2.23. Volatile compounds of lanthanides (for Comprehensive Inorganic chemistry)
A. Drozdov, N. Kuzmina, Moscow State University
The term volatility, first suggested by Lewis in 1901, characterizes the tendency of a substance to vaporise. The strict determination precise definition of the term volatility is based on determination of vapor pressure at different temperatures. Here we define volatile compounds as those that can evaporate and condense without changes at moderate temperatures.
During sublimation and condensation, the composition of volatile substance remains unchanged. Besides, in some cases we call compounds, that during sublimation dissociate loose neutral ligands, but during condensation the initial composition forms, volatile as well. The preliminary experimental criterion of volatility is the compound’s ability to sublime. Sublimation in vacuo with detectible rate at fixed temperature means that vapor pressure of a substance exceeds 0.01-1Pa. It should be noted that the sublimation temperature is indicative of volatility.
It is the temperature at which a substance sublimes with detectible rate under given conditions, i.e. in a given reactor and at fixed pressure. In literature, the sublimation temperature (that in reality changes within some limits even during the experiment) is widely used as an approximate characteristic of volatility. More strictly, the volatility can be proved using mass-spectrometry by the presence of the complete molecules of the substance or the metal containing fragmentized ions formed from the ionization of the molecules directly in the vapor phase, in mass spectra.
What fully charaterizes a volatile compound is the dependence of its vapor pressure on temperature. However, only some lanthanide compounds are have been characterized in this way. For most volatile compounds of lanthanides only the sublimation temperature range is known.
The volatile compounds of lanthanides can be divided into several groups. The first one is complexes with covalent sigma-donor ligands such as halides, amides, alkoxides, including covalent organometallic compounds without pi-bonding. The second group is represented by metal chelates, among which the derivatives of beta-diketones being the most important. This group also includes dialkyldithiocarbamates, carboxylates and others. Pi-complexes with Cp and others ligands should be discussed separately. This classification fails to sign mixed ligand complexes, containing, for example, both diketonate and alkoxide ligands, to a separate group.
Volatility significantly depends on the structure of a compound. It is logical that it favors molecular packing with weak intermolecular interactions, for example, complexes with bulky groups. On the other hand, large substituents (such as tertial tret-butyl) facilitate the intermolecular interactions that increase significantly with the increase in molecular weight, thus suppressing the predicted increase in volatility partially or completely.

The lanthanide cations are hard Pyrson acids that have affinity to fluoride and oxygen rather then to chlorine, sulfur and phosphorus donor atoms. Most lanthanide compounds including complexes are predominantly ionic with high coordination numbers (from six to eight or even higher) which is in accordance with their large ionic radii. Volatility can also be expected in the case of weak intermolecular interactions that occur between molecules with fully occupied, coordination sphere. Taking into account high coordination numbers and oxidation state of the metal (generally +3, in some cases +2 or +4) one can predict that the saturation of coordination sphere requires, besides two to four negatively single-charged anionic ligands
(usually bidentate such as nitrate or diketonate), some (from one to three) monodentate neutral ligands from the solvent or of other nature, added to a reaction mixture. Such compounds are called adducts, i.e. neutral ligand weakly linked to a metal center. While heating or even storing this neutral ligand can become free and leave the coordination sphere of a metal, which causes polymerization of the species. To prevent it, in the absence of additional neutral ligands chelating ligands with bulky groups that restrain polymerization by steric hindrances can be used.
Yttrium, lanthanum and lanthanides (also called rare-earth elements, REE) have similar properties due to the proximity of their ionic radii. Because of lanthanide сontraction, the metalligand bonding strengthens in the REE row from La to Lu, yttrium being similar to the heavy lanthanides (of the so called yttrium family). This results in the weakening of the intermolecular interactions and increase in volatility. Calculated molecular contractions for molecular trihalides and hydrated cations show the contribution of the relativistic effects by participation of a 4f-shell in bonding only in lutetium fluoride [1]. The augmentation of the oxidation state, for example, in cerium compounds diminishes ionic radius and consequently leads to the increase in volatility.
The volatile compounds must be stable up to the temperature range of sublimation.
Nevertheless, a large number of compounds dissociate while heating, usually losing additional ligands or solvent molecules. It results in polymerization of the remaining part of the molecule.
The resulting polymer can be often sublimed without decomposition that allows to use such complexes as precursors of volatile compounds and to include them in further discussion [2].
1. Halides
The fluorides of REE are high-melting ionic compounds, pour soluble in water and hydrofluoric acid [3].
The unhydrous chlorides can be prepared from hydrated products by treatment with desiccating agents such as oxochloride of sulphur(IV) or by heating with ammonia chloride, or in the atmosphere of dry chlorine, hydrogen chloride or phosgene. Crude products containing

oxochlorides can be prepared by chlorination of oxides using carbon tetrachloride, hydrogen chloride, phosgene, phosphorus pentachloride, sulphur monochloride together with chlorine or by chlorination of oxide mixed with carbon. They can be purified by distillation. The REE chlorides are solids with high melting points (from 500 to 957

