Synthesis and Crystal Chemistry of Octahedral Rhodium(III) Chloroamines ^†

Abstract

Rhodium(III) octahedral complexes with amine and chloride ligands are the most common starting compounds for preparing catalytically active rhodium(I) and rhodium(III) species. Despite intensive study during the last 100 years, synthesis and crystal structures of rhodium(III) complexes were described only briefly. Some [RhCl_x(NH₃)_6-x] compounds are still unknown. In this study, available information about synthetic protocols and the crystal structures of possible [RhCl_x(NH₃)_6−x] octahedral species are summarized and critically analyzed. Unknown crystal structures of (NH₄)₂[Rh(NH₃)Cl₅], trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O, and cis–[Rh(NH₃)₄Cl₂]Cl are reported based on high quality single crystal X-ray diffraction data. The crystal structure of [Rh(NH₃)₅Cl]Cl₂ was redetermined. All available crystal structures with octahedral complexes [RhCl_x(NH₃)_6-x] were analyzed in terms of their packings and pseudo-translational sublattices. Pseudo-translation lattices suggest face-centered cubic and hexagonal closed-packed sub-cells, where Rh atoms occupy nearly ideal lattices.

1. Introduction

Rhodium(III) coordination compounds play an important role in Rh chemistry. Many Rh(III) species are suggested to be active homogeneous catalysts for practically important reactions. Rhodium organometallic chemistry has been investigated in details to support progress in chemical technology and catalysis [1,2]. Rhodium amines and acidocomplexes such as chlorides, nitrites, and sulfates play a fundamental role as intermediates in rhodium refinery from other platinum group metals (especially iridium). Rhodium(III) has an octahedral coordination, and as a result, substitution in its amine-coordinated compounds is kinetically difficult. Despite intensive research, not all theoretically possible rhodium(III) complexes have been isolated and structurally investigated in details, so far. Rhodium(III) coordinated with Cl and NH₃ ligands can form 7 possible types of octahedral species [3]:

i)

[RhCl₆]^3–;

ii)

[Rh(NH₃)Cl₅]^2–;

iii)

cis– and trans–[Rh(NH₃)₂Cl₄]⁻;

iv)

fac– and mer–[Rh(NH₃)₃Cl₃];

v)

cis– and trans–[Rh(NH₃)₄Cl₂]⁺;

vi)

[Rh(NH₃)₅Cl]²⁺;

vii)

[Rh(NH₃)₆]³⁺.

Among all possible compounds, salts with [RhCl₆]^3– and [Rh(NH₃)₅Cl]²⁺ are the most common and have been investigated in great details. [Rh(NH₃)₅Cl]Cl₂ seems to be the first investigated Rh(III) coordination compound [4] and was one of the first Rh(III) compound that was structurally characterized [5]. Crystal structures of fac–[Rh(NH₃)₃Cl₃] and diverse salts with [Rh(NH₃)₆]³⁺ cations, as well as the first complexes with known anions [Rh(NH₃)Cl₅]^2– and cis–[Rh(NH₃)₂Cl₄]⁻ were just recently reported [6,7,8]. No previous study have looked at the crystal structures of complexes with known cis–[Rh(NH₃)₄Cl₂]⁺. Synthesis and properties of trans–[Rh(NH₃)₂Cl₄]⁻ as well as mer–[Rh(NH₃)₃Cl₃] have never been reported in the literature. As soon as the crystal structures of Rh(III) chloroamines were investigated during the past 80 years and briefly described in several sporadic publications, there was a need to perform a critical analysis of available experimental data and formulate general trends in their synthesis and crystal structures, especially to understand their packings in the context of stereochemistry.

Among the coordination compounds of transition metals with “simple” inorganic ligands, only crystal structures of [Pt(NH₃)_xCl_1−x] complexes containing Pt(IV) were systematically investigated and assessed from the crystallographic points of view [9]. At that time (70 years ago), all possible [Pt(NH₃)_xCl_1−x] complexes were synthesized, but not all crystal structures were known. Nevertheless, there were no efforts made to complete the Pt(IV) series or extend our knowledge about crystallography of coordination compounds of transition metals through a systematic investigation of other transition metals. Series of coordination compounds of other metals were investigated sporadically. In a later review [10], authors postulated the following. The main attention so far was paid to solve crystal structures of coordination compounds with organic ligands. At the same time, many crystal structures of coordination compounds with “simple” inorganic ligands were still unknown. Nevertheless, such complexes are important intermediates and target compounds that might be used as good models in understanding chemical reactions and processes for investigation of the chemical and structural aspects of coordination chemistry. The main idea of the current study was to fill this gap in experimental data regarding Rh(III) chloroamine complexes, obtain missing crystallographic information, and critically analyze existing experimental data on the synthetic procedures and crystal structures of Rh(III) chloroamines. The main goal was to understand the general trends in the Rh(III) crystal chemistry, as well as analyze the available crystal structures in terms of their packings and pseudo-translational sublattices. New crystal structures of (NH₄)₂[Rh(NH₃)Cl₅], trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O and cis–[Rh(NH₃)₄Cl₂]Cl were solved and refined, and a crystal structure of the Claus salt, [Rh(NH₃)₅Cl]Cl₂ was re-determined as well, based on high-quality single crystal X-ray diffraction data.

2. Review of the Available Synthetic Routines to Access Rh(III) Chloroamines

RhCl₃⋅xH₂O is traditionally used as a starting chemical for the preparation of most Rh(III) complexes. All Rh(III) chloroamines could be in principle prepared by a reaction of RhCl₃⋅xH₂O with ammonia or ammonium chloride. Nevertheless, not all compounds are easily accessible due to the fast substitution of the first ligands in [RhCl₆]³⁻ and [Rh(NH₃)Cl₅]²⁻ and difficulties in the substitution of the last Cl-ligand in [Rh(NH₃)₅Cl]²⁺. High interest of the [Rh(NH₃)₅Cl]²⁺ species indicates its practical importance for Rh(III) coordination chemistry. Schematically, synthesis of various rhodium(III) chloroamines are summarized in Scheme 1.

