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 [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.

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

1st row + 1st transition seriesComplexcolourelectron config.structuregeometrycomments TiCl4 colourless (t2g)0 tetrahedral [Ti2Cl9]− white/colourless d0d0 face-sharing bioctahedron Ti–Cl(terminal) = 2.23 Å, 2.45 (terminal) salt [Ti2Cl9]3− orange (t2g)1(t2g)1 face-sharing bioctahedron Ti–Ti = 3.22 Å Ti–Cl(terminal) = 2.32–2.35 Å,Ti–Cl(bridge) = 2.42–2.55 Å salt [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–Cl(terminal) = 2.36 Å (terminal),Ti–Cl(bridge) = 2.50 Å salt [TiCl6]2− yellow d0 octahedral Ti–Cl = 2.33 Å salt VCl4 red (t2g)1 tetrahedral V−Cl = 2.29 Å V2Cl10 violet (t2g)0 edge-shared bioctahedron V−Cl(bridging) = 2.48 ÅV−Cl(terminal) = 2.16–2.21 Å [VCl6]2− red (t2g)1 octahedral V−Cl = 2.29 Å [CrCl6]3− pink (t2g)3 octahedral [Cr2Cl9]3− red (d3)2 face-sharing bioctahedron Cr–Cl(terminal) = 2.31 Å, 2.42 (terminal) salt [MnCl4]2− pale pink to white (eg)2(t2g)3 tetrahedral Mn–Cl bond length = 2.3731–2.3830 Å [MnCl6]2− dark red (t2g)3(eg)1 octahedral Mn–Cl distance = 2.28 ÅK+ salt – salt is isostructural with K2PtCl6 [MnCl6]3− brown (t2g)3(eg)1 octahedral [Mn2Cl6]2− yellow-green (eg)2(t2g)3 bitetrahedral Mn–Cl(terminal) bond length = 2.24 ÅMn–Cl(terminal) bond length = 2.39 Å salt [Mn3Cl12]6− pink (t2g)3(eg)2 cofacial trioctahedron salt [FeCl4]2− cream (eg)3(t2g)3 tetrahedralEt4N+ salt [FeCl4]− (eg)2(t2g)3 tetrahedral Fe–Cl bond length = 2.19 Å [FeCl6]3− orange (t2g)3(eg)2 octahedral [Fe2Cl6]2− pale yellow (eg)2(t2g)3 bitetrahedral Fe–Cl(terminal) bond length = 2.24 ÅFe–Cl(terminal) bond length = 2.39 Å(PPN+)2 salt [CoCl4]2− blue (eg)4(t2g)3 tetrahedral [Co2Cl6]2− blue (eg)4(t2g)3 bitetrahedral Mn–Cl(terminal) bond length = 2.24 ÅCo–Cl(terminal) bond length = 2.35 Å(PPN+)2 salt [NiCl4]2− blue (eg)4(t2g)4 tetrahedral Ni–Cl bond length = 2.28 Å(Et4N+)2 salt [Ni3Cl12]6− orange (t2g)6(eg)2 confacial trioctahedral saltdouble salt with two Cl−Ni–Cl bond length = 2.36–2.38 Å [CuCl4]2− orangeyellow (flattened tetrahedral)green (square planar) (t2g)6(eg)3 flattened tetrahedralor square planar Cu–Cl bond length = 2.24 Å [Cu2Cl6]2− red [(t2g)6(eg)3]2 edge-shared bis(square planar) Cu–Cl(terminal) = 2.24 ÅCu–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 seriesComplexcolourelectron config.structuregeometrycomments [ZrCl6]2− yellow d0 octahedral Zr–Cl distance = 2.460 Å(Me4N+)2 salt [Zr2Cl10]2− colorless (d0)2 edge-shared bioctahedral Zr–Cl = 2.36 Å (terminal), 2.43 