C) that can be sublimed in vacuo at high temperatures. They form infinite frameworks of predominantly ionic character, in which
REE metal cations are linked by bridging chlorides. The boiling points, temperatures of vapor at different pressures and enthalpies of evaporation are given in Table 1.
Table 1 The vapor pressare, entahlpies of vaporization and bowling points of REE chlorides
[3]
Compound B.p.,

C Vapor temp at pressure, mm Hg
4 2 1 0,1

Hv, kJ/mol
YCl
3
LaCl
3
CeCl
3
PrCl
3
NdCl
3
SmCl
2
EuCl
3
GdCl
3
TbCl
3
DyCl
3
HoCl
3
1510
1750
1730
1710
1690
2030 dec
1580
1550
1530
1510
1050
1027
1195
1144
1166
1400
998
1048
1121
979
986
ErCl
3
TmCl
3
YbCl
3
LuCl
3
1500
1490 dec
1480

extrapolated values
1155
951
1091
998
975
997
1125
1085
1106
1229
930
995
1068
939
953
1076
925
1039
959
909
969
1065
1031
1048
1087
869
947
1010
899
919
1000
899
994
926
892
764
703
808
735
886
888
878
854
779
827
809
824
(856)

(819)

129,2
329,8
170,5
187,3
185,6
83,2
129,2
183,9
175,6
201,5
262,1
137,5
324,0
199,4
239,1
To summarize, the halides of REE are low volatile at moderate temperatures. For example, to sublime neodymium trichloride at reduced pressure we need ca 700

C. One of the possible ways to diminish the temperature range of sublimation is to use mixed complexes.
In the molten KCl medium the vaporization of LnCl
3
occurs better that in the single phase.
The investigation of LnCl
3
-KCl equimolar molten mixtures at 1018-1273 K by means of
Knudsen effusion mass spectrometry shows the presence of vapor species KLnCl
4
over the melt.
The volatility enhancement of NdCl
3
by the formation of the vapor complex KNdCl
4
decreases with increase in temperature. A relatively small enthalpy change, -10 ± 21 kJ/mol, of the gas

phase exchange KNdCl
4
(g) + KCl(g) = NdCl
3
(g) + K
2
Cl
2
(g) allows to suggest that the structural change during the reaction is not drastic and that the KNdCl
4
(g) complex has two bridging and two terminal chlorine atoms [4].
It is well known that aluminum trichloride (as well as FeCl
3
, GaCl
3
) due to the strong chloride ion affinity forms gaseous complexes with different metal chlorides including REE. The solution of REE chlorides in molten aluminum trichloride doesn’t contain any products of interaction between these compounds, but in the vapor phase the mixed metal complexes form:
LnCl
3
(s) + n/2Al
2
Cl
6
(g) = LnCl
3
 nAlCl
3
(g)
The thermodynamic parameters of such reactions have been calculated. For example, for neodymium compounds

H is 157,0

13 kJ/mol,

S is 110,3

13 J/mol

K for n = 1 and 45,2

1 kJ/mol, 8,4

1 J/mol

K, respectively, for n = 3 [5]. The vapor pressure of neodymium compound determined from electron spectra by the intensity of Nd
3+
band at 17030 cm
-1
showed the significant increase in volatility (ca 10
13
times at 600K and 3

10
7
times at 800K, p(AlCl
3
) =
10
5
Pa. This increase is due to the depolymerization of solid REE chlorides as a result of interaction between chloride-group of the former with aluminum chloride giving [AlCl
4
] unit that prevents polymerization by means of Cl-bridges. From the band at 17030 cm
-1
in the electron spectra of NdCl
3
-AlCl
3
vapor the octahedral coordination of neodymium [NdCl
6
] can be proposed. Partial pressure of such compounds with general formula LnCl
3
 nAlCl
3
has a very complicated dependence on temperature and AlCl
3
vapor pressure. At fixed p(AlCl
3
) the vapor pressure of LnCl
3
 nAlCl
3
with temperature has a maximum that shifts at high temperatures with increase of p(AlCl
3
). The similar effects of gas-phase transport are also known for anhydrous ferric chloride and gallium chloride, the complexes of former being less stable in comparison with aluminum derivatives discussed above. The enhancement of REE halide vapor pressure in the melts containing alkali metal halides or aluminum halides has been used in the manufacturing of REE sulphides. The process comprises heat-treating a mixure of a rare earth halide and an alkali metal halide or aluminum halide in the presence of H2S at a temperature high enough to volatilize the halides [6].
The tribromides and triiodides of REE are similar to trichlorides discussed above. The synthesis of tribromides can be carried out by the ways discussed above for trichlorides or by heating of hydrated salts in vacuo at 70

