Transition metal chloride complex explained

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.

Bonding

Halides are X-type ligands in coordination chemistry. They are both σ- and π-donors. Chloride is commonly found as both a terminal ligand and a bridging ligand. The halide ligands are 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 [CrCl<sub>6</sub>]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.[1]

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
TiCl4colourless(t2g)0tetrahedral
[Ti<sub>2</sub>Cl<sub>9</sub>]white/colourlessd0d0face-sharing bioctahedronTi-Cl(terminal) = 2.23 Å, 2.45 (terminal)
(N(PCl3)2)+ salt)[2]
[Ti<sub>2</sub>Cl<sub>9</sub>]3-orange(t2g)1(t2g)1face-sharing bioctahedronTi-Ti =3.22 Å
Ti-C1(terminal) = 2.32-2.35 Å,
Ti-Cl(bridge) = 2.42-2.55 Å
((NEt4+)3)3 salt)[3]
[Ti<sub>2</sub>Cl<sub>10</sub>]2−colourlessd0d0bioctahedral
[Ti<sub>3</sub>Cl<sub>12</sub>]3-green(t2g)1(t2g)1(t2g)1face-sharing trioctahedronTi-Ti = 3.19, 3.10 Å (terminal)
Ti-C1(terminal) = 2.36 Å (terminal),
Ti-Cl(bridge) = 2.50 Å
((PPh4+)3)3 salt)[4]
[TiCl<sub>6</sub>]2−yellowd0octahedralPPh4+ salt
Ti-Cl = 2.33 Å[5]
VCl4red(t2g)1tetrahedralV1−Cl = 2.29 Å
V2Cl10violet(t2g)0edge-shared bioctahedronV1−Cl(bridging) = 2.48 Å
V1−Cl(terminal) = 2.16-2.21 Å
[VCl<sub>6</sub>]2-red(t2g)1octahedralV1−Cl = 2.29 Å[6]
[CrCl<sub>6</sub>]3−pink(t2g)3octahedral[7]
[Cr<sub>2</sub>Cl<sub>9</sub>]3−red(d3)2face-sharing bioctahedronCr-Cl(terminal) = 2.31 Å, 2.42 (terminal)
(Et2NH2+ salt)[8]
[MnCl<sub>4</sub>]2−[9] pale pink to while(eg)2(t2g)3tetrahedralMn-Cl bond length = 2.3731-2.3830 Å[10]
[MnCl<sub>6</sub>]2−dark red(t2g)3(eg)1octahedralMn-Cl distance = 2.28 Å
K+ salt[11])
salt is isostructural with K2PtCl6
[MnCl<sub>6</sub>]3−brown(t2g)3(eg)1octahedral
[Mn<sub>2</sub>Cl<sub>6</sub>]2−yellow-green(eg)2(t2g)3bitetrahedralMn-Cl(terminal) bond length = 2.24 Å
Mn-Cl(terminal) bond length = 2.39 Å[12]
(PPN+)2 salt
[Mn<sub>3</sub>Cl<sub>12</sub>]6−pink(t2g)3(eg)2cofacial trioctahedronMn-Cl distance = --- Å
[(C(NH<sub>2</sub>)<sub>3</sub>]+6 salt[13]
[FeCl<sub>4</sub>]2−cream(eg)3(t2g)3tetrahedral((Et4N+)2 salt)
[FeCl<sub>4</sub>](eg)2(t2g)3tetrahedralFe-Cl bond length = 2.19 Å[14]
[FeCl<sub>6</sub>]3−orange(t2g)3(eg)2octahedral[15]
[Fe<sub>2</sub>Cl<sub>6</sub>]2−pale yellow(eg)2(t2g)3bitetrahedralFe-Cl(terminal) bond length = 2.24 Å
Fe-Cl(terminal) bond length = 2.39 Å
(PPN+)2 salt
[CoCl<sub>4</sub>]2−blue(eg)4(t2g)3tetrahedral
[Co<sub>2</sub>Cl<sub>6</sub>]2−blue(eg)4(t2g)3bitetrahedralMn-Cl(terminal) bond length = 2.24 Å
Co-Cl(terminal) bond length = 2.35 Å
(PPN+)2 salt
[NiCl<sub>4</sub>]2−blue(eg)4(t2g)4tetrahedralNi-Cl bond length = 2.28 Å
(Et4N+)2 salt[16]
[Ni<sub>3</sub>Cl<sub>12</sub>]6−orange[17] (t2g)6(eg)2confacial trioctahedral((Me2NH2+)2)8 salt
double salt with two Cl
Ni-Cl bond length = 2.36-2.38 Å
[CuCl<sub>4</sub>]2−orange[18]
yellow (flattened tetrahedral)[19]
green (square planar)[20]
(t2g)6(eg)3flattened tetrahedral
or square planar[21] [22]
Cu-Cl bond length = 2.24 Å
[Cu<sub>2</sub>Cl<sub>6</sub>]2−red[(t<sub>2g</sub>)<sup>6</sup>(e<sub>g</sub>)<sup>3</sup>]2edge-shared bis(square planar)[23] Cu-Cl(terminal) = 2.24 Å
Cu-Cl(bridging) = 2.31 Å
[ZnCl<sub>4</sub>]2−white/colorlessd10tetrahedralZn-Cl bond length = 2.27 Å
(Et4N+)2 salt