In general, ammonia substitution in Rh(III) is a relatively slow process and can be controlled by pH. pH control can be achieved using buffer solutions such as (NH₄Cl + CH₃COONH₄) or (NH₄Cl + NH₄CO₃), with a pH~7 and (NH₄Cl + NH₃) with a pH~8.2. (NH₄)₂[Rh(NH₃)Cl₅] first forms at neutral pH. In more basic media, [Rh(NH₃)₃Cl₃] and [Rh(NH₃)₄Cl₂]Cl as well as [Rh(NH₃)₅Cl]Cl₂ can be isolated [11]. Due to the low trans-effect of coordinated ammonia, in comparison to Cl⁻ and NO₂⁻, reaction of [RhCl₆]³⁻ or [Rh(NO₂)₆]³⁻ with ammonia, results in a formation of cis–[Rh(NH₃)₂Cl₄]⁻ and cis–[Rh(NH₃)₂(NO₂)₄]⁻, respectively [3]. Further nitration should result in a formation of fac–[Rh(NH₃)₃Cl₃] and fac–[Rh(NH₃)₃(NO₂)₃], which was confirmed experimentally [6,7,8]. Due to the high stability of [Rh(NH₃)₅Cl]Cl₂, its deamination cannot be performed to access mer–[Rh(NH₃)₃Cl₃]. Only trans–[Rh(NH₃)₂Cl₄]⁻ can be prepared from [Rh(NH₃)₅Cl]Cl₂, using reactions with reducing agents (see synthesis 2.5 below). As far as we know, synthesis and properties of trans–[Rh(NH₃)₂Cl₄]⁻ salts as well as mer–[Rh(NH₃)₃Cl₃] were not described in the literature. Reaction with bidentate C₂O₄²⁻ was successfully applied for the substitution of amine- to acido-ligand: [Rh(NH₃)₅Cl]²⁺ + C₂O₄²⁻ → [Rh(NH₃)₄(C₂O₄)]⁺ + Cl⁻ + NH₃ and further [Rh(NH₃)₄(C₂O₄)]⁺ + 2Cl⁻ → cis–[Rh(NH₃)₄Cl₂]⁺ + C₂O₄²⁻. trans–[Rh(NH₃)₄Cl₂]⁺ has been obtained by a direct reaction of [RhCl₆]³⁻ with ammonia at high pH. It seems that, [Rh(NH₃)₅Cl]Cl₂ is the most stable Rh(III) ammine complex and it can be prepared by deep amination of any rhodium compounds. The sixth chloride-ligand cannot be easily substituted and [Rh(NH₃)₆]Cl₃ can only be prepared in an autoclave using concentrated water ammonia as a reaction medium. cis–[Rh(NH₃)₄Cl₂]Cl has been synthetized by Hancock, using oxalate substitution [12] but its crystal structure was unknown.

Reducing agents such as ethanol, Zn-dust, and especially hydrazinium salts speed up a ligand’s substitution in Rh(III) complexes, due to a possible formation of highly reactive Rh(I) intermediates. Such catalytic reaction has been utilized to prepare trans–[Rh(NH₃)₄Cl₂]Cl and [Rh(NH₃)₅Cl]Cl₂ with high yields [11]. High stability of the Rh(III) complexes with nitrito-ligand can be used to access Rh(III) chloroamines, which can be can be obtained using a reaction of [Rh(NO₂)₆]³⁻ with ammonia and further with HCl [8,13,14]. In particular, such reaction can be utilized for preparation of fac–[Rh(NH₃)₃Cl₃], cis–[Rh(NH₃)₂Cl₄]⁻, and [Rh(NH₃)Cl₅]²⁻. Complicity of synthesis of cis–[Rh(NH₃)₂Cl₄]⁻ salts can be explained by their relatively high solubility, in comparison with other Rh(III) chloroamines. Nevertheless, it seems that, cis–[Rh(NH₃)₂Cl₄]⁻ has a high stability in solid-state and in solution. [Rh(NH₃)₆]Cl₃ can be isolated from hydrothermal reaction in concentrated ammonia solutions.

Available synthetic protocols for known rhodium(III) chloroamines are briefly summarized below.

2.1. Ammonium Hexachlororhodate(III)

(NH₄)₃[RhCl₆]⋅H₂O can be isolated by slow evaporation of a 3 M HCl solution containing the stoichiometric amounts of RhCl₃⋅xH₂O and NH₄Cl, with quantitative yield [15]; alternatively, according to [16,17], (NH₄)₃[RhCl₆]⋅H₂O can also be prepared by a direct reaction of H₃[RhCl₆] with NH₄Cl water solution, or by evaporation of ammonium chloronitrorhodate(III) with HCl water solution. Water-free (NH₄)₃[RhCl₆] is prepared by gentle heating of (NH₄)₃[RhCl₆]⋅H₂O in vacuum. For (NH₄)₃[RhCl₆]⋅H₂O, the IR-spectrum (cm^–1) was: 3560, 3471 (broad) (ν(H₂O)); 3174 (strong) (ν(NH₃)); 1630 (broad) (δ_d(H₂O)); 1587 (δ_a(NH₄⁺)); 1381 (δ_s(NH₄⁺)); 325_ssh, 318_s, 314_ssh (ν(RhCl)); 205_m, 196_sh, 185_m (δ(ClRhCl)); 157_s, 145_s (lattice) [15]; and the Raman-spectrum (cm^–1) was: 339 (ν₁(RhCl)); 300 (ν₂(RhCl)); 283 (δ(ClRhCl)).

2.2. Ammonium Pentachloroaminerhodate(III)

(NH₄)₂[Rh(NH₃)Cl₅] can be prepared from Na₃[RhCl₆] or H₃[RhCl₆] through reaction with a mixture of ammonium chloride and ammonium acetate, in water [18]. In the current study, (NH₄)₂[Rh(NH₃)Cl₅] is prepared from (NH₄)₃[RhCl₆]⋅H₂O. 0.25 g of (NH₄)₃[RhCl₆]⋅H₂O and is dissolved in a minimum amount of hot water (several ml). A total of 2 mL of saturated water solution of NH₄Cl is added. After heating for 5 min, 1.2 mL of ammonium acetate in water is added (ammonium acetate solution is prepared by neutralization of concentrated water ammonia with acetic acid). After heating for 1 h, dark-orange crystals are isolated. An additional portion is isolated by addition of 1 mL of ammonium acetate solution and evaporation until crystallization of orange needles. After cooling to room temperature, equal amounts of concentrated NH₄Cl solution is added to the reaction mixture. Precipitate is filtered, washed with diluted NH₄Cl water solution, and dried in air. Total yield was above 45%. Single crystals for the current single-crystal X-ray diffraction study were collected from the first portion. For (NH₄)₂[Rh(NH₃)Cl₅], the IR-spectrum (cm^–1) was: 3370 (broad) (ν(NH₃)); 3039 (shoulder) (ν_s(NH₄⁺)); 1680 (shoulder) (δ_d(NH₄⁺)); 1604, 1518 (δ_d(NH₃)); 1388 (δ_d(NH₄⁺)); 1279 (δ_s(NH₃)); and the Raman-spectrum (cm^–1) was: 502 (ν(RhN)); 322, 305 (ν(RhCl)); 284, 218, 190, 130 (skeletal bending).

Crystals of K₂[Rh(NH₃)Cl₅] can be prepared from K₂[Rh(NH₃)(NO₂)₅]∙H₂O [8], by heating in 6 M HCl. After 24 h of crystallization at room temperature, dark-red crystals are filtered. For K₂[Rh(NH₃)Cl₅], the IR-spectrum (cm^–1) was: 3310, 3236 (ν(NH₃)); 1604, 1519 (δ_a(NH₃)); 1292 (δ_s(NH₃)); 516 (ν(RhN)) [8].