C followed by gradually increasing the temperature up to
120

C [7]. The triiodides are generally prepared by heating of anhydrous trichlorides in the stream of hydrogen iodide and hydrogen. This method does not work for europium, and in the case of samarium and ytterbium, the temperature control should be very accurate. All the compounds except unstable iodides of europium(III) and samarium(III) are volatile at high

temperatures. Aluminum bromide was used to accelerate the volatility of anhydrous LnBr3 in a way already discussed above for trichlorides.
Among the binary tetrahalides only fluorides of cerium, praseodymium and terbium are known, all of them are easily decomposed upon heating.
The dihalides that are stable for Nd, Sm, Eu, Dy and Yb are high-melting poorly-volatile ionic solids prepared by reduction of trihalides with hydrogen or ammonia. The diiodides can be made by thermal decomposition of triiodides, EuI
2
forms as olive-green precipitate by the addition of sodium iodide to the solution of europium(III) salts. The dichlorides of samarium and europium can be transferred into a vapor phase at lower temperatures by the addition of aluminum chloride.
2. Complexes with amides
Among the amide derivatives of REE only complexes with donor-functionalised amido ligands contating bulky groups are stable. Dialkylamides such as lithium diisopropylamide give homoleptic tetracoordinate anionic complexes with REE, e.g. [Li(THF)
4
][Ln(NR
2
)
4
] are stable, but non volatile due to ionic character. The steric saturation of the metal center in such compounds is achieved by interaction between REE and carbon atoms of the amide [8]. The complexes with di-isopropylamide, i
Pr
2
N−, ( i
Pr = (CH
3
)
2
CH) can be sublimed only with decomposition, e.g., [Ln(N i
Pr
2
)
3
] decomposes at 80

C and 10
−4
Torr [9]. Complexes with the dimethylamide ligand can be stabilized only in the presence of MMe
3
(M=Ga or Al) to form
[Ln(NMe
2
)
3
(MMe
3
)
3
].
To make the compound more volatile, sterically hindered bulky ligands such as silylamides can be introduced [10]. The large size of the silylamide ligands is effective in filling the coordination sphere of the Ln 3+ ion. The (trimethylsilyl)amide group due to the trivalent nitrogen directly linked to a metal center, has a unique ability to shield REE from attacks of other ligands. As a matter of fact, in homoleptic silylamido-Ln(III) compounds the coordination number of REE is three even in solid state. For instance, in Nd(N(SiMe
3
)
2
)
3
the angle N-Nd-N is
117,8

, that is close to planar triangular geometry of the coordination sphere [11]. Such compounds, first reported in 1973 [12], show the unique example of three-coordinate lanthanide complexes. To prepare such compounds the reaction of unhydrous chloride with lithium hexamethyldisilylamide is used:
LnCl
3
+ 3LiN(SiMe
3
)
2
= Ln(N(SiMe
3
)
2
)
3
+ 3LiCl
The products are moisture sensitive and easily hydrolize.

Fig. 1. Crystal structure of Nd(N(SiMe
3
)
2
)
3
[11]
The compounds without solvent molecules are volatile due to non-polar nature of the exterior of a complex and weak intermolecular interactions, and can be sublimed in vacuo 10
-2
Torr at temperature range 353 – 373 K; the sublimation temperature poorly dependent on the atomic number of REE and being close to that for analogous complexes of d-metals. If the synthesis or recrystallization is performed in solvent which molecules contain electron-dnor atoms such as THF, the adducts, like Ln(N(Me
3
Si)
2
)
3
(THF) form. The TPPO adducts are also reported [13]. The adducts are more stable in the case of less sterically hindered amide ligands such as (Me
2
HSi)
2
N− [14]. The monomeric nature of these compounds provides corresponds to their volatility that changes from 100 - 105

C/ 10
-4
Torr for La (and Y) to 75-80

C/ 10
-4
Torr for
Lu [12] .
Lithium hexamethyldisilylamide reacts with Eu(II) and Yb(II) derivatives forming anionic tricoordinate complexes MLn(N(SiMe
3
)
2
)
3
or molecular adducts with ethers. The latter are volatile according to their mass-spectra [15].
Bulky alkyl ligand −CH(SiMe
3
)
2
, similar to silylamide one, also gives monomeric threecoordinate complexes, similar to those of silylamides [16], but no volatility data are available.
3. Alkoxides and siloxides
The interest in volatile alkoxides, including the derivatives of alkylsubstituted phenols and heterobimetallic compounds, is linked to the search for soluble and volatile precursors for laser and superconducting materials, sensors, catalysts etc. The results of such studes are summarized in numerous books and reviews by D. Bradley [17], Mehrotra et al [18-21],
Bochkarev et al. [22], Turova and Turevskaya [23], Hubert-Pfalzgraf [24]. Derivatives with a general formula “Ln(OR)
3
” in realty have more complex composition, containing oxo-groups.
For instance, for a family of crystalline RE isopropoxides the crystal structure determination shows Ln
5
O(O i Pr)
13 composition, where Ln= Sc, Y, Er, Yb. The oxo-group forms due to the desolvation of the very unstable [Ln(O i Pr)
3
( i PrOH)]
2
solvates (perfectly soluble and rather

