Chemistry of Precious Metals - CNTQ

All of these probably have the AlCl3 structure (unconfirmed for RhI3) with
bond lengths (EXAFS) of 2.337 A (Rh-Cl) [13] and 2.48 A (Rh-Br) [14].
'Soluble' chlorides and bromides are made by dissolving the oxide in the
appropriate acid.
Rhodium trihalides (and complexes like K3RhBr6) are frequently added to
photographic emulsions in trace quantities to improve the gradation of the
emulsion (the 'rhodium effect') [15].
Only fluorides are known in higher oxidation states with tetra-, penta- and
hexafluorides isolated.
RhBr3 -^X RhF4.2BrF3 -^ RhF4
Rh > RhF5
40O 0 C, 6 atm
RhF3 F2 ) RhF5
40O 0 C
RhF5
25O 0 C, 30 atm
> RhF6
Little is known about the structure of purple paramagnetic RhF4
(Meff — 1-1 MB) but it may be similar to PdF4 [16]. RhF5 has a tetrameric
structure [17] similar to RuF5 and OsF5 (section 1.3.4); the terminal Rh-F
bonds are 1.808 A and the bridges 2.01 A. The ruby red solid (m.p. 95.5 0 C)
has //eff = 2.39/IB. Rhodium hexafluoride is a very reactive black solid
(attacking glass at room temperature) vaporizing to a deep brown gas
(triple point c. 7O 0 C). EXAFS measurements indicate a Rh-F bond length
of 1.838 A [18].
Various ill-defined binary halides have been reported but not characterized,
such as RhI2.
2.5.2 Iridium halides
The pattern of iridium halides resembles rhodium, with the higher oxidation
states only represented by fluorides. The instability of iridium(IV) halides,
compared with stable complexes IrCl4L2 and the ions IrX6" (X = Cl, Br,
I), though unexpected, finds parallels with other metals, such as plutonium.
Preparations of the halides include [19]
IrF4 J??!£U IrF3 (black)
40O 0 C
IrF5 + SF4 -^-^ IrF3 + SF6
Ir —^-> 0-IrCl3 (brown) 75 °" C . /3-IrCl3 (deep red)
60O 0 C V vacuum J V K /