2.3. Potassium cis–Tetrachlorodiamminerhodate(III)

According to [8,15], cis–K[Rh(NH₃)₂Cl₄]⋅KCl can be prepared from (NH₄)₂Na[Rh(NO₂)₆]. As the first step, 5 g of RhCl₃⋅xH₂O is nitrated with 12 g of NaNO₂ in 200 mL water. After heating and filtration, (NH₄)₂Na[Rh(NO₂)₆] is crystallized by addition of saturated NH₄Cl water solution. Precipitate is filtered and added to ca. 5 mL of concentrated water solution of NH₃, and boiled until its complete dissolution. The solution is cooled and filtrated from the fac–[Rh(NH₃)₃(NO₂)₃] precipitate. The residual clear solution is mixed with ca. 10 g of KCl and filtered immediately. After 3–4 days at room temperature, crystals of cis–K[Rh(NH₃)₂(NO₂)₄] are filtered. Both parts, crystals and solution, can be treated with HCl water solution (1:1). After short heating, yellow–orange solution are obtained; cis–K[Rh(NH₃)₂Cl₄]⋅KCl can be crystallized by evaporation of the solution at room temperature [8]. For cis–K[Rh(NH₃)₂Cl₄]⋅KCl, the IR-spectrum (cm^–1) was: 3283 (strong), 3181 (weak) (ν(NH₃)); 1604, 1548 (δ_a(NH₃)); 1287 (δ_s(NH₃)); 848, 793 (ρ_r(NH₃)); 506 (ν(RhN)) [8].

2.4. Fac–Trichlorotriaminerhodium(III)

According to [6,14], fac–[Rh(NH₃)₃Cl₃] can be prepared by heating of fac–[Rh(NH₃)₃(NO₂)₃] with HCl (25 mL of 1:1 aqueous solution), for 3 h. After cooling the solution to room temperature, a light-yellow powder is isolated with a 91% yield. The crystalline structure of fac–[Rh(NH₃)₃Cl₃] was solved and refined using powder X-ray diffraction data (PXRD). Fac–[Rh(NH₃)₃(NO₂)₃] can be prepared according to [19], by reaction of Na₃[Rh(NO₂)₆] with sulfanilic acid and ammonia, or alternatively as a precipitate of heating of (NH₄)₂Na[Rh(NO₂)₆] with concentrated ammonia solution (see synthesis of cis–K[Rh(NH₃)₂Cl₄]⋅KCl above). For fac–[Rh(NH₃)₃Cl₃], the IR-spectrum (cm^–1) was: 1590 (δ_a(NH₃)); 1318 (δ_s(NH₃)); 837 (ρ_r(NH₃)) [6].

2.5. Trans–Dichlorotetraminerhodium(III) Chloride

Synthesis of trans–[Rh(NH₃)₄Cl₂]Cl was described by Lebedinskii [3] and later modified by other authors [11,20,21]. The method reported by Lebedinskii includes a reaction of RhCl₃⋅xH₂O with a water solution of NH₄Cl and (NH₄)₂CO₃. The mixture is heated on a steam bath for 3 h and cooled to room temperature for crystallization. After crystallization of a golden-yellow powder, the solid product is extracted by boiling with HCl (1:1 water solution). [Rh(NH₃)₅Cl]Cl₂ is insoluble in HCl water solution but trans–[Rh(NH₃)₄Cl₂]Cl can be easily dissolved and further recrystallized from diluted HNO₃ water solution as trans–[Rh(NH₃)₄Cl₂](NO₃)⋅H₂O [20].

Later, catalytic reaction with N₂H₆Cl₂ was suggested to speed up ammonium substitution and improve the yield of trans–[Rh(NH₃)₄Cl₂]Cl [9]. Briefly, (NH₄)₃[RhCl₆]⋅H₂O is dissolved in water after addition of NH₄Cl powder, the mixture is boiled until a complete dissolution of the precipitate. Further addition of the CH₃COONH₄ water solution and a few milligrams of N₂H₆Cl₂ powder, results in an immediate color change from red to golden-yellow. Additional portions of CH₃COONH₄ should be added to the reaction mixture to keep pH~7. After cooling to room temperature, a golden-yellow precipitate is filtered. For recrystallization, the trans–[Rh(NH₃)₄Cl₂]Cl powder is dissolved in hot HCl (2:1) and evaporated until crystallization. trans–[Rh(NH₃)₄Cl₂]Cl is dissolved in hot water and recrystallized by addition of HCl water solution. Yield is above 30%. It should be noted, that authors reported water-free salt [11]; nevertheless, later studies suggested that only trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O can be isolated. Such a catalytic reaction seems to be quite easy and reproducible to access trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O and its derivatives.

Alternatively, according to [21], trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O can by prepared from [Rh(NH₃)₅Cl]Cl₂. A mixture of [Rh(NH₃)₅Cl]Cl₂ and Zn dust is heated in a solution of 12 M ammonia and water (10:13) at 50 °C, for 5 min with stirring. The resultant mixture is filtered. The pale-yellow solution is cooled in an ice-bath. pH should be adjusted to ~6–7 by adding of 12 M HCl. NaCl, HCl, and H₂O₂ are sequentially added. The reaction mixture is rapidly boiled under reflux for 30 min; an orange solution is filtered. The precipitate is washed with 0.1 M HCl and the solution is kept in a refrigerator, overnight at 5 °C. The orange-yellow crystals are isolated by filtration and further recrystallized from 0.1 M HCl (50 °C), after addition of 6 M HCl and cooling; yield is above 70%.

For trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O, the IR-spectrum (cm^–1) was: 3268, 3178, 3155 (ν(NH₃)); 1604, 1542 (δ_a(NH₃)); 1298 (strong) (δ_s(NH₃)); 837, 820 (ρ_r(NH₃)); the Raman-spectrum (cm^–1) was: 491, 506 (ν(RhN)); 306, 320(shoulder) (ν(RhCl)); 270, 259, 216, 205, 187, 158, 148 (skeletal bending).

2.6. Cis–Dichlorotetraminerhodium(III) Chloride

Cis–[Rh(NH₃)₄Cl₂]Cl can be prepared from [Rh(NH₃)₄(C₂O₄)]ClO₄⋅H₂O by boiling it for 1 min, with 6 M HCl [13]. Bright-yellow solution is cooled in ice for crystallization of bright-yellow crystals. An additional portion can be obtained by addition of methanol. The product can be recrystallized from hot water by addition of 3 M HCl; yield is above 90%. [Rh(NH₃)₄(C₂O₄)]ClO₄⋅H₂O can be prepared from [Rh(NH₃)₅Cl]Cl₂ [13]. A mixture of [Rh(NH₃)₅Cl]Cl₂ (6.8 mmol), Na₂C₂O₄ (6.8 mmol), and H₂C₂O₄ (3.4 mmol) is dissolved in 70 mL of water, placed in a stainless steel autoclave, and heated at 120 °C for 24 h. Solution of [Rh(NH₃)₄(C₂O₄)]⁺ is filtered out and cooled to room temperature. Small volume of 70% HClO₄ is added to the solution for a crystallization. Pale-yellow crystals are further purified from hot water by addition of water solution of LiClO₄; yield is above 55%.