reactive), characterized for Nd. Desolvation of Ln(OBut)
3
•2L (Ln=Y, La; L=tBuOH, THF, Py) also leads to the formation of oxocomplexes.
The volatility of RE alkoxides depends on the nature of the substituent in the alkoxygroup. The simplest alkoxides – methoxide and ethoxide – give unvolatile derivatives due to the formation of the stable unsoluble polymeric species. The better results show the derivatives with branched or fluorinated groups such as isopropyl, tret-butyl, neo-pentyl, triethylmethyl and their fluorinated analogues including phenoxides (Scheme 1).
Scheme 1. The most important alkoxy-groups providing volatility of RE derivatives
O
O
O i-PrO-
Et
3
CO-
O t-Bu t-BuO-
F
3
C
CF
3
O
(CF
3
)
2
MeCO-
MeEt(i- Pr)O-
OEt t-Bu
O t-Bu
EtOCH
2
(t-Bu)
2
COt-Bu
O O t-Bu t-Bu
(t-Bu)
2
-2,6-C
6
H
3
O-
(t-Bu)
2
-2,6-Me-4-C
6
H
2
O-
The REE alkoxides can be prepared by exchange reactions between REE chlorides and alkali metal alkoxides in molar ratio 1:3. As a by-product in such reaction, mixed-ligand alkoxide chloride can be formed. Only few of such compounds have been characterized crystallographically (e.g. Nd
6
Cl(O i
Pr)
17
[25], Y
3
Cl(O t
Bu)
8
•2THF [26, 27] ). The incorporation of chloride in a multinuclear complex has been also reported for alkaline-earth metals, such as barium [28]. The fact is, that after being incorporated, chloride cannot be easily removed even by the excess of alkoxide. In such cases, bimetallic alkoxide complexes such as
[LiNdCl(OC t
Bu
3
)
3
(THF)
3
] [29] form. Instead of REE halide, REE carboxylates can be used. The synthesis of isopropoxides was performed by reaction of acetates or bensoates with L i
OPr, in

order to obtain t-buthoxides KO t
Bu, oxalates of REE have been used [30]. Another synthetic route is to start from easily accessible anhydrous Ln(OCOCCl
3
)
3
[31, 32], or to use the adducts of Ln(NO
3
)
3
with glycols or polyethers [33, 34] or alcoholysis of Ln[N(SiR
3
)
2
]
3
[35]. The latter reaction in the presence of excess fluoroalcohol can give ammonia derivatives due to the reaction of released silylamine (Me
3
Si)
2
NH [36]:
Ln[N(SiMe
3
)
2
]
3
+ 3ROH = Ln(OR)
3
+ 3(SiMe
3
)
2
NH
(SiMe
3
)
2
NH + 2ROH = NH
3
+ 2Me
3
SiOR
The common method for the preparation of REE isopropoxides is the electrochemical (anodic) dissolution of a metal. Outside of the electrolytic cell, REE metals react with alcohols very slowly even under reflux and in the presence of the mercury [37, 38]. In the case of primary alcohols, unsoluble products that are formed, prevent the reaction.
Fig. 2. The crystal structures of REE alkoxides with bulky ligands
Volatile RE alkoxides are crystalline molecular products, consisting of oligonuclear species, or polymers. The products obtained from solvents with electron donor atoms, usually contain coordinated solvent molecules, that can be easily removed by heating before sublimation. The low temperatures of their transition into the gas phase and their solubility in nonpolar organic solvents correlate with the small size of their molecules. The coordination numbers in these compounds are rather low for RE and usually do not exceed six. That is why the structure of RE alkoxides depends on the nature of substituent rather than the radius of the central atom; the derivatives of different REE with the same alkoxide are usually isostructural for the whole RE family. Complexes with sterically hindered ligands such as tret-butyl or substituted aryl-groups contain tri- or tetracoordinated metal atoms. Such species are monomeric or dimeric. The compounds with solvent molecules such as THF, MeCN are usually monomeric with CN = 5. Good examples of octaherdal coordination are isopropoxides and t-buthyl-oxides.
In Ln
5
O(O i
Pr)
13
that is Ln
5
(

5
-O)(

3
-O i
Pr)
4
(

-O i
Pr)
4
(O i
Pr)
5
, Ln atoms form tetragonal pyramid linked by four

3
-O i
Pr-groups located over the side faces and four

-O i
Pr groups located in the basal plane. The remaining alkoxy-groups are monodentate. All the metal atoms are in

octahedral [LnO
6
] environment. It is interesting to note that in the [Ln
5
] core one of the atoms can be substituted by another RE, forming bimetallic species such as [Y
4
Pr], [Nd
4
Ti], [Sm
4
Ti]
[23, 39].
A family of t-butylates consist of trimers [Ln
3
(O t Bu)
9
L
2
], where L = THF, py, t BuOH etc.
The [Ln
3
] triangles are linked by two

3
-O t
Bu groups and three

2
-O t
Bu groups. The four remaining alkoxy-groups are monodentate, saturating the coordination sphere of Ln together with L molecules up to CN = 6. The solvate [Nd(O i
Pr)
3
( i
PrOH)]
4
has a unique structure that belongs to the [Ti(OMe)
4
]
4
type. Some examples of heptacoordination are known, among them the monomer [La(OC
6
H
3 i
Pr
2
-2,6)
3
(NH
3
)
4
] containing seven monodentate ligands, four of which are rather small.
The volatility of RE alkoxides does not correlate exactly with the nature of REE that makes it impossible to map out any correlations with the change of the ionic radius of a metal.
The literature data even for the most common isopropoxide precursor Ln
5
O(O i
Pr)
13
are rather scarce and can not be compared “as is”, taking into account different experimental conditions.
As an example, the sublimation and decomposition temperatures obtained in [23] are presented in (Table 2).
Table 2. The sublimation and decomposition temperatures for Ln
5
O(O i
Pr)13 [23]
Metal Y La Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Temp 200 170 175 250 200 200 200 190 190 195 180 185 195 198 subl,

