Transition metal chloride complex

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Octahedral molecular geometry is a common structural motif for homoleptic metal chloride complexes. Examples include MCl6 (M = Mo, W), [MCl6]− (M = Nb, Ta, Mo, W, Re), [MCl6]2- (M = Ti Zr, Hf, Mo, Mn, Re, Ir, Pd, Pt), and [MCl6]3- (M = Ru Os, Rh, Ir).

In chemistry, a transition metal chloride complex is a coordination complex that consists of a transition metal coordinated to one or more chloride ligand. The class of complexes is extensive.[1]

Bonding

Halides are X-type

weak field ligands. Due to a smaller crystal field splitting energy, the homoleptic
halide complexes of the first transition series are all high spin. Only [CrCl6]3− is exchange inert.

Homoleptic metal halide complexes are known with several stoichiometries, but the main ones are the hexahalometallates and the tetrahalometallates. The hexahalides adopt

octahedral coordination geometry
, whereas the tetrahalides are usually tetrahedral. Square planar tetrahalides are known for Pd(II), Pt(II), and Au(III). Examples with 2- and 3-coordination are common for Au(I), Cu(I), and Ag(I).

Due to the presence of filled pπ orbitals, halide ligands on transition metals are able to reinforce

π-backbonding onto a π-acid. They are also known to labilize cis-ligands.[2]
[3]

Homoleptic complexes

Homoleptic complexes (complexes with only chloride ligands) are often common reagents. Almost all examples are anions.

1st row

1st Transition Series
Complex colour electron config. structure geometry comments
TiCl4 colourless (t2g)0 tetrahedral
[Ti2Cl9]− white/colourless d0d0 face-sharing bioctahedron Ti-Cl(terminal) = 2.23 Å, 2.45 (terminal)
(N(PCl3)2)+ salt)[4]
[Ti2Cl9]3- orange (t2g)1(t2g)1 face-sharing bioctahedron Ti-Ti =3.22 Å
Ti-C1(terminal) = 2.32-2.35 Å,
Ti-Cl(bridge) = 2.42-2.55 Å
((NEt4+)3)3 salt)[5]
[Ti2Cl10]2− colourless d0d0 bioctahedral
[Ti3Cl12]3- green (t2g)1(t2g)1(t2g)1 face-sharing trioctahedron Ti-Ti = 3.19, 3.10 Å (terminal)
Ti-C1(terminal) = 2.36 Å (terminal),
Ti-Cl(bridge) = 2.50 Å
((PPh4+)3)3 salt)[6]
[TiCl6]2− yellow d0 octahedral PPh4+ salt
Ti-Cl = 2.33 Å[7]
VCl4 red (t2g)1 tetrahedral V1−Cl = 2.29 Å
V2Cl10
 violet (t2g)0 edge-shared bioctahedron V1−Cl(bridging) = 2.48 Å
V1−Cl(terminal) = 2.16-2.21 Å[8]
[VCl6]2- red (t2g)1 octahedral V1−Cl = 2.29 Å[9]
[CrCl6]3− pink[3] (t2g)3 octahedral[10][3]
[Cr2Cl9]3− red (d3)2 face-sharing bioctahedron Cr-Cl(terminal) = 2.31 Å, 2.42 (terminal)
(Et2NH2+ salt)[11]
[MnCl4]2−[12] pale pink to while (eg)2(t2g)3 tetrahedral Mn-Cl bond length = 2.3731-2.3830 Å[13]
[MnCl6]2− dark red (t2g)3(eg)1 octahedral Mn-Cl distance = 2.28 Å
K+ salt[14])
salt is isostructural with K2PtCl6
[MnCl6]3− brown[3] (t2g)3(eg)1 octahedral[10][3]
[Mn2Cl6]2− yellow-green (eg)2(t2g)3 bitetrahedral Mn-Cl(terminal) bond length = 2.24 Å
Mn-Cl(terminal) bond length = 2.39 Å[15]
(PPN+)2 salt
[Mn3Cl12]6− pink (t2g)3(eg)2 cofacial trioctahedron Mn-Cl distance = --- Å
[(C(NH2)3]+6 salt[16]
[FeCl4]2−[12] cream (eg)3(t2g)3 tetrahedral((Et4N+)2 salt)[12]
[FeCl4]− (eg)2(t2g)3 tetrahedral Fe-Cl bond length = 2.19 Å[17]
[FeCl6]3− orange (t2g)3(eg)2 octahedral[3]
[Fe2Cl6]2− pale yellow (eg)2(t2g)3 bitetrahedral Fe-Cl(terminal) bond length = 2.24 Å
Fe-Cl(terminal) bond length = 2.39 Å[15]
(PPN+)2 salt
[CoCl4]2−[12] blue[12] (eg)4(t2g)3 tetrahedral
[Co2Cl6]2− blue[15] (eg)4(t2g)3 bitetrahedral Mn-Cl(terminal) bond length = 2.24 Å
Co-Cl(terminal) bond length = 2.35 Å[15]
(PPN+)2 salt
[NiCl4]2−[12] blue[12] (eg)4(t2g)4 tetrahedral Ni-Cl bond length = 2.28 Å
(Et4N+)2 salt[18]
[Ni3Cl12]6− orange[19] (t2g)6(eg)2 confacial trioctahedral ((Me2NH2+)2)8 salt
double salt with two Cl−
Ni-Cl bond length = 2.36-2.38 Å[19]
[CuCl4]2−[12] orange[20]
yellow (flattened tetrahedral)[21]
green (square planar)[22] 		(t2g)6(eg)3 flattened tetrahedral
or square planar[23][24] 		Cu-Cl bond length = 2.24 Å
[Cu2Cl6]2− red [(t2g)6(eg)3]2 edge-shared bis(square planar)[25] Cu-Cl(terminal) = 2.24 Å
Cu-Cl(bridging) = 2.31 Å
[ZnCl4]2− white/colorless d10 tetrahedral