Å (bridging) salt Nb2Cl10 yellow (d0)2 edge-shared bioctahedral [Nb2Cl10] 3.99 Å [NbCl6]− yellow d0 octahedral Nb–Cl = 2.34 Å salt [Nb6Cl18]2− black (d2)4(d3)2 (14 cluster electrons) cluster Nb···Nb bonding Nb–Cl = 2.92 Å (K+)2 salt MoCl6 black d0 octahedron Mo−Cl = 2.28–2.31 Å [MoCl6]2− yellow (t2g)2 octahedron Mo−Cl = 2.37, 2.38, 2.27 Å [MoCl6]3− pink (t2g)3 octahedral [Mo2Cl8]4− purple 2(d4) Mo-Mo quadruple bond [Mo2Cl9]3− 2(d3) face-shared bioctahedralMo–Mo (triple) bond length = 2.65 ÅMo–Cl (terminal) bond length = 2.38 ÅMo–Cl (bridging) bond length = 2.49 Å Mo2Cl10 green (d1)2 edge-sharing bioctahedra [Mo2Cl10]2− (d2)2 edge-sharing bioctahedra [Mo5Cl13]2− brown d2d2d2d2d3 incomplete octahedron [Mo6Cl14]2− yellow d4 octahedral cluster (4-HOPyH+)2 salt [TcCl6]2− yellow (t2g)3 octahedron Tc–Cl = 2.35 Å for salt [Tc2Cl8]2− green (t2g)4 Tc–Tc quadruple bond Tc–Tc = 2.16, Tc–Cl = 2.34 Å for salt [RuCl6]2− brown (t2g)4 octahedral salt [Ru2Cl9]3− red [(t2g)5]2 cofacial bioctahedral Ru–Ru bond length = 2.71 ÅRu–Cl(terminal) = 2.35 Å, Ru–Cl(bridging) = 2.36 Å ((Et4N)+)3 salt [Ru3Cl12]4− green (d5)2(d6) cofacial trioctahedral Ru–Ru bond lengths = 2.86 ÅRu–Cl bond lengths = 2.37–2.39 Å salt [RhCl6]3− red (t2g)6 octahedral salt [Rh2Cl9]3− red-brown (t2g)6 octahedral Rh–Cl(terminal) = 2.30 Å, Rh–Cl(terminal) = 2.40 Å ((Me3CH2Ph)+)3 salt [PdCl4]2− brown d8 square planar [Pd2Cl6]2− red ((Et4N+)2 salt) d8 square planar [Pd3Cl8]2− orange brown ((Bu4N+)2 salt) d8 square planar [PdCl6]2− brown d6 octahedral Pd(IV) [Pd6Cl12] yellow-brown d8 square planar [AgCl2]− white/colorless d10 linear salt of [K(2.2.2-crypt)]+ [CdCl4]2− white/colorless d10 tetrahedral Et4N+ saltCd–Cl distance is 2.43 Å [Cd2Cl6]2− white/colorless d10 edge-shared bitetrahedron4- salt [Cd3Cl12]6− white/colorless d10 octahedral (central Cd)pentacoordinate (terminal Cd)cofactial trioctahedral4- salt(3,8-Diammonium-6-phenylphenanthridine3+)2 [Cd6Cl19]7− white/colorless d10 octahedron of octahedra(4,4′-2- salt

3rd row

+ 3rd transition seriesComplexcolourelectron config.structuregeometrycomments [HfCl6]2− white d0 octahedral Hf–Cl distance = 2.448 Å(Me4N+)2 salt [Hf2Cl10]2− colorless/white d0 edge-shared bioctahedral [Hf2Cl9]− colorless/white (d0)2 face-shared bioctahedral [TaCl5] white d0 edge-shared bioctahedral [TaCl6]− white/colourless d0 octahedral Ta–Cl = 2.34 Å salt [Ta6Cl18]2− green d0 octahedral Ta–Ta = 2.34 Å salt hexahydrate WCl6 blue d0 octahedral 2.24–2.26 Å [WCl6]2− (t2g)2 octahedral W–Cl distances range from 2.34 to 2.37 Å salt [WCl6]− (t2g)1 octahedral W–Cl