C
P,
Torr
0,1 0,01 0,04 0,1 0,1 0,1 0,01 0,1 0,17 0,18 0,01 0,06 0,2 0,1
Temp. 280 260 200 250 200 200 200 200 250 250 260 250 250 250
Dec.,

C
The mononuclear and oligonuclear alkoxides without additional ligands regardless of their nuclearity sublime without decomposition. The Ln
3
(O t Bu)
9
and Ln
5
O(O i Pr)
13
molecules are stable even in the gas phase. The polymeric species partly decompose during the sublimation.
The compounds containing additional ligands transform into gas phase with dissociation. For instance, La
3
(hftb)
9
Et
2
O, where htfb is anion of (CF
3
)
2
(CH
3
)COH sublimes at 90 - 130

C in vacuo 0,01 Torr losing ether molecules. In mass-spectra the La
2
F
3
(hftb)
2
+
, La
2
F
4
(hftb)
+
, La
2
F
5
+

and LaF
3
O
+
ions have been found [40]. Neopentyl derivatives under heating usually decompose according to equation:
Ln(OC t Bu)
3

CH
2
=CH(CH
3
)
2
+ Ln(OCH t Bu
2
)
3
.
For Nd decomposition occurs at 160

C (10
-3
Torr).
The branching of a substituent in the alkyl or aryl group results in the increase in volatility. For this reason the derivatives of 3-ethylpentanol-3 are among the most volatile ones.
For example, Y(OCEt
3
)
3
sublimes with partial decomposition at 224

C at atmospheric pressure.
The derivatives of fluorinated ligands have better volatility in comparison with nonfluorinated ones. The most studied are derivatives of hexafluoroisoproponanol and perluorotertbutanol. The complex La(OCH(CF
3
)
2
)
3
sublimes at 130

C in vacuo 0,01 Torr that is
40 degrees lower than non-fluorinated product. It should be taken into account that fluorination also decreases the decomposition temperature: polymeric La(OC(CH
3
)(CF
3
)
2
)
3
sublimes at
130

C/0,01 Torr with decomposition.
The fluorinated alkoxides of RE form stable adducts with donor molecules [41]. In some cases such adducts transform into vapor without decomposition. It is well known that hexafluoroisopropoxides form complexes with two molecules of ammonia, such compounds can be sublimed at 120 - 140

C/11 Torr. On the contrary, La(OC(CH
3
)(CF
3
)
2
)
3
(NH
3
)
2
loses ammonia while subliming at 85-105

C/0,01 Torr. In the gas phase, the peaks corresponding to
La
2
F
3
(OR)
3
+
, La
2
F
4
(OR)
+
, La
2
F
5
+
have been detected. The sublimation product in some cases depends on the way of heating. Under slow heating at moderate temperatures,
Ln(OCH(CF
3
)
2
)
3
(NH
3
)
2
loses ammonia and its volatility diminishes. The complex with diglyme upon sublimation loses the neutral ligand partly:
La
2
(OCMe(CF
3
)
2
)
6
Diglyme
3
(s)

2La(OCMe(CF
3
)
2
)
3
Diglyme(g) + Diglyme(g)
In the mass-spectra, La
2
(OCMe(CF
3
)
2
)
6
Diglyme
+ peak is detected.
Studies of heterometallic alkoxides have been initiated by their potential use as a volatile single-source heterobimetallic precursor for different solid-state materials (HTSC, perovskites,
LnAlO
3
etc). It is well known that alkoxide ligands can easily act as bridging between different metal centers.
All bimetallic alkoxides can be divided into three groups. The first one represents the complexes with alkoxides acting as efficient assembling ligands between different metallic centers, the second one includes the derivatives of functionalized (e.g., ether or amino) alkoxides and the third one – the mixed-ligand heterobimetallic complexes such as alkoxydiketonates. The complexes of the second group are non volatile [42].
The complexes of the first group in some cases can be sublimed without decomposition.
A good example is the isopropoxides Ln[Al(O i
Pr)
4
]
3
that forms in good yield by the interaction

between aluminium and RE alkoxides [43 - 45]. The molecular weight determination in boiling benzene shows the monomeric structure of complexes with aluminium atoms occupying the tetrahedrally coordinated sites. Such molecules have been detected in mass-spectra. All the REE including cerium (for which a simple alkoxide is not air-stable due to oxidation) in the form of heterobimetallic alkoxide have been prepared and studied. They sublime at 180 - 200