2nd row

Some homoleptic complexes of the second row transition metals feature metal-metal bonds.

2nd Transition Series
Complex colour electron config. structure geometry comments
[ZrCl6]2− yellow d0 octahedral Zr-Cl distance = 2.460 Å
(Me4N+)2 salt[27]
[Zr2Cl10]2− colorless (d0)2 edge-shared bioctahedral Zr-Cl = 2.36 Å (terminal), 2.43 Å (bridging)
N(PCl3)2)+ salt[4]
Nb2Cl10
 yellow (d0)2 edge-shared bioctahedral [Nb2Cl10] 3.99 Å[28]
[NbCl6]− yellow d0 octahedral Nb-Cl = 2.34 Å
N(PCl3)2)+ salt[4]
[Nb6Cl18]2− black (d2)4(d3)2 (14 cluster electrons) cluster Nb---Nb bonding Nb-Cl = 2.92 Å
(K+)2 salt[29]
MoCl6
 black d0 octahedron Mo−Cl = 2.28 -2.31 Å[8]
[MoCl6]2− yellow (t2g)2 octahedron Mo−Cl = 2.37, 2.38, 2.27 Å[30]
[MoCl6]3− pink (t2g)3 octahedral
[Mo2Cl8]4− purple[31] 2(d4) Mo-Mo quadruple bond
[Mo2Cl9]3− 2(d3) face-shared bioctahedral Mo-Mo (triple) bond length = 2.65 Å
Mo-Cl (terminal) bond length = 2.38 Å
Mo-Cl (bridging) bond length = 2.49 Å[32][33]
Mo2Cl10 green (d1)2 edge-sharing bioctahedra[34]
[Mo2Cl10]2− (d2)2 edge-sharing bioctahedra[35]
[Mo5Cl13]2− brown[31] d2d2d2d2d3 incomplete octahedron[36]
[Mo6Cl14]2− yellow d4 octahedral cluster (4-HOPyH+)2 salt[37]
[TcCl6]2− yellow (t2g)3 octahedron Tc-Cl = 2.35 Å for As(C6H5)4+ salt[38]
[Tc2Cl8]2− green (t2g)4 Tc-Tc quadruple bond Tc-Tc = 2.16, Tc-Cl = 2.34 Å for NBu4+ salt[39]
[RuCl6]2− brown (t2g)4 octahedral (EtPPh3+)2 salt[40]
[Ru2Cl9]3− red [(t2g)5]2 cofacial bioctahedral Ru-Ru bond length = 2.71 Å; Ru-Cl(terminal) = 2.35 Å, Ru-Cl(bridging) = 2.36 Å ((Et4N)+)3 salt[41]
[Ru3Cl12]4− green (d5)2(d6) cofacial trioctahedral Ru-Ru bond lengths = 2.86 Å
Ru-Cl bond lengths = 2.37-2.39 Å
(Et4N+)2(H7O3+)2 salt[42]
[RhCl6]3− red (t2g)6 octahedral H2N+(CH2CH2NH3+)2 salt)[43]
[Rh2Cl9]3− red-brown (t2g)6 octahedral Rh-Cl(terminal) = 2.30 Å, Rh-Cl(terminal) = 2.40 Å
((Me3CH2Ph)+)3 salt)[32]
[PdCl4]2− brown d8 square planar
[Pd2Cl6]2−[44] red ((Et4N+)2 salt) d8 square planar
[Pd3Cl8]2−[45] orange brown ((Bu4N+)2 salt) d8 square planar
[PdCl6]2− brown d6 octahedral Pd(IV)
[Pd6Cl12] yellow-brown d8 square planar[46]
[AgCl2]− white/colorless d10 linear salt of [K(2.2.2-crypt)]+[47]
[CdCl4]2− white/colorless d10 tetrahedral Et4N+ salt, Cd-Cl distance is 2.43 Å[26]
[Cd2Cl6]2− white/colorless d10 edge-shared bitetrahedron (C6N3(4-C5H4N)33+ salt[48]
[Cd3Cl12]6− white/colorless d10 octahedral (central Cd)
pentacoordinate (terminal Cd's)
cofactial trioctahedral 		(C6N3(4-C5H4N)33+ salt[48]
(3,8-Diammonium-6-phenylphenanthridine3+)2[49]
[Cd6Cl19]7− white/colorless d10 octahedron of octahedra 4,4'-(C6H3(2-Et)NH3+)2 salt[50]