According to our experience, cis–[Rh(NH₃)₄Cl₂]Cl can be prepared without intermediate crystallization of [Rh(NH₃)₄(C₂O₄)]ClO₄⋅H₂O. A mixture of [Rh(NH₃)₅Cl]Cl₂ (6.8 mmol), Na₂C₂O₄ (6.8 mmol), and H₂C₂O₄ (3.4 mmol) is dissolved in 70 mL of water, placed in a stainless steel autoclave, and heated at 120 °C for 48 h. the reaction mixture should be heated with 1 mL of concentrated HCl water solution. After cooling to room temperature, light-yellow precipitate is filtered. Precipitate contains mainly unreacted [Rh(NH₃)₅Cl]Cl₂. A light-orange solution is evaporated for several days on air at room temperature. Orange crystals are contaminated with minimum light-yellow [Rh(NH₃)₅Cl]Cl₂ crystals, as well as with large light-yellow needles of an unknown crystal structure and composition. The procedure can be recommended for laboratory synthesis of cis–[Rh(NH₃)₄Cl₂]Cl. Nevertheless, for preparation of large amounts of the salt, it should be further optimized.

For cis–[Rh(NH₃)₄Cl₂]Cl, the IR-spectrum (cm^–1) was: 3270, 3140 (ν(NH₃)); 1600 (δ_a(NH₃)); 1300, 1290 (δ_s(NH₃)); 798, 818 (ρ_r(NH₃)). The Raman-spectrum (cm^–1) was: 489, 506 (ν(RhN)); 305 (ν(RhCl)); 252, 217, 203 (skeletal bending).

2.7. Chloropentaminerhodium(III) Chloride (Claus Salt)

Similar to trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O, [Rh(NH₃)₅Cl]Cl₂ can be prepared from RhCl₃⋅xH₂O, with high yield, using hydrazinium salts as catalysts [22,23]. In comparison with the synthesis of trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O, reaction should be performed in a more basic medium. Briefly, 1 g of RhCl₃⋅xH₂O powder is dissolved in 10 mL of concentrated HCl and heated for 0.5 h. After complete dissolution, 10–20 mg of solid N₂H₆Cl₂ is added together with 20 mL of hot ammonia buffer, with pH~8.2. Immediately, the solution turns to light-yellow with a formation of [Rh(NH₃)₅Cl]Cl₂ precipitate (precipitate might also contain minor quantities of Rh, [Rh(NH₃)₃Cl₃] and trans–[Rh(NH₃)₄Cl₂]Cl). The precipitate is filtered and washed from [Rh(NH₃)₄Cl₂]Cl with hot HCl water solution (2:1). [Rh(NH₃)₅Cl]Cl₂ can be purified by dissolution in hot water (insoluble Rh and [Rh(NH₃)₃Cl₃] are left on the filter) and recrystallized by addition of an equal volume of 10 wt. % water solution of HCl. After 2–3 h, light yellow crystals of [Rh(NH₃)₅Cl]Cl₂ are filtered and washed with ethanol and dried in air; yield is above 75 %. For [Rh(NH₃)₅Cl]Cl₂, IR-spectrum (cm^–1): 3288 (strong), 3173 (weak) (ν(NH₃)); 1559 (δ_a(NH₃)) was; 1308(strong), 1273(weak) (δ_s(NH₃)); 850 (ρ_r(NH₃)); 492 (ν_a(RhN)); 275, 266 (δ(NRhN)); 200 (ν(RhCl)); 102shs, 96s (lattice). The Raman-spectrum (cm^–1) was: 506, 486 (ν(RhN)); 319 (ν(RhCl)); 266 (π(RhN)+ δ(NRhN)) [22,24].

2.8. Hexaminerhodium(III) Chloride

[Rh(NH₃)₆]Cl₃ can be prepared from [Rh(NH₃)₅Cl]Cl₂, according to [7,23]. [Rh(NH₃)₅Cl]Cl₂ is heated with concentrated NH₃ water solution in a 10-mL Teflon autoclave at 150 °C for 100 h. After natural cooling, the reaction mixture is washed with water and evaporated in air for several days. Dry colorless powder contains pure [Rh(NH₃)₆]Cl₃. The procedure is quite general, any other rhodium(III) chloroamines, as well as rhodium(III) chloride can be used to prepare [Rh(NH₃)₆]Cl₃ with nearly quantitative yield.

For [Rh(NH₃)₆]Cl₃, the IR-spectrum (cm^–1) was: 3276 (strong), 3181 (weak) (ν(NH₃)); 1616, 1572, 1546 (δ_a(NH₃)); 1302 (strong), 1279 (weak) (δ_s(NH₃)); 855, 821, 755 (ρ_r(NH₃)); 470 (ν_a(RhN)); 310 (δ_a(NRhN)) [7]. The Raman-spectrum (cm^–1) was: 515 (ν_s(RhN)); 480 (ν_a(RhN)); 240 (δ(NRhN)) [25].

Protocols 2.7 and 2.8 have been tested many times by several independent research groups and can be considered to be the easiest methods for a preparation of [Rh(NH₃)₅Cl]Cl₂ and [Rh(NH₃)₆]Cl₃ from various starting materials, including Rh(III) refinery as [Rh(NH₃)₅Cl]Cl₂ from mixtures of transition metal chlorides. Both protocols give high yields of pure crystalline products.

3. Materials and Methods

For the current study, (NH₄)₂[Rh(NH₃)Cl₅] has been prepared according to [18], starting from (NH₄)₃[RhCl₆]⋅H₂O (see procedure 2.2 above). A total of 0.25 g (NH₄)₃[RhCl₆]⋅H₂O was dissolved in 5 mL hot water. A total of 2 mL of saturated water solution of NH₄Cl was added. After heating for 5 min, 1.2 mL of ammonium acetate in water was added. After heating for 1 h and cooling to room temperature, dark-orange crystals were isolated.

Light-orange, plate-shaped, single crystals of trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O were prepared using catalytic reaction with hydrazinium chloride ([11], see procedure 2.5 above). A total of 0.64 g NH₄Cl powder was added to 0.21 g (NH₄)₃[RhCl₆]⋅H₂O and dissolved in 10 mL water; mixture was boiled until a complete dissolution of the precipitate. Further addition of 0.27-g solid CH₃COONH₄ and a few milligrams of N₂H₆Cl₂ powder, resulted in an immediate color change from red to golden-yellow. Additional portions of CH₃COONH₄ were added (max 0.1 g) to the reaction mixture to keep pH~7. After cooling to room temperature, a golden-yellow precipitate was filtered. Single crystals were collected from the stock solution, after its evaporation. A total of 1-mm light-yellow needles were collected from the surface of the solution, after completion of the reaction and cooling to room temperature (room temperature X-ray diffraction experiment). An additional portion of the crystals was obtained after evaporation of the solution after two weeks (crystals were used for low temperature X-ray diffraction experiment).