C in vacuo
0,1 Torr
Upon coordination with aluminium the volatility of the alkoxide rises, but not dramatically, as it has been demonstrated for mixed metal halides. In contrast to the latter, alkoxides are rather stable in solid state, solution and vapor. They can be distilled without decomposition. Similar complexes have been prepared for gallium [46] respectively.
A series of bimetallic isopropoxides Ln
2
Al
2
(O i
Pr)
12
( i
PrOH)
2
, Ln = Y, La, Nd, Pr, Er prepared by the interaction of [47 - 52] of REE and aluminium alkoxides in the isopropanoltoluene mixture or by dissolving rare-earth metal and aluminium isopropoxide in the same medium (using mercury(II) chloride or iodine as initiator), are volatile, but they dissociate into
LnAl
3
(O i
Pr)
12
and Ln
5
O(O i
Pr)
13
upon evaporation
15Ln
2
Al
2
(O i
Pr)
12
( i
PrOH)
2

4Ln
5
O(O i
Pr)
13
+ 10LnAl
3
(O i
Pr)
12
+ 4 i
Pr
2
O + 30 i
PrOH
In solution, however, these molecules remain intact according to NMR data, that allows their application as single-source precursors in the synthesis of Ln.
Complexes containing alkali metals have different structures and stechiometry, e.g.
[Na
2
Gd
4
(μ
6
-O)(O t Bu)
12
] is an octahedron [53, 54], Li
5
Sm(O t Bu)
8
has a structure Li
5
Sm(

4
-
OR)
2
(

3
-OR)
4
(OR)
2
consisting of two cubes with a common Li
2
O
2
face [55]. The latter complex sublimes at 135

C/0,5 Torr. Synthesis of a group of bimetallic alkoxides containing additional anionic ligands such as chloride or hydroxide has been reported. Such compounds, e.g.
[Na
8
EuX(O t
Bu)
10
]], X = Cl, OH sublime without decomposition at 125

C/10
-3
Torr [56 - 58].
The use of fluorinated ligands increases stability of the heterobimetallic complex due to the possible M...F interactions that prevent the dissociation of molecule in the vapor phase [59].
The third group of heterobimetallic complexes is based on alkoxides and b-diketonates.
The first volatile Y -Ba species Y
2
Ba[OCH(CF
3
)
2
]
4
(thd) has been reported in 1993 [60].
The donor-functionalized alkoxide ligands HOC(R
1
)(R
2
)CH
2
X, where R
1
is H or alkyl group, R
2 is an optionally substituted alkyl group and X is OR or NR
2
(R ia alkyl) give highly volatile and n-hexane-soluble complexes with lanthanides [61]. The volatility of these complexes is the highest achieved for RE alkoxides (sublimation < 100

C/10
-3
Torr) [62]. The neodimium complexes with Et
2
NCH
2
C(H)( t
Bu)O
−
, EtOCH
2
C t
Bu
2
O
− and EtOCH
2
C i
Pr
2
O
− sublime at
150
◦
C, 125
◦
C and 115
◦
C respectively [63].

Potentially tridentate alcohols HOC t
Bu(CH
2
O i
Pr
2
)
2
and HOC i
Pr
2
CH
2
OCH
2
OMe afford tris-alkoxides of reduced volatility. The most common of this family of ligands is 1-methoxy-2methylpropan-2-ol (Hmmp), giving dinuclear tris-complexes [Ln
2
(mmp)
6
] with significantly polarized metal centers originating from unsymmetrical ligand association (triple-bridging) [64].
A common synthetic route to them is the reaction of alkylamid or silylamide with stechiometric quantity of (Hmmp) ligand in toluene in the presence of tetraglyme. In this reaction tetraglyme acts as a stabilizing Lewis base that prevents the formation of condensed unvolatile oxoalkoxides [{Ln(mmp)
3-n
}
2
O n
]. Salt metathesis reactions using M(mmp) (M = Li or Na) and
Ln(NO
3
)
3
(tetraglyme) is also used by it can give some amount of heterobimetallic species. [65].
Unintentional employment of water-contaminated reagents give poorly-volatile oxo-species, such as Lu
4
(O)(OH)(OCMe
2
CH
2
OMe)
9
.
Fig. 3. Crystal structure of [Lu(mmp)
3
]
2
The RE complexes with bulky siloxide ligands such as HOSi(CMe
3
)
3-n
[(CH
2
)
3
NMe
2
] n
(n
= 1, 2) are analogous to those donor-functionalized alkoxides. The complexes with silanols are more thermally stable compared with alcohols as the decomposition of siloxide complexes is not autocatalytic [63].
The tris(siloxide) complexes are made by the reaction between the silanol
(prepared starting from chlorosilanes) and silylamide of REE. They are viscous slowly crystallizing oils of high solubility in hexane. Y{OSi(CMe
3
)
2
[(CH
2
)
3
NMe
2
]} complex can be sublimed at 115 /10
-4
Torr without decomposition [66]
Among the RE in oxidation states higher than three, only for cerium(IV) such volatile alkoxides are known. Due to the oxidizing power of Ce
4+
the complexes with aryloxides do not form. Due to the higher charge and smaller radius of Ce
4+
compared with Ce
3+
one can expect the monomeric complexes to be more common. Meanwhile, the higher acidity of Ce4+ causes hydrolysis and oligomerization. Thus, monomeric [Ce(O t
Bu)
4
(THF)
2
] condenses on standing in solution at room temperature for 3 days giving the oxo-bridged cluster [Ce
3
(O t
Bu)
10
O] that sublimes at 140