distance = 2.32 Å salt W2Cl10 black (t2g1)2 bioctahedral W–W distance = 3.814 Å [W2Cl8]4− blue 2(d4) W–W quadruple bond W–W distance = 2.259 Å[Na(tmeda)+]4 salt [W2Cl9]2− d3d2 face-sharing bioctahedral W–W distance = 2.54 ÅW–Cl(terminal) = 2.36 Å, W–Cl(bridge) = 2.45 Å(PPN+)2 salt [W2Cl9]3− d3d3 octahedral W–Cl distance = 2.32 ÅEt4N+ salt [W3Cl13]3− d3, d3, d4 [W3(μ3-Cl)(μ-Cl)3Cl9]3− W–W distances = 2.84 Å [W3Cl13]2− d3, d4, d4 [W3(μ3-Cl)(μ-Cl)3Cl9]2− W–W distances = 2.78 Å [W6Cl14]2− yellow (d4)6 see Mo6Cl12 [ReCl6]− red-brown (t2g)2 octahedral Re–Cl distance = 2.24–2.31 Å salt) [ReCl6] (t2g)1 octahedral Re–Cl distance = 2.263(6) Å [ReCl6]2− green (t2g)3 octahedral Re–Cl distance = 2.35–2.38 Å(PPN+)2 salt [Re2Cl9]2− (t2g)3(t2g)4 face-sharing bioctahedral Re–Re distance = 2.48 ÅRe–Cl distances = 2.42 Å (bridge), 2.33 Å (terminal) (Et4N+)2 salt [Re2Cl9]− ((t2g)3)2 face-sharing bioctahedral Re–Re distance = 2.70 ÅRe–Cl distances = 2.41 (bridge), 2.28 Å (terminal)Bu4N+ salt [OsCl6]− dark green (t2g)3 octahedral Os–Cl = 2.30 Å for Et4N+ and Ph4P+ salts [OsCl6]2− yellow-orange (t2g)4 octahedral Os–Cl distance 2.33 Å [Os2Cl8]2− green (d5)2 square antiprism Os–Os = 2.182 ÅOs–Cl = 2.32 Å(Bu4N+)2 salt [Os2Cl10]2− green (d4)2 octahedral Os–Cl(terminal) = 2.30 Å Os–Cl(bridging) = 2.42 Å (Et4N+)2 salt [IrCl6]3− red (t2g)6 octahedral Ir–Cl = 2.36 Å [IrCl6]2− brown (t2g)5 octahedral Ir–Cl = 2.33 Å [Ir2Cl9]3− - ((t2g)6)2 bioctahedral [PtCl4]2− pink d8 square planar [PtCl6]2− yellow d6 octahedral Pt–Cl distance = 2.32 ÅEt4N+ salt, ((Me4N+)2 salt) [Pt2Cl9]− red (Bu4N+ salt) ((t2g)6)2 octahedral Pt–Clt and Pt–Clbridge = 2.25, 2.38 Å [Pt2Cl10]2− yellow-brown (PPN+ salt) ((t2g)6)2 edge-shared bioctahedral Pt–Clt and Pt–Clbridge = 2.27, 2.37 Å [Pt6Cl12] yellow-brown (d8)6 square planar Pt–Cl = 2.31 [AuCl2]− white/colorless d10 linear Au–Cl distances of 2.28 Å salt black (d10)2(d8)2 linear and square planar rare example of mixed valence, molecular chloride [AuCl4]− yellow d8 square planar Au–Cl distances of 2.26 Å salt [HgCl4]2− white/colorless d10 tetrahedral Hg–Cl distance is 2.46 ÅEt4N+ salt [Hg2Cl6]2− white/colorless d10 edge-shared bitetrahedral Hg–Cl distance is 2.46 ÅBu4N+ 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. 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 Coordinationsphere of the metal TiCl3(H2O)6 trans-[TiCl2(H2O)4]+ VCl3(H2O)6 trans-[VCl2(H2O)4]+ 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+ MnCl2(H2O)6 trans-[MnCl2(H2O)4] MnCl2(H2O)4 cis-[MnCl2(H2O)4] FeCl2(H2O)6 trans-[FeCl2(H2O)4] FeCl2(H2O)4 trans-[FeCl2