Light-orange, plate-shaped single crystals of cis–[Rh(NH₃)₄Cl₂]Cl were prepared using an autoclave reaction of [Rh(NH₃)₅Cl]Cl₂ with Na₂C₂O₄ and H₂C₂O₄, without isolation of the [Rh(NH₃)₄(C₂O₄)]ClO₄⋅H₂O intermediate ([13], see procedure 2.6 above). A mixture of 50 mg [Rh(NH₃)₅Cl]Cl₂, 23 mg Na₂C₂O₄, and 13 mg H₂C₂O₄ was dissolved in 5 mL of water, placed in a stainless steel autoclave and heated at 120 °C, for 48 h. After finishing the first step, the reaction mixture was heated with 1 mL of concentrated HCl water solution. After cooling to room temperature, the light-yellow precipitate (unreacted [Rh(NH₃)₅Cl]Cl₂) was filtered. Light-orange stock solution was evaporated for several days in air, at room temperature. Orange crystals of cis–[Rh(NH₃)₄Cl₂]Cl were contaminated with minimum light-yellow [Rh(NH₃)₅Cl]Cl₂ crystals, as well as with large light-yellow needles of an unknown crystal structure and composition.

[Rh(NH₃)₅Cl]Cl₂ was prepared using a catalytic procedure with hydrazinium chloride ([7,23], see procedure 2.7 above). A total of 1 g of RhCl₃⋅xH₂O powder was dissolved in 10 mL of concentrated HCl and heated for 0.5 h. After a complete dissolution, a few milligrams of solid N₂H₆Cl₂ was added together with 20 mL of hot ammonia buffer, with pH~8.2. Immediately, the solution turned to light-yellow, with a formation of [Rh(NH₃)₅Cl]Cl₂ precipitate. The precipitate was filtered and washed from [Rh(NH₃)₄Cl₂]Cl, with hot HCl water solution (2:1). A total of 10 mg of [Rh(NH₃)₅Cl]Cl₂ powder was dissolved in hot water and evaporated at room temperature to get [Rh(NH₃)₅Cl]Cl₂ single crystals.

Fourier transform infrared spectroscopy of (NH₄)₃[RhCl₆]⋅H₂O, (NH₄)₂[Rh(NH₃)Cl₅], and trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O (FTIR, Digi-lab Excalibur FTS 3000 FTIR) was recorded in transmission in the range of 4000–500 cm^-1, using the ART optic (MK11 Golden Gate system). Raman spectra were collected on a Raman ART optic spectrometer MK11 Golden Gate system.

The single crystal X-ray diffraction (SC-XRD) study of trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O and [Rh(NH₃)₅Cl]Cl₂ single crystals was performed on an automated Bruker APEX-II CCD diffractometer (MoKα radiation, graphite monochromator, and two-dimensional CCD detector) at 150 K at the BAM. The structure was refined in the anisotropic approximation. Hydrogen atoms were set geometrically. All calculations were performed using the SHELXTL software [26]. For the refinement of trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O at 150 K, in the final full-matrix refinement of 88 structural parameters, the total number of reflexes used was 3813 and the divergence factors were: R_all = 2.78%, wR_ref = 8.49%; for 3747 reflections with I ≥ 2σ(I), R_gt = 2.75%, wR_gt = 8.48%, S factor against F² was 1.179. For the refinement of cis–[Rh(NH₃)₄Cl₂]Cl at 150 K, in the final full-matrix refinement of 43 structural parameters, the total number of reflexes used was 1186 and the divergence factors were: R_all = 4.19%, wR_ref = 12.27%; for 1185 reflections with I ≥ 2σ(I), R_gt = 4.19%, wR_gt = 12.27%, S factor against F² was 1.359. For the refinement of [Rh(NH₃)₅Cl]Cl₂ at 150 K, in the final full-matrix refinement of 49 structural parameters, the total number of reflexes used was 2347 and the divergence factors were: R_all = 5.99 %, wR_ref = 13.35 %; for 2099 reflections with I ≥ 2σ(I), R_gt = 5.11 %, wR_gt = 12.85 %, S factor against F² was 1.085.

The X-ray diffraction study of trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O and (NH₄)₂[Rh(NH₃)Cl₅] single crystals was performed on an automated Bruker APEX-II CCD diffractometer (MoKα radiation, graphite monochromator, and two-dimensional CCD detector), at room temperature, at the Nikolaev Institute. For the refinement of trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O at room temperature, in the final full-matrix refinement of 94 structural parameters, the total number of reflexes used was 3928 and the divergence factors were: R_all = 4.64%, wR_ref = 10.56%; for 3308 reflections with I ≥ 2σ(I), R_gt = 3.79%, wR_gt = 10.04%, S factor against F² was 1.038. For the refinement of (NH₄)₂[Rh(NH₃)Cl₅] at room temperature, in the final full-matrix refinement of 66 structural parameters, the total number of reflexes used was 1886 and the divergence factors were: R_all = 1.93%, wR_ref = 4.29%; for 1748 reflections with I ≥ 2σ(I), R_gt = 1.71%, wR_gt = 4.20%, S factor against F² was 1.078.

4. Crystal Structures of Rh(III) Chloroamines

Crystallographic characteristics of Rh(III) chloroamines are summarized in

Table 1. Crystallographic data for [RhCl_x(NH₃)_6−x] complexes.