C/0,1 Torr [67], isopropoxide Ce(O i Pr)
4
•iPrOH gives Ce
4
(μ
4
-O)(O i Pr)
14
[68]. As in the case of Ln 3+ , the volatility of Ce(IV) alkoxides increase in the case of branched
(Ce(OCMeEt
2
)
4
140

C/0,06 Torr, boil.; Ce(OCMe
2
Et)
4
120

C/0,1 Torr, subl.; Ce(OCMe
2 n
Pr)
4

146

C/0,05 Torr, boil., Ce(OCEt
3
)
4
154

C/0,05 Torr, boil.) or fluorinated (Ce(OCH(CF
3
)
2
)
4
70

C/10 -4 Torr, subl., 200

C dec.) ligands .[69, 70 ].
A common preparation route to Ce(IV) alkoxides is the interaction between nitrato-
(NH
4
)
2
Ce(NO
3
)
6
[71] or chlorocerrates (IV) ((PyH)
2
CeCl
6
[72]), with alcohol in the alkaline media. In some cases the products of incomplete substitution for the nitrate-ligands, such as
Ce(NO
3
)
2
(O t Bu)
2
( t BuOH)
2
] can be isolated.
A number of volatile mixed ligand Ce(IV) complexes contatining alkoxide and different neutral ligands such as 2,2’-bipyridine, N,N,N’,N’-tetramethylethane-1,2-diamine, iso-propanol and N,N,N’,N’’,N’’-pentamethyldiethylenetriamine (pmdien) have been prepared [73]. A peculiar ionic product [Hpmdien]
2
[Ce(hfip)
6
] has been reported, in which the counter-ions are associated by a short F … C contacts. The compound can be sublimed at 70

C/10
-4
Torr in 85% yield [73].
4. Borohydrides
The tetrahedral borohydride-ion acts as di-, tri- and bridging tetradentate ligand towards different metal centers [74]. A possibility to occupy three or even four positions in the coordination sphere of metal together with small weight and small size leads to the volatility of metal hydrides. However, the lanthanide complexes of BH4 - and B3H8- are poorly volatile. As an example, borohydrides LnCl(BH
4
)
2
of heavy REE can be sublimed in vacuo with low yield only, for other REE such hydrides are unvolatile [74]. It was shown that the adduct
Y(BH
4
)
3
(THF)
2
obtained by the reaction between yttrium chloride and LiBH
4
in THF can be sublimed in vacuo at 90

C. The compound is ionic [Y(BH
4
)
2
(THF)
4
][Y(BH
4
)
4
] with all BH
4
groups being tri-coordinated. The analogous La compound with three molecules of THF is unvolatile [75].
One of the ways to increase the volatility is to use substituted hydrides. The methylborohydrides isolated as adducts with diethyl ether and pyridine M(BH
3
CH
3
)
3
·O(C
2
H
5
)
2 and M(BH
3
CH
3
)
3
·2C
5
H
5
N can be sublimed in vacuo at 100

C with the loss of the solvent [76].
A new class of borohydrides is the aminodiboranates - multidentate (often chelating) borohydride ligands binding to metal centers via M-H-B bridges. The N , Ndimethylaminodiboranate H
3
BNMe
2
BH
3
(DMADB) is involved in synthesis as a sodium salt
Na(DMADB). Its interaction with anhydrous RE chloride in THF or in solid gives triscomplexes Ln(H
3
BNMe
2
BH
3
)
3
, that can be sublimed under a dynamic vacuum sublime at low temperatures in vacuum with yields greater than 90%, despite the fact that some of them are polymeric [77]. The sublimation temperatures at 10
-2
Torr decrease steadily across the period, from 120˚C (La) to 65˚C (Lu). Treatment of the trichlorides EuCl
3
and YbCl
3
with Na(DMADB

results in a reduction of the RE to a divalent state. The resulting complexes can be separated from trivalent byproducts by extraction and crystallization from pentane. In contrast with the trivalent ones, they are thermally unstable and decompose at 107 - 115

C without sublimation
[78].
Fig. 4. Crystal structures of Er(H
3
BNMe
2
BH
3
)
3
and Pr(H
3
BNMe
2
BH
3
)
3
[77]
5. Organolanthanide Complexes
Lanthanides are capable of forming stable organometallic complexes predominantly with

-bonding [79 - 81]. The first compounds of this type were tris-cyclopentadienyls [LnCp
3
] prepared in 1956 [82] by the interaction between RE chloride and cyclopentadienyl derivative of alkaline metal in an aromatic hydrocarbon solvent or THF. The structures of these compounds differ from polymeric such as PrCp
3
to tetrameric (Nd
4
(Cp*)
12
) and monomeric for Ln around the middle of the 4f-series, such as Sm [83]. These compounds are stable indefinitely in inert atmosphere, but are readily decomposed with protolytic solvents and are highly reactive towards oxygen .They also react with CS
2
, halogenated hydrocarbons, ketones and even ferrous chloride, instantly giving ferrocene. All the compounds are thermally robust and can be heated without decomposition up to 300

C except europium, poor stability of which is caused by reduction of metal to divalent state. In vacuo 10
− 4
Torr they can be sublimed without decomposition at 200–
250˚C; volatility being increased along the 4f-series from :La to Eu, i.e., with decreasing ionic radius. Bulky groups in the Cp ring increase the volatility of tris-complexes as shown below
(temperatures of sublimation for Nd tris-complexes at 7,5

10 -4 Torr) [63, 84].