3rd row

3rd Transition Series
Complex colour electron config. structure geometry comments
[HfCl6]2− white d0 octahedral Hf-Cl distance = 2.448 A
((Me4N+)2 salt)[27]
[Hf2Cl10]2− colorless/white d0 edge-shared bioctahedral[51]
[Hf2Cl9]− colorless/white (d0)2 face-shared bioctahedral[52]
[TaCl5] white d0 edge-shared bioctahedral
[TaCl6]− white/colourless d0 octahedral Ta-Cl = 2.34 Å
(N(PCl3)2)+ salt)[4]
[Ta6Cl18]2- green d0 octahedral Ta-Ta = 2.34 Å
(H+2 salt hexahydrate[53]
WCl6 blue d0 octahedral 2.24–2.26 Å[54]
[WCl6]2− (t2g)2 octahedral W-Cl distances range from 2.34 to 2.37 Å
(PPh4+ salt)[55]
[WCl6]− (t2g)1 octahedral W-Cl distance = 2.32 Å
(Et4N+ salt)[56]
W2Cl10 black[57] (t2g1)2 bioctahedral W-W distance = 3.814 Å[58]
[W2Cl8]4− blue 2(d4) W-W quadruple bond dW-W = 2.259 Å [Na(tmeda)+]4 salt[59]
[W2Cl9]2− d3d2 face-sharing bioctahedral W-W distance = 2.54 Å
W-Cl(terminal) = 2.36 Å, W-Cl(bridge) = 2.45 Å
((PPN+)2 salt)[60]
[W2Cl9]3− d3d3 octahedral W-Cl distance = 2.32 Å
(Et4N+ salt)[60]
[W3Cl13]3− d3,d3,d4 [W3(μ3-Cl)(μ-Cl)3Cl9]3- W-W distances = 2.84 Å[61]
[W3Cl13]2− d3,d4,d4 [W3(μ3-Cl)(μ-Cl)3Cl9]2-[61] W-W distances = 2.78 Å[61]
[W6Cl14]2- yellow[62] (d4)6 see Mo6Cl12
[ReCl6]− red-brown (t2g)2 octahedral Re-Cl distance = 2.24-2.31 Å
(PPh4+ salt)[63]
[ReCl6] (t2g)1 octahedral Re-Cl distance = 226.3(6) Å[8]
[ReCl6]2− green (t2g)3 octahedral Re-Cl distance = 2.35-2.38 Å
((PPN+)2 salt)[64]
[Re2Cl9]2− (t2g)3(t2g)4 face-sharing bioctahedral Re-Re distance = 2.48 Å
Re-Cl distances = 2.42 Å (bridge), 2.33 Å (terminal)
((Et4N+)2 salt)[65]
[Re2Cl9]− ((t2g)3)2 face-sharing bioctahedral Re-Re distance = 2.70 Å
Re-Cl distances = 2.41 (bridge), 2.28 Å (terminal)
(Bu4N+ salt)[65]
[OsCl6]− dark green (t2g)3 octahedral dOs-Cl = 2.30 Å for Et4N+[66] and Ph4P+[67] salts
[OsCl6]2− yellow-orange (t2g)4 octahedral[67] Os-Cl distance 2.33 Å
[Os2Cl8]2− green (d5)2 square antiprism dOs-Os = 2.182 Å, dOs-Cl = 2.32 Å (Bu4N+)2 salt[68]
[Os2Cl10]2− green (d4)2 octahedral dOs-Cl(terminal) = 2.30 Å dOs-Cl(bridging) = 2.42 Å (Et4N+)2 salt[66]
[IrCl6]3− red (t2g)6 octahedral Ir-Cl = 2.36 Å[69]
[IrCl6]2− brown (t2g)5 octahedral Ir-Cl = 2.33 Å[70]
[Ir2Cl9]3− - ((t2g)6)2 bi-octahedral[71]
[PtCl4]2− pink d8 square planar
[PtCl6]2− yellow d6 octahedral Pt-Cl distance = 2.32 Å
Et4N+ salt, ((Me4N+)2 salt)[27]
[Pt2Cl9]− red (Bu4N+ salt) ((t2g)6)2 octahedral Pt-Clt and Pt-Clbridge = 2.25, 2.38 Å[72]
[Pt2Cl10]2− yellow-brown (PPN+ salt) ((t2g)6)2 edge-shared bioctahedral Pt-Clt and Pt-Clbridge = 2.27, 2.37 Å[72]
[Pt6Cl12] yellow-brown (d8)6 square planar Pt-Cl = 2.31[73]
[AuCl2]− white/colorless d10 linear Au-Cl distances of 2.28 Å
NEt4+ salt[74]
Au4Cl8 black (d10)2(d8)2 linear and square planar rare example of mixed valence, molecular chloride[75]
[AuCl4]− yellow d8 square planar Au-Cl distances of 2.26 Å
NBu4+ salt[76]
[HgCl4]2− white/colorless d10 tetrahedral Hg-Cl distance is 2.46 Å[26]
Et4N+ salt
[Hg2Cl6]2− white/colorless d10 edge-shared bitetrahedral Hg-Cl distance is 2.46 Å[77]
Bu4N+ salt