Composition	(NH4)3[RhCl6]⋅H2Oa SC-XRD, RT	(NH4)2[Rh(NH3)Cl5] SC-XRD, RT	K2[Rh(NH3)Cl5] SC-XRD, RT	cis–K[Rh(NH3)2Cl4]⋅KCl SC-XRD, RT	fac–[Rh(NH3)3Cl3] PXRD, RT	trans–[Rh(NH3)4Cl2]Cl SC-XRD, RT	trans–[Rh(NH3)4Cl2]Cl⋅H2O SC-XRD, RT	trans–[Rh(NH3)4Cl2]Cl⋅H2O SC-XRD, 150 K	cis–[Rh(NH3)4Cl2]Cl SC-XRD, 150 K	[Rh(NH3)5Cl]Cl2 SC-XRD, 150 K	[Rh(NH3)6]Cl3 SC-XRD, 150 K
[Ref.]	[16,17]	[current study]	[8]	[8]	[6]	[27]	[current study]	[current study]	[current study]	[current study]b	[6]
Colour	ruby–red	orange–red	dark–red	red	light–yellow	pale–yellow	pale–yellow	light–orange	yellow	light–yellow	colourless
Space group	Pnma (62)	Pnma (62)	Pnma (62)	C2/m (12)	P21/m (11)	P$\overline{1}$ (2)	P$\overline{1}$ (2)	P$\overline{1}$ (2)	R3m (160)	Pnma (62)	C2/m (12)
a, Å	12.213(3); 12.193(4)	13.3982(3)	13.464(6)	11.822(5)	6.72793(26)	6.35(2)	5.8697(7)	5.8341(6)	19.600(4)	13.2770(13)	12.6244(10)
b, Å	7.012(1); 7.007(4)	10.2926(2)	9.989(4)	11.608(5)	9.78879(38)	5.77(2)	6.4606(7)	6.4469(7)	19.600(4)	10.4930(9)	21.499(2)
c, Å	14.151(3); 14.167(10)	6.8177(2)	6.932(3)	8.196(4)	5.46106(20)	13.81(5)	12.5870(13)	12.5189(12)	6.3357(13)	6.6578(6)	12.9439(16)
α, °	90	90	90	90	90	115.3(2)	85.412(5)	94.430(4)	90	90	90
β, °	90	90	90	107.56(2)	90	99.6(2)	89.721(6)	90.029(4)	90	90	113.188
γ, °	90	90	90	90	90	77.5(2)	77.767(5)	102.407(4)	120	90	90
Z	4	4	4	4	2	2	2	2	9	4	12
V, Å3	1211.9(5); 1210.38(2)	940.18(4)	932.23(7)	1072.42(8)	358.2	445.0(5)	464.95(9)	458.42(8)	2107.9(9)	927.54(15)	3229.33
ICSD	61129; 202826	1965928	430397	430395	426435	—	1965934	1965942	1965940	1965941	187073
h1k1l1	0 1 1	0 1 1	0 −1 1	−1 1 1	1 1 0	0 0 −2	0 1 0	0 1 0	2 0 2	0 1 1	−1 −3 1
h2k2l2	–1 –2 0	0 1 −1	0 –1 –1	–1 –1 1	1 −1 0	1 0 0	1 0 1	1 0 1	0 2 2	0 1 −1	−1 3 1
h3k3l3	2 0 0	−2 −1 0	2 1 0	−2 1 0	0 0 1	0 1 0	−1 0 1	−1 0 1	0 0 3	−2 −1 0	0 0 −2
aT	– a	−a/4 + b/2 + c/2	a/4 − b/2 − c/2	b/2 + c/2	a/2 + b/2	c/2	b	b	a/2	−a/4 + b/2 + c/2	−a/2 − b/6
bT	−b/2−c/2	− a/4 + b/2 − c/2	a/4 − b/2 + c/2	−b/2 + c/2	a/2 + b/2	a	a/2 + c/2	a/2 + c/2	b/2	a/4 − b/2 + c/2	−a/2 + b/6
cT	a/2 − b/4 − c/4	−a/2	a/2	−a/2 − c/2	c	b	−a/2 + c/2	−a/2 + c/2	a/3 + b/3 - c/3	−a/2	−a/2 − c/2
aT, Å	7.01	7.02	6.95	7.11	5.94	6.91	6.46	6.44	9.80	7.04	7.26
bT, Å	7.90	7.02	6.95	7.11	5.94	6.35	6.96	6.90	9.80	7.04	7.26
cT. Å	7.27	6.70	6.73	6.09	5.46	5.77	6.93	6.90	6.87	6.64	7.04
$\overline{a_T,b_T{c}_T}$, Å	7.39	6.91	6.88	6.77	5.78	6.34	6.78	6.75	-	6.91	7.19
αT, °	57,1	61.5	61.03	102.7	92.9	77.5	50.0	49.9	61.6	61.9	59.2
βT, °	76,1	61.5	61.03	102.7	92.9	64.7	91.0	91.1	61.6	61.9	62.3
γT, °	63,6	58.1	59.84	109.6	111.0	80.4	80.7	80.8	120	56.4	59.2
$\overline{{\alpha }_T,{\beta }_T{\gamma }_T}$, °	65.6	60.4	60.6	105.0	98.9	74.2	73.9	73.9	-	60.1	60.2

Bold correspond to vectoctor, all letters in the line represent corresponding translational vectors. a) no experimental data for (NH₄)₃[RhCl₆], it is probably isostructural to (NH₄)₃[IrCl₆] [28]: P$\overline{1}$ (2), a = 12.602(30), b = 7.226(20), c = 7.502(20) Å, α = 119.18(59), β = 91.60(50)°, γ = 106.70(50)°. For (NH₄)₃[IrCl₆], a_T = (−2 0 0) = −a/2 − c/2 = 7.24, b_T =(0 1 0) = b = 7.23, c_T = (1 0 −1) = −c = 7.50 Å, α_T = 60.8, β_T = 60.4, γ_T = 59.8°; b) previous data for [Rh(NH₃)₅Cl]Cl₂ (SC-XRD, RT [29,30]): Pnma (62), a = 13.36(1), b = 10.46(1), c = 6.741(8) Å, V = 942.0 ų, ICSD 1524334.

.

Coordination of Rh(III) in all crystal structures are shown in Figure 1. Crystal structures of all coordination compounds mentioned above are summarized in Figure 2. Almost all salts had a relatively low crystal symmetry and crystallized in triclinic, monoclinic, and especially in orthorhombic (Pnma) space groups. Only cis–[Rh(NH₃)₄Cl₂]Cl crystallized in the R3m space group. It should be noted that in all cases the highest symmetry of Rh(III) octahedron is m, even in potentially symmetric fac–[Rh(NH₃)₃Cl₃], [RhCl₆]³⁻, and [Rh(NH₃)₆]³⁺. Nevertheless, in all described complexes, Rh(III) had a nearly ideal octahedral coordination. Deviations of valence angles from 90° between Cl and NH₃ ligands were relatively small and usually below 5°. All crystals structures had relatively dense packing, without formation of any channels or cages. As a result, almost all complexes crystallized without crystal water. Only (NH₄)₃[RhCl₆]⋅H₂O had crystal water in its structure. In all existing examples, such as (NH₄)₂[Rh(NH₃)Cl₅] and K₂[Rh(NH₃)Cl₅] as well as (NH₄)₃[RhCl₆]⋅H₂O and K₃[RhCl₆]⋅H₂O, ammonium and potassium salts seemed to be isomorphous to each other, but the sodium salts usually showed different crystal structures [15,28]. It should be noted that previously reported water-free trans–[Rh(NH₃)₄Cl₂]Cl, according to [21] and the current study seemed to have crystal water and should be written as trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O.

In the crystal structure of (NH₄)₃[RhCl₆]⋅H₂O, [RhCl₆]^3– anions occupy special positions corresponding to mirror planes, where Rh and two Cl ligands sit on the plane. The crystal structure is quite regular with only N…Cl short contacts of about 3.21–3.28 Å.

