CLIFFORD E. MYERSAND ALAN W. SEARCY
52G
silver is in the fl oxidation state is shown by its diamagnetism (X, = -194 X lo+'). Stabilization of Ag(T1) by (AO) should occur, for the structure would be planar and have the extra energy of the hydrogen bond. However, treatment with HzO2 or SzO* = does not appear to oxidize [Ag(AO)zI+'. Palladium(I1) in dilute acid solution rapidly fornis compounds of the type [Pd(AO)Clz]O with all of the (-40)'s studied. In neutral media a second ligand is added which predominantly attaches cis. It has been shown" that [Pd(40),-H] C104 and [Ni(A0)~-H]C104 are isomorphous with one axis of the former's unit cell slightly enlarged. Platinum(I1) slowly forms the intermediate [Pt(AiO)Clz]o in acid solution and in neutral solution [Pt(,40)~-H]+lmay be isolated. I n this case, however, the yield of the cis form is low and the trans may be present but difficult to isolate. In every Pt(I1) or Pd(I1) compound containing two (AO) groups the characteristic infrared absorption of the hydrogen bond was observed, and was absent in all mono-chelated complexes. The action of free (AO) on [Co(HzO)a](C104)2in water lowers the p H and yields a slightly soluble I I1 I
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I)ri\ate r o m r n u n i c ~ t i o n
Vol. 79
brown precipitate of [CO(AO)~ - H]C104 which oxidizes rapidly with air to give a mixture of Co(111) compounds. The octahedral coordination compounds of Co(Ir1) can be prepared using essentially the same procedures as for the corresponding ethylenediamine compounds. However, unlike the ethylenediamine and (DMG) compounds there is no evidence that a tris-(AO) compound exists and pII changes do not rupture the liydrogen bond as they appear to do in the [Co(DMG)L -2H Xz1-l series. The properties of the cobalt-(AO) compounds suggest that the (A0)'s have a planar configuration and that the monodentate ligands assume trtrizs positions. Sterically, the formation of a hydrogen bond would appear to be impossible if this were not the case. I n addition the green color of [Co(AO)n-HCl2lfl is similar to many other tuumdichloro Co(II1) compounds as opposed to the purple cis isomers. Attempts to prepare cis isomers by procedures analogous to those used in thc ethylenediamine series have been unsuccessful. Acknowledgment.--The author expresses his appreciation to George F. Svatos for the infrared spectra measurements. STORRS, COY\FCTICI r
[ C O Y T R I F j ' J T I O V F R O M T I I E DEPARTMENT OF CHEMISTRY, P 3 R D U 6 UNIVERSITY, A N D THE D I V I s t O S O F >fINRRAI,
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UNIVERSITY OF CALIFORSIA, BERKELEY]
The Dissociation Pressures of the Tantalum Silicides's3 BY CLIFFORD E.MYERSAND ALAN W. SEARCY RECEIVED AUGUST27, 1956 The dissociation pressures of the tantalum silicides have been measured by the Knudsen effusion method. Phase modifications in the temperature ranges covered were established by quenching experiments and X-ray diffraction investigation. The heats of dissociation were combined with the heat of vaporization of silicon to obtain, for the heats of formation per silicon atom a t 298'K. : 4.5TaSio.2?,,0 03, -34.4 kcal.; Ta?Si, -29.3 kcal.; 1.67TaSio,eo,o.o~, -26.7 kcal.; and l/*TaSiz, - 11.6 kcal.
The silicides of t a n t a l ~ m ~have - ~ been shown by Kieffer, Benesovsky, Nowotny and Schachner6 to have melting points in excess of 2000". From a consideration of the trends in stability of carbides, nitrides and other related compounds as well as from stabilities demonstrated in high temperature chemical reaction,' the silicides of tantalum could be predicted to be among the most stable of all silicides from a thermodynamic standpoint. Recause of the expected high stabilities, measurement of heats and free energies of formation of the tantalum silicides seemed of particular interest. Accordingly, stabilities of all four of the stable tantalum silicide phases : TaSi0.22~f0.03, TazSi, TaSio.60f0.08 and TaSiz have been determined from (1) From a thesis by >Ir. Myers in partial fulfillment of t h e requirements for the P h . D . degree a t Purdue University. ( 2 ) This research w a s supported by the Office of Naval Research. ( 3 ) 0 . Honigschmid, M o n o t s h . , 28, 1017 (1907). (-1) I T . J . Walbaum, Z. Metallkunde, 33, 378 (1941). (.i)I.. Brewer. A . W. Searcy, D . H . Templeton and C. €I. Dauben. J A m . C e r a m . Soc., 33, 291 (1950). (ti) I
