Aug. 20, 1954
NOTES
of the melting point of a polymer by added diluent in terms of the heat of fusion Hr of the polymer
duced molybdenum was determined by titration against the same reagent. The stock solution of exact quinquevalent molybdenum was prepared by mixing the solution containing molybdenum ions, reduced to an oxidation state 7 N agree quite closely with this value. This indicates that, in this range of HC1 concentrations, molybdenum(V) ions exist as monomer entities. As the HCl concentration is lowered, the magnetic susceptibility begins to drop and approaches null value as the 2 N concentration of HC1 is approached. This shows that a t acid concentration lower than 7 3,electron pairing takes place, reaching the completion at HC1 concentration near 2.5 N. Below this value, 100% of molybdenum(V) ions exist as dimers containing two mo(4) (a) w. Klemm, “Magnetochemie,” Akademische Verlag, Leipzig, 1936,p. 150; (b) W. KIemm and H. Steinberg, 2. anorg. allgem. Chcm., 227, 202 (1936). ( 5 ) Cf. P. W. Selwood, “Magnetochemlstry,” Interscience Publishers, Inc., New York, N. Y., 1943,p. 106; W.Klemm, ref. 4a, p. 193; W. Klemm and H. Steinberg, ref. 4b, p. 193. (6) Gf. W. Klcmm, ref. 4a, P. 111; P. W.Sclwood, ref, 8, p. 62.
4240
NOTES
MAGNETIC SUSCEPTIBILITY NO.
1 2 3 4
5 6
7 8 9 10
MO,
c1
%
%'
2.670 2.736 2.752 2.767 2.783 2.835 2.859 2.900 2.929 2.958
33.13 31.08 27.08 24.94 22.79 20.55 18.24 16.03 13.73 11.21
HzO,
%
64.20 66.18 70.17 72.30 74.43 76.62 78.90 81.07 83.34 85.83
OF
Vol. i 6
TABLE I HYDROCHLORIC SOLUTIONS O F MOLYBDENUM( V ) IONS
HCI, moles/l.
10.29 9.46 7.99 7.19 6.39 5.61 4.82 4.05 3.27 2.45
xg x 100 -0.3138 - .3106 - ,3155 - ,3182 - .3194 - ,4387 - ,5723 - .6560
P
1.209 1.196 1.178 1.166 1.154 1.142 1.134 1.120 1.111 1.098
-
26
-0.6712 - ,6733 - ,6778 - .G795 ,6815 - .6838 - .G862 ,6882
-
-
.G740
-
,0876
- .6930
,6910
X I X 106
+1284 +1272 +1263 +1253 +1146 +830 382 107 +52.4 +17.5
+ +
Mo(Vj monomer,
70
100 100 99.2 08.4 90.0 64.8 29.1 7.2 2.8 0.0
TABLE I1 MAGNETIC SUSCEPTIBILITTES O F MOLYBDENUM-BLUE SOLUTIONS Solution I containing Mo(V) and Mo(V1) ions MOW. MOW), 0
a
7c 1.59 1.50
70
c1, 70
xg x i o 0 -0.6585
95% ethanol
x g x 100
% (Kt.) of sol. I
11.57 2.98 -0.7276" 41.10 1 9.87 4.32 - .G583 2 .7281" 41.19 These values refer to samples from two different standard 95vo ethnnol.
-
lybdenum atoms linked by an electron-pair bond. The little value of +17.5 X lop6 for X A a t 2.45 N HCI, can be considered as due to a small temperature independent paramagnetism usual in the molybdenum compound^.^ At lower acid concentration, the values of X A remain roughly constant but such values are scarcely accurate since the diamagnetic contribution of the solvent reaches nearly the value of the susceptibility of the solution.
1200
lo00
Concn. o f HCL (moles/l.). Fig. l . - x A
of molybdenum(V) ion as a function of HCI concentration.
Both monomer and dimer forms of mobbdenum(V) ions are in equilibrium, as is confirmed by the reversible change in color from green to brown and v k , of the solutions. (7) C/. P. W. Selwood, ref. 5 , p. 150; W. K l e m m , ref. 4a. p . 151.
?Jolybdenum-blue solution Calcd. expt. xg x io0 Xg X 106
-0,6992 .G993
-
-0,7001 & 5 . 1 0 - 4 .G089zk55.10-4
-
It must be remembered, a t least, that the sharing of a pair of electrons by two molybdenum(V) atoms was already assumed by Klemm8 in order to cxplain the diamagnetism or the feeble paramagnetism of certain complexes of quinquevalent molybdenum. Present investigations correlate with those on amorphous molybdenum-blue which was found to be diamagnetic.9 In fact, the measurements above reported show that, in hydrochloric solutions of quinque- and sexavalent molybdenum ions, the disappearance of paramagnetism of molybdenum(V) ions, at concentration of HCI near 2 N , precedes the formation of molybdenum-blue which, as is well known, takes place in solution less than 2 N in hydrochloric acid ~ o n t e n t . ~ As a confirmation the susceptibilities of colloidal solutions of molybdenum-blue, obtained by mixing the brown solutions containing quinque- and sexavalent molybdenum ions with 9570 ethanol, lo arc found to be, within the experimental errors, strictly additive to the susceptibilities of both the mixed solutions. This affords evidence that the diamagnetism which accompanies the appearance of Mo(V)-Mo(V) electron-pair bonds takes place before molybdenum blue is formed. Table I1 reports the data referring to thesc mcnsurements carried out with a very sensitive torsion balance already described elsewhere. I Acknowledgment.-The financial assistance of the Italian Research Council (C.N.R.) which supported this research is gratefully acknowledged. INSTITUTE O F PHYSICAL CHEMISTRY UNIVERSITYOF FLORENCB
FLORENCE, ITALY (8) W Klemm and H. Steinberg, ref. 4b. p . 211. ~y s. . , 18, 1124 (inso); L. (9) I,. Sacconi and R . Cini, J . c ~ wp h Sncconi a n d R. Cini, AN^. chim. ( R o m e ) , 4 2 , 706 (1952). (10) 95% ethanol w a s used as diluent since in aqueous alcoholic solution i t can t e reached a concentration in molybdenum-blue larger t h a n in aqueous one before flocculation takes place. ( 1 1 ) I t . Cini and I,. Sacconi. J . Scz. Znslv., 31, 5 6 (1954).
