atomic number rule then we may equate the total enthalpy of disruption to the sum (12T+8B+ 11M). A J. Evans B. F. G. Johnson J. Lewis J. R. Norton and F.A. Cotton J.C.S. Chern. Cornrn. 1973 807. GBNBPAL DISCUSSION simultaneous solution with the equation relating to Rh4(C0)12 leads to a negative value of M so that one concludes that either M or T changes on going from one molecule to another in contrast to our earlier basic assumption regarding transfer- ability of these quantities.If M is assumed constant in Rh4(CO)1 and Rh6(C0)'6 then T = 34 kcal mol-l in Rh6(C0)1 6 ; if the opposite assumption is made then M = 20 kcal mol-' in R~~(CO)~~. We have shown that there is an apparent linear relation between T and the enthalpy of formation of the gaseous metal atom in that T N (0.28+O.O4)[bHj(M,g)]. Further consideration of the results suggests that M -2[AH;(M,g)]/n where n is the coordination number of M in the bulk metal. Finally the results suggest that M is approximateIy 60-70 % of T. While we do not suggest that there is any direct and necessary relation between M T and AH;(M,g) we believe that these empirical relationships provide a useful index of the bond strengths in polynuclear systems.Employing these relationships it is then possible to make an estimate of the value of T in the binary carbonyls of Pd Pt and Cu ; the values (which are expected to be minima) are approximately 24 35 and 21 kcal mol-l respectively. Carbonyl com- pounds of these elements have been detected spectrometrically in matrices at very low temperatures.' The binary compounds are unstable with respect to [M(c)+ CO(g)]at room temperature. Dr. D. S. Barnes (University of Manchester) said Measurements are being made using a " hot-zone " calorimeter of the enthalpies of iodination of metallic carbonyls to confirm by macro-scale studies some of the results obtained using the micro- calorimeter as described by Connor.It has been confirmed that the product formed by reaction of Cr(CO)6 with excess iodine is essentially the tri-iodide analyzing for CrIn,n > 2.95 < 3.05. With W(CO)6 a product analyzing as W12.,4 was obtained. Dr. H. A. Skinner (Universityof Manchester) said The assumption that the metal carbonyl and metal-metal bond enthalpies are transferable unchanged from Fe(CO) to Fe,(C0)9 and to Fe,(CO), was made of necessity in order to extract values for three unknowns (T and M) from three items of information. The M value so obtained is the average Fe-Fe bond contribution from four Fe-Fe bonds of different lengths (2.46A in Fe,(CO), 2.56A and 2.67A (twice) in Fe,(CO),,). A more sophisticated approach might attempt to discriminate between metal-metal bonds in respect of their lengths as a given bond is expected to be stronger the shorter it is.The problem is complicated however by the association of bridging carbonyls with these metal-metal bonds; thus e.g. the Fe-Fe (2.46A) in Fe,(CO) has three bridging CO molecules assisting it the short Fe-Fe (2.56A) in Fe3(C0)12 has two associated bridging CO molecules but the longer Fe-Fe bonds (2.67A) are acting alone. The latter are longer than the Fe-Fe " bonds " in a-Fe (b.c.c. Fe-Fe = 2.48A) ; in Ru3(C0)12 and OS~(CO)~~ all the metal-metal bonds are " unassisted " and are significantly longer than in the metals themselves (Ru-Ru = 2.85A in RU,(CO)~, against 2.65-2.71A in ruthenium metal ; 0s-0s = 2.88A against 2.67-2.73A in osmium metal).The dissociation energy in the diatomic gaseous molecule Fe, is reported as 25 kcal mol-l but the bond length is not known. The Co-Co bond lengths in CO,(CO)~ (2.52A) and CO~(CO)~~ (2.49A) are similar to the bond length (2.51 A) in cobalt metal-and there would appear to be E. P. Kiindig D. McIntosh M. Moskvits and G. A. Ozin J. Amer. Chem. Suc. 1973 95 7234 (and references therein). S. S. Lin and A. Kant J. Plzys. Chem. 1969 73 2450. GENERAL DISCUSSION little scope for variability of M in these compounds. In Rh4(C0)12 the Rh-Rh bond lengths vary over the range 2.7-2.8A (average 2.73A) and are marginally shorter than in Rh6(C0)16 (Rh-Rh = 2.78 A). The metal has Rh-Rh = 2.69 A. Cocke and Gingerich have recently reported a value 66+6 kcal mol-l for the dissociation energy of Rh,(g)-substantially larger than the value (26.5 kcal mol-l) adopted for M in the present paper.It is probable that the bond length in Rh is very short (the estimated value = 2.28A) compared with Rh-Rh bond lengths in the carbonyls. Analogously the bond length in RhC(g) of 1.61 A is very short com- pared with the bridging Rh-C- bond length in Rh6(C0)16 (2.17 A). II 0 In view of the limited amount of data yet available on dimetal molecules M2 the only general starting point for estimation of M-M bond contributions is from the heats of atomization of the pure metals. For example Rh metal has AHsub-133 kcal mol-1 at 298 K and is f.c.c. with C.N = 12 in the solid state at room tempera-ture supposing that the major contribution to the cohesive energy of the metal TABLE1 -metal "metallic " bond M Mn 11 16 Re 30.7 30.5 Fe 16.6 19.2 Ru 25.6 28 0s 31.5 31.1 co 16.9 22 Rh 22.2 26.5 Ir 26.5 31 originates in the metallic bonding between adjacent Rh atoms in the lattice then each Rh-Rh bond (2.69 A) contributes -133/6 N 22 kcal mol-I.The "metallic bond " is electron deficient with respect to a normal covalent bond M-M which might therefore be expected to be both shorter and stronger than its metallic counter- part. " Metallic '' bond-strengths (values in kcal mol-I) calculated in this way compare with the M values given in the paper are given in table 1. D. L.Cocke and K. L. Gingerich J. Chem. Phys. 1972,57,3654.