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
[ZrCl<sub>6</sub>]2−yellowd0octahedralZr-Cl distance = 2.460 Å
(Me4N+)2 salt[24]
[Zr<sub>2</sub>Cl<sub>10</sub>]2−colorless(d0)2edge-shared bioctahedralZr-Cl = 2.36 Å (terminal), 2.43 Å (bridging)
N(PCl3)2)+ salt
Nb2Cl10yellow(d0)2edge-shared bioctahedral [Nb<sub>2</sub>Cl<sub>10</sub>]3.99 Å[25]
[NbCl<sub>6</sub>]yellowd0octahedralNb-Cl = 2.34 Å
N(PCl3)2)+ salt
[Nb<sub>6</sub>Cl<sub>18</sub>]2−black(d2)4(d3)2 (14 cluster electrons)cluster Nb---Nb bondingNb-Cl = 2.92 Å
(K+)2 salt[26]
MoCl6blackd0octahedronMo−Cl = 2.28 -2.31 Å
[MoCl<sub>6</sub>]2−yellow(t2g)2octahedronMo−Cl = 2.37, 2.38, 2.27 Å[27]
[MoCl<sub>6</sub>]3−pink(t2g)3octahedral
[Mo<sub>2</sub>Cl<sub>8</sub>]4−purple2(d4)Mo-Mo quadruple bond
[Mo<sub>2</sub>Cl<sub>9</sub>]3−2(d3)face-shared bioctahedralMo-Mo (triple) bond length = 2.65 Å
Mo-Cl (terminal) bond length = 2.38 Å
Mo-Cl (bridging) bond length = 2.49 Å[28]
Mo2Cl10green(d1)2edge-sharing bioctahedra[29]
[Mo<sub>2</sub>Cl<sub>10</sub>]2−(d2)2edge-sharing bioctahedra[30]
[Mo<sub>5</sub>Cl<sub>13</sub>]2−brown[31] d2d2d2d2d3incomplete octahedron[32]
[Mo<sub>6</sub>Cl<sub>14</sub>]2−yellowd4octahedral cluster(4-HOPyH+)2 salt[33]
[TcCl<sub>6</sub>]2−yellow(t2g)3octahedronTc-Cl = 2.35 Å for As(C6H5)4+ salt[34]
[Tc<sub>2</sub>Cl<sub>8</sub>]2−green(t2g)4Tc-Tc quadruple bondTc-Tc = 2.16, Tc-Cl = 2.34 Å for NBu4+ salt[35]
[RuCl<sub>6</sub>]2−brown(t2g)4octahedral(EtPPh3+)2 salt[36]
[Ru<sub>2</sub>Cl<sub>9</sub>]3−red[(t<sub>2g</sub>)<sup>5</sup>]2cofacial bioctahedralRu-Ru bond length = 2.71 Å; Ru-Cl(terminal) = 2.35 Å, Ru-Cl(bridging) = 2.36 Å ((Et4N)+)3 salt[37]
[Ru<sub>3</sub>Cl<sub>12</sub>]4−green(d5)2(d6)cofacial trioctahedralRu-Ru bond lengths = 2.86 Å
Ru-Cl bond lengths = 2.37-2.39 Å
(Et4N+)2(H7O3+)2 salt[38]
[RhCl<sub>6</sub>]3−red(t2g)6octahedralH2N+(CH2CH2NH3+)2 salt)[39]
[Rh<sub>2</sub>Cl<sub>9</sub>]3−red-brown(t2g)6octahedralRh-Cl(terminal) = 2.30 Å, Rh-Cl(terminal) = 2.40 Å
((Me3CH2Ph)+)3 salt)[40]
[PdCl<sub>4</sub>]2−brownd8square planar
[Pd<sub>2</sub>Cl<sub>6</sub>]2−[41] red ((Et4N+)2 salt)d8square planar
[Pd<sub>3</sub>Cl<sub>8</sub>]2−[42] orange brown ((Bu4N+)2 salt)d8square planar
[PdCl<sub>6</sub>]2−brownd6octahedralPd(IV)
[Pd<sub>6</sub>Cl<sub>12</sub>]yellow-brownd8square planar[43]
[AgCl<sub>2</sub>]white/colorlessd10linearsalt of [K(2.2.2-crypt)]+[44]
[CdCl<sub>4</sub>]2−white/colorlessd10tetrahedralEt4N+ salt, Cd-Cl distance is 2.43 Å[45]
[Cd<sub>2</sub>Cl<sub>6</sub>]2−white/colorlessd10edge-shared bitetrahedron(C6N3(4-C5H4N)33+ salt[46]
[Cd<sub>3</sub>Cl<sub>12</sub>]6−white/colorlessd10octahedral (central Cd)
pentacoordinate (terminal Cd's)
cofactial trioctahedral
(C6N3(4-C5H4N)33+ salt
(3,8-Diammonium-6-phenylphenanthridine3+)2[47]
[Cd<sub>6</sub>Cl<sub>19</sub>]7−white/colorlessd10octahedron of octahedra4,4'-(C6H3(2-Et)NH3+)2 salt[48]