Heteroleptic complexes

Heteroleptic complexes containing chloride are numerous. Most hydrated metal halides are members of this class.

Hexamminecobalt(III) chloride and Cisplatin
(cis-Pt(NH3)2Cl2) are prominent examples of metal-ammine-chlorides.

Hydrates

"Nickel dichloride hexahydrate" consists of the chloride complex trans-[NiCl2(H2O)4 plus water of crystallization.

As indicated in the table below, many hydrates of metal chlorides are molecular complexes.[78][79] These compounds are often important commercial sources of transition metal chlorides. Several hydrated metal chlorides are not molecular and thus are not included in this tabulation. For example the dihydrates of manganese(II) chloride, nickel(II) chloride, copper(II) chloride, iron(II) chloride, and cobalt(II) chloride are coordination polymers.

Formula of
hydrated metal halides		 Coordination
sphere of the metal
TiCl3(H2O)6 trans-[TiCl2(H2O)4]+[80]
VCl3(H2O)6 trans-[VCl2(H2O)4]+[80]
CrCl3(H2O)6 trans-[CrCl2(H2O)4]+
CrCl3(H2O)6 [CrCl(H2O)5]2+
CrCl2(H2O)4 trans-[CrCl2(H2O)4]
CrCl3(H2O)6 [Cr(H2O)6]3+[81]
MnCl2(H2O)6 trans-[MnCl2(H2O)4]
MnCl2(H2O)4 cis-[MnCl2(H2O)4][82]
FeCl2(H2O)6 trans-[FeCl2(H2O)4]
FeCl2(H2O)4 trans-[FeCl2(H2O)4]
FeCl3(H2O)6 one of four hydrates of
ferric chloride,[83]
FeCl3(H2O)2.5 cis-[FeCl2(H2O)4]+[84]
CoCl2(H2O)6 trans-[CoCl2(H2O)4]
CoCl2(H2O)4 cis-[CoCl2(H2O)4]
NiCl2(H2O)6 trans-[NiCl2(H2O)4]
NiCl2(H2O)4 cis-[NiCl2(H2O)4]

Adducts

Metal chlorides form adducts with ethers to give transition metal ether complexes.

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