NMe
2
MeO
220 C t-Bu
210 C 200 C t-Bu i-Pr
175 C
135 C 115 C
95 C 80 C
For some derivatives the vapour pressure data at different temperatures are available. The authors showed that isopropylcyclopentadienyl derivatives are more volatile in comparison with non-substituted ones. It can be explained by weaker intermolecular interactions that make the polymeric structure unstable.
The coordination sphere of metal in tris-cyclopentadienyl complexes of Ln is unsaturated that explains the formation of adducts with THF, ammonia, pyrazine (Pzn), pyridines [85]. They sublime with evolution of neutral ligand. (YbCp
3
)
2
Pzn compound sublimes in vacuo at 80 -
105

C [74].
The “lanthanocene” derivatives LnCp
2
X have been studied for a number of anionic ligands X including halides, alkoxy, phenoxy, carboxy, amino, dialkylditiocarbamato and other substituents including “classical” chelating groups as β-diketones [86]. Lanthanide chlorides
LnCp
2
Cl are known for heavy REE from Sm. They can be prepared by interaction between Ln trichloride with two equivalents of cyclopentadienylsodium in THF or by treating the LnCp
3 with LnCl
3
[87].The complexes have been isolated by sublimation at 150-250

C/10
-5
Torr. Di-
(methylcyclopentadieny1)-lanthanide chlorides sublime at somewhat lower temperatures. In benzene and in solid state these compounds are dimeric with two chloride bridging group [88].
Phenoxides and carboxylates are more stable to oxidation and hydrolysis. In vapour both dimmers and monomers are detected. Th interaction between Ln(Cp)
2
Cl and LiAlMe
4
gives bimetallic compounds LnCp
2
AlMe
4
with two bridging methyl-groups. Yb complex sublimes at
100

C/7Pa while Gd complex under the same conditions gives GdCp
3
[89]. The borohydide derivatives of Ln bis-cyclopentadienyls obtained as adducts with THF can be sublimed at
190

C/0,13 Pa for Er and Yb [74].
The existence of CeCp
4
is under question [90], while CeCp
3
(O i Pr) prepared from cerium(IV) isopropoxide and MgCp2 can be sublimed in high vacuo at 150

C [91].

The cyclopentadienyl, cyclooctatetraenyl and cyclononatetraenyl derivatives of divalent
Yb, Sm, Eu have been studied [92]. The LnCp
2
were prepared by dissolving of Ln metal in liquid ammonia with further addition of HCp. The compounds are air and moisture sensitive.
They can be sublimed in vacuo only at 400

C due to their polymeric structure. The substituted cp-derivatives of divalent RE are more volatile. The Yb complex with trimethylsilylcyclopentadiene sublimes at 300

C/0,13 Pa [93], while SmCp*
2
sublimes at 85

C
[94].
Besides cyclopentadienyls, the alternative ligand sets which are able to satisfy the coordination requirements of the large Ln
3+
cations are amidinate and guanidinate anions [95].
R'N
R
NR'
N
R C
R'N R
NR' guanidinate amidinate
R = H, alkyl, aryl
R' = alkyl, cycloalkyl, aryl, SiMe
3
R = alkyl, SiMe
3
R' = alkyl, cycloalkyl, aryl, SiMe
3
Two such ligands coordinate to Ln
3+
giving a metallocene-like coordination environment.
Alkyl-substituted lanthanide tris(amidinates) and tris(guanidinates) with branched substituents such as N,N’-diisopropyl-2-dimethylamidoguanidinate were found to have good thermal stability and high volatily [96 - 98], some of the complexes sublime at 90

C at low pressure [99].
6. Phtalocyanines
The bis-phalocyanine derivatives of REE LnPc
2
known since 1967 [100] are prepared by the interaction between solid REE acetate with o-phtalodinitrile in molar ratio 1 : 8 at 280 -
290ºC. Other synthetic approaches are described in [101]. The complexes have a sandwich-type molecular structure and are soluble in hexane. The complexes are radical species containing Ln in oxidation state (+3) and one of the phtalocyanine anions in unusual Pc
2-
form. The properties of these compounds and their uses as sensors are discussed in [102]. The LnPc
2
complexes with different substituents are stable in vacuo up to 500ºC. Most of them can be sublimed with decomposition at 560ºC/0,01 Torr [103], at 450ºC/0,001 Torr [104] or even at ca 300 - 400ºC in high vacuum [102]. This procedure is used for the deposition of Langmuir-Blodgett films.