3rd row

3rd Transition Series!Complex!!colour!!electron config.!!structure!!geometry!!comments
[HfCl<sub>6</sub>]2−whited0octahedralHf-Cl distance = 2.448 A
((Me4N+)2 salt)
[Hf<sub>2</sub>Cl<sub>10</sub>]2−colorless/whited0edge-shared bioctahedral[49]
[Hf<sub>2</sub>Cl<sub>9</sub>]colorless/white(d0)2face-shared bioctahedral[50]
[TaCl<sub>5</sub>]whited0edge-shared bioctahedral
[TaCl<sub>6</sub>]white/colourlessd0octahedralTa-Cl = 2.34 Å
(N(PCl3)2)+ salt)
[Ta<sub>6</sub>Cl<sub>18</sub>]2-greend0octahedralTa-Ta = 2.34 Å
(H+2 salt hexahydrate[51]
WCl6blued0octahedral2.24–2.26 Å[52]
[WCl<sub>6</sub>]2−(t2g)2octahedralW-Cl distances range from 2.34 to 2.37 Å
(PPh4+ salt)[53]
[WCl<sub>6</sub>](t2g)1octahedralW-Cl distance = 2.32 Å
(Et4N+ salt)[54]
W2Cl10black[55] (t2g1)2bioctahedralW-W distance = 3.814 Å[56]
[W<sub>2</sub>Cl<sub>8</sub>]4−blue2(d4)W-W quadruple bonddW-W = 2.259 Å [Na(tmeda)<sup>+</sup>]4 salt[57]
[W<sub>2</sub>Cl<sub>9</sub>]2−d3d2face-sharing bioctahedralW-W distance = 2.54 Å
W-Cl(terminal) = 2.36 Å, W-Cl(bridge) = 2.45 Å
((PPN+)2 salt)[58]
[W<sub>2</sub>Cl<sub>9</sub>]3−d3d3octahedralW-Cl distance = 2.32 Å
(Et4N+ salt)
[W<sub>3</sub>Cl<sub>13</sub>]3−d3,d3,d4[W<sub>3</sub>(μ<sub>3</sub>-Cl)(μ-Cl)<sub>3</sub>Cl<sub>9</sub>]3-W-W distances = 2.84 Å[59]
[W<sub>3</sub>Cl<sub>13</sub>]2−d3,d4,d4[W<sub>3</sub>(μ<sub>3</sub>-Cl)(μ-Cl)<sub>3</sub>Cl<sub>9</sub>]2-W-W distances = 2.78 Å
[W<sub>6</sub>Cl<sub>14</sub>]2-yellow[60] (d4)6see Mo6Cl12
[ReCl<sub>6</sub>]red-brown(t2g)2octahedralRe-Cl distance = 2.24-2.31 Å
(PPh4+ salt)[61]
[ReCl<sub>6</sub>](t2g)1octahedralRe-Cl distance = 226.3(6) Å[62]
[ReCl<sub>6</sub>]2−green(t2g)3octahedralRe-Cl distance = 2.35-2.38 Å
((PPN+)2 salt)[63]
[Re<sub>2</sub>Cl<sub>9</sub>]2−(t2g)3(t2g)4face-sharing bioctahedralRe-Re distance = 2.48 Å
Re-Cl distances = 2.42 Å (bridge), 2.33 Å (terminal)
((Et4N+)2 salt)[64]
[Re<sub>2</sub>Cl<sub>9</sub>]((t2g)3)2face-sharing bioctahedralRe-Re distance = 2.70 Å
Re-Cl distances = 2.41 (bridge), 2.28 Å (terminal)
(Bu4N+ salt)
[OsCl<sub>6</sub>]dark green(t2g)3octahedraldOs-Cl = 2.30 Å for Et4N+ and Ph4P+ salts
[OsCl<sub>6</sub>]2−yellow-orange(t2g)4octahedral[65] Os-Cl distance 2.33 Å
[Os<sub>2</sub>Cl<sub>8</sub>]2−green(d5)2square antiprismdOs-Os = 2.182 Å, dOs-Cl = 2.32 Å (Bu4N+)2 salt[66]
[Os<sub>2</sub>Cl<sub>10</sub>]2−green(d4)2octahedraldOs-Cl(terminal) = 2.30 Å dOs-Cl(bridging) = 2.42 Å (Et4N+)2 salt[67]