Crystal structures of (NH₄)₂[Rh(NH₃)Cl₅] and K₂[Rh(NH₃)Cl₅] [8] are very similar and belong to a large series of isostructural isoformular A₂[ML₅X] complexes. In their crystal structures, [Rh(NH₃)Cl₅]^2– cations lie on the mirror plane. Rh, NH₃, trans-Cl, and two cis-Cl ligands occupy positions in the mirror plane; two cis-Cl ligands are symmetrically connected by the plane. Rh...Rh distances are 6.364–7.274 Å. Isostructural similarity of (NH₄)₂[Rh(NH₃)Cl₅] and [Rh(NH₃)₅Cl]Cl₂ salts is the main important structural feature within the series. If in [Rh(NH₃)₅Cl]Cl₂ all Cl positions were filled with nitrogen ligands and all nitrogen positions were filled with Cl atoms, nearly ideal crystals structures of (NH₄)₂[Rh(NH₃)Cl₅] can be obtained. Both crystal structures had similar cell parameters. Such a phenomenon has been also found for isomorphous (NH₄)₂[Ir(NH₃)Cl₅] and [Ir(NH₃)₅Cl]Cl₂ salts, as well as (NH₄)₂[V(NH₃)Cl₅] and [Rh(NH₃)₅Cl]Cl₂ complexes [29,31]. In both types of crystal structures, the Rh(III) octahedra occupy positions on the mirror plane. Both structures have deformed CaF₂ fluorite structure types, where coordination ions occupy Ca positions and contra-ions correspond to the F positions. (NH₄)₂[Rh(NH₃)Cl₅] and [Rh(NH₃)₅Cl]Cl₂ can be considered as anti-isomorphic pairs of crystal structures. As soon as both (NH₄)₂[Rh(NH₃)Cl₅] and [Rh(NH₃)₅Cl]Cl₂ have similar crystal structures, it would be interesting to prepare their co-salts to check a possibility to get isomorphous double salts, without miscibility such as (NH₄)_2–2x[Rh(NH₃)Cl₅]_1–x[Rh(NH₃)₅Cl]_xCl_2+2x, double salts with a fixed composition with NH₄Cl in its structure, similar to cis–K[Rh(NH₃)₂Cl₄]⋅KCl, such as [Rh(NH₃)Cl₅][Rh(NH₃)₅Cl]⋅2NH₄Cl, or double complex salt such as [Rh(NH₃)Cl₅][Rh(NH₃)₅Cl], similar to [Rh(NH₃)Cl₅][IrCl₆] [32,33]. It would be also interesting to investigate temperature-induced ligand exchange in such double salts, in the solid-state.

Anion cis–[Rh(NH₃)₂Cl₄]⁻ has been crystallized only as a double salt with KCl, namely cis–K[Rh(NH₃)₂Cl₄]⋅KCl [8]. The Rh central atoms occupied a mirror plane as deformed octahedrons [RhN₂Cl₄]. The valence angles divided from 90° less than 3.7°. The shortest Rh...Rh distances in the structure were ca. 5.727 Å. K⁺ cations occupied two different positions—inversion center with six Cl⁻ neighbors and mirror planes with seven Cl⁻ neighbors. The shortest Rh…K distances were 3.835 and 3.914 Å.

Crystal structure of fac–[Rh(NH₃)₃Cl₃] has been solved and refined using powder X-ray diffraction data [6]. The monoclinical crystal structure was solved using the direct space approach and was refined using the Rietveld method. In its crystal structure, Rh(1), Cl(1), and N(1) atoms were arranged on a mirror plane. Rh—Cl and Rh—N bond lengths that lay on the mirror plane, were 0.016 (4) and 0.074 (6) Å longer than the others. Deviations of the valence of cis angles on the Rh(1) atom from the ideal value of 90° did not exceed 6.8°. Isolated fac–[Rh(NH₃)₃Cl₃] octahedrons formed nearly ideal flat hexagonal layers. Layers were oriented perpendicular to the c axis, with the distances between the layers being equal to c. All layers followed a simple arrangement without shirts (AA packing type).

The crystal structure of trans–[Rh(NH₃)₄Cl₂]Cl was obtained based on poor experimental data [21]. Positions of water molecules were not found and refined. So, details of the crystal structure of trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O were described here for the first time. The crystal structure contained two independent trans–[Rh(NH₃)₄Cl₂]⁺ cations; both cations were located in inversion centers. The shortest Cl⁻…N distances were 3.44–3.48 Å, Cl…N were 3.28 Å, and the crystal water showed O…N (3.26 and 3.45 Å) and O…Cl (3.38 Å) contacts.

Cis–[Rh(NH₃)₄Cl₂]Cl was synthetized in the early 1960s but its crystal structure was unknown. The crystal structure of the salt had a high symmetry with only one crystallographically independent cis–[Rh(NH₃)₄Cl₂]⁺ cation located on the mirror plane. Rh and two N ligands sat on the mirror plane. Cl⁻….N contacts were 3.32–3.36 Å. The cations formed chains parallel to the c-direction with relatively short Cl…N contacts of 3.30 Å (Figure 3). The crystal structure had three-layered packing of cis–[Rh(NH₃)₄Cl₂]⁺ cations with empty space. Cl⁻ anions occupied hexagonal voids.

In the crystal structure of [Rh(NH₃)₆]Cl₃ [7], there were four non-equal [Rh(NH₃)₆]³⁺ cations with Rh–N distances of 2.043–2.072 Å. The N–Rh–N angles were close to 90°. There were close contacts between the coordinated NH₃ ligands and the Cl⁻ anions, of about N…Cl 3.27–3.50 Å.

All crystal structures had a relatively high packing density and could be described as a closed packing of coordination cations or anions with contra-ions placed in the octahedral or tetrahedral voids of such packings. To find the general orientation of the closed packed layers, a pseudo-translational sublattice could be applied. Pseudo-translation sublattices can be applied to find a minimum sublattice with highest symmetry. Pseudo-translation ordering of structural fragments that can be corresponded to heavy atoms or groups of atoms, such as polyhedral for coordination compounds with the presence of smaller sublattices with higher symmetry, in comparison to the actual symmetry of the unit cell. To find these pseudo-translation sublattices, at least three intersecting crystallographic planes should be found. Crystallographic independent planes should correspond to the heaviest structural fragments, where their concentration is high [34,35]. The suitable planes can be found from a powder diffraction pattern as diffraction reflexes with the strongest intensity. Therefore, the general accurate motif of the crystal structure can be described as the most symmetric pseudo-translation sublattice. Briefly, the procedure for selecting a pseudo-translation sublattice should include a choice of the triad of diffraction lines with the strongest diffraction intensity. The diffraction indices of these reflexes should be for a matrix with a determinant (D) equal to the number of heavy fragments in the unit cell (N_hf). Corresponding intersections between such plains (h₁ k₁ l₁), (h₂ k₂ l₂), and (h₃ k₃ l₃) form a sublattice with a number of heavy fragments equal to D. The corresponding vectors (a_T, b_T and c_T) should give the nearest sites of the initial lattice and angles (α_T, β_T and γ_T) between the most symmetric arrangement of the crystal structure. Their length a_T, b_T, c_T, should correspond to the minimal distances between the heavy fragments in the crystal structure (Table 1). Detailed mathematics behind the technique can be found in [34,35,36]. Finally, the shortest sublattice should be found with the corresponding angles close to 60°, 90°, 109.5°, and 120°, which is usually a sign for the most symmetric sub-cells. Orientation of pseudo-translational sub-lattices should correspond to the layers with the highest packing density in the crystal structure.

Among all crystal structures, only (NH₄)₃[RhCl₆]⋅H₂O corresponds to the hexagonal closed packed lattice of [RhCl₆]^3– anions, with hexagonal layers arranged with AB stacking perpendicular to the a-direction in the crystal structure. Distances between the layers correspond to a_T = 7.01 Å (Figure 2). Two ammonia cations occupy tetrahedral voids in the packing. Crystal water and the third ammonia cation corresponds to the octahedral positions of the packing. Water-free (NH₄)₃[RhCl₆] seems to be isostructural to (NH₄)₃[IrCl₆] [30]. In the crystal structure of (NH₄)₃[IrCl₆], translational sublattice correspond to ideal pseudo-rhombohedral sub-cells with angles close to α_T ≈ 60.3° and three nearly equal axis of 7.32 Å. Such an arrangement correspond to the face-centered cubic packings of three hexagonal layers with ABC stackings. Ammonia occupy all tetragonal and all hexagonal voids in the structure.

In the crystal structure of fac–[Rh(NH₃)₃Cl₃], the hexagonal layers of isolated fac–[Rh(NH₃)₃Cl₃] octahedrons was observed, perpendicular to the c-direction (Figure 4). The shortest distances between the octahedrons in the layers corresponded to a_T = b_T = 5.94 Å. Hexagonal layers formed quite unusual AA stacking along the c-direction, which could be driven by the interactions between the triangle {….[(NH₃)₃RhCl₃]…[(NH₃)₃RhCl₃]…} faces (Figure 4, left).

In all other structures, the metrics obtained corresponded to the pseudo-rhombohedral sub-cells with angles close to α_T ≈ 60–70° and two nearly equal longer axis (6.90–7.30 Å) and one shorter axis (6.40–7.10 Å). Such an arrangement corresponded to the face-centered cubic packings of three hexagonal layers with ABC stackings. As an example, the correspondence of closed-packed layers and pseudo-rhombohedral sub-cell for (NH₄)₂[Rh(NH₃)Cl₅] is shown in Figure 2. In the crystal structures of (NH₄)₂[Rh(NH₃)Cl₅], trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O, [Rh(NH₃)₅Cl]Cl₂, and [Rh(NH₃)₆]Cl₃, the hexagonal layers corresponded to the packings of the coordination ions, ammonia in (NH₄)₂[Rh(NH₃)Cl₅] could be found in the tetrahedral voids; coordinated ammonia was placed in the octahedral voids of the closed-packed structure. Cl⁻ in the crystal structure of [Rh(NH₃)₅Cl]Cl₂ was located in the tetrahedral voids and coordinated Cl was placed in the octahedral voids of the packing. Similar, Cl⁻ anions in trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O could be found in the tetrahedral voids, crystal water was located in the octahedral positions of the packing. In [Rh(NH₃)₆]Cl₃, two Cl⁻ anions were located in the tetragonal voids, one Cl⁻ anion corresponded to the octahedral void in the ABC packing of [Rh(NH₃)₆]³⁺ cations.

The crystal structure of cis–[Rh(NH₃)₄Cl₂]Cl showed a relatively low packing density, where the hexagonal layers had empty space. Cl⁻ anions occupied hexagonal voids in the structure. N…Cl interactions between the cis–[Rh(NH₃)₄Cl₂]⁺ cations should be mentioned. The structure had a hexagonal sublattice with angles α_T = β_T = 60.1°, γ_T = 120°. The shortest pseudo-translational vector, c_T = 6.87 Å, corresponded to the distances between the pseudo-hexagonal layers.

5. Conclusions

Rh(III) is known as a metal with rich coordination chemistry in solution and in solid-state. In the last decade, nearly all possible octahedral Rh(III) chloroamine complexes were synthesized and structurally characterized. Some rare compounds were prepared just recently. Synthetic procedures available in the Rh(III) chemistry included direct substitution of Cl⁻ to NH₃ in solution, as well as catalytic reactions. In some cases, NO₂⁻- and C₂O₄²⁻-coordinated derivatives could be used as intermediates. Such approaches were never successful in Ir(III) chemistry, where only the direct substitution can be applied. For synthesis of Pt(IV), oxidation of the Pt(II) species with Cl₂ in the solution is typical. Such routes are impossible in Rh(III) chemistry.

Synthetic and crystal chemistry of Cr(III), Co(III), Pt(IV), Ir(III), and Ir(IV) compounds have been investigated in many details; nevertheless, only [Rh(NH₃)_xCl_1-x] complexes containing Rh(III) give a nearly complete series available for detailed systematic analysis of their transformations and details of their crystal chemistry. For iridium, the existence of Ir(III) and Ir(IV) complexes with identical coordination spheres was experimentally confirmed. [IrCl₆]³⁻ and [IrCl₆]²⁻, as well as pairs of [Ir(NH₃)Cl₅]²⁻/[Ir(NH₃)Cl₅]⁻ and [Ir(NH₃)₄Cl₂]²⁺/[Ir(NH₃)₄Cl₂]⁺ with Ir(III) and Ir(IV), respectively, were available for investigation [31]. Among all Rh(III) [Rh(NH₃)_xCl_1−x] complexes only mer–[Rh(NH₃)₃Cl₃] was not described in the literature; further efforts to synthetize and characterize such coordination compounds should be made.

[Rh(NH₃)_xCl_1−x] complexes did not show a large variety in their crystal structures. All crystal structures could be described to have a closed-packing coordination of ions, with the contra-ions and crystal water located in octahedral or tetrahedral voids. (NH₄)₃[RhCl₆]⋅H₂O showed hexagonal closed-packed structure, fac–[Rh(NH₃)₃Cl₃] showed a relatively rare, one-layer packing due to the formation of specific contacts between non charged fac–[Rh(NH₃)₃Cl₃] octahedrons. Such one-layer packing might be stable only under curtain P–T conditions and small changes in pressure or temperature, as well as crystallization from other solvents might be revealed in the formation of other polymorphic modifications of fac–[Rh(NH₃)₃Cl₃], similar to several known polymorphic modifications isolated for fac–[Rh(NH₃)₃(NO₂)₃].

The crystal structures of (NH₄)₂[Rh(NH₃)Cl₅], trans–[Rh(NH₃)₄Cl₂]Cl⋅H₂O, [Rh(NH₃)₅Cl]Cl₂, and [Rh(NH₃)₆]Cl₃ had face-centered cubic arrangement of closed, packed hexagonal layers. Such an arrangement could be easily found, based on pseudo-translational sublattices. In all mentioned salts, pseudo-translational sublattices had similar metrics. Directions of pseudo-translational vectors gave the orientation of the closed-packed layers in their crystal structures. It is important to note that the crystal structures of (NH₄)₂[Rh(NH₃)Cl₅] and [Rh(NH₃)₅Cl]Cl₂ were similar. It is possible that this is a general tendency in coordination chemistry, which should be proven for other classes of isoformular coordination compounds. Such a phenomenon might be useful as a tool for crystal structure prediction and preparation of double complexes.

The Rh(III) chloroamine complexes did not realize the maximal possible symmetry of coordination polyhedrons. Deformations of the coordination polyhedrons depended not only on the ligand arrangements, but also possibly on the packings of deformed octahedrons in the crystal structures. Closed-packings are typical for such structurally rigid coordination species, but local structure and symmetry of coordination polyhedrons usually fit the packings of hexagonal packed layers. Changes in the interatomic distances were also high due to trans-effect in cis- and trans-isomers. If specific interactions could be realized in the crystal structures (similar to fac–[Rh(NH₃)₃Cl₃]) due to sterically-driven factors, then close-packed arrangements could possibly distort with a formation of other types of crystal packings.