[IrCl<sub>6</sub>]3−red(t2g)6octahedralIr-Cl = 2.36 Å[68]
[IrCl<sub>6</sub>]2−brown(t2g)5octahedralIr-Cl = 2.33 Å[69]
[Ir<sub>2</sub>Cl<sub>9</sub>]3−-((t2g)6)2bi-octahedral[70]
[PtCl<sub>4</sub>]2−pinkd8square planar
[PtCl<sub>6</sub>]2−yellowd6octahedralPt-Cl distance = 2.32 Å
Et4N+ salt, ((Me4N+)2 salt)
[Pt<sub>2</sub>Cl<sub>9</sub>]red (Bu4N+ salt)((t2g)6)2octahedralPt-Clt and Pt-Clbridge = 2.25, 2.38 Å[71]
[Pt<sub>2</sub>Cl<sub>10</sub>]2−yellow-brown (PPN+ salt)((t2g)6)2edge-shared bioctahedralPt-Clt and Pt-Clbridge = 2.27, 2.37 Å
[Pt<sub>6</sub>Cl<sub>12</sub>]yellow-brown(d8)6square planarPt-Cl = 2.31[72]
[AuCl<sub>2</sub>]white/colorlessd10linearAu-Cl distances of 2.28 Å
NEt4+ salt[73]
black(d10)2(d8)2linear and square planarrare example of mixed valence, molecular chloride[74]
[AuCl<sub>4</sub>]yellowd8square planarAu-Cl distances of 2.26 Å
NBu4+ salt[75]
[HgCl<sub>4</sub>]2−white/colorlessd10tetrahedralHg-Cl distance is 2.46 Å
Et4N+ salt
[Hg<sub>2</sub>Cl<sub>6</sub>]2−white/colorlessd10edge-shared bitetrahedralHg-Cl distance is 2.46 Å[76]
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

As indicated in the table below, many hydrates of metal chlorides are molecular complexes.[77] [78] 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
trans-[TiCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]+
trans-[VCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]+[79]
trans-[CrCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]+
[CrCl(H<sub>2</sub>O)<sub>5</sub>]2+
trans-[CrCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]
[Cr(H<sub>2</sub>O)<sub>6</sub>]3+[80]
trans-[MnCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]
cis-[MnCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>][81]
trans-[FeCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]
trans-[FeCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]
trans-[FeCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]+ one of four hydrates of ferric chloride,[82]
cis-[FeCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]+[83]
trans-[CoCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]
cis-[CoCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]
trans-[NiCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]
cis-[NiCl<sub>2</sub>(H<sub>2</sub>O)<sub>4</sub>]

Adducts

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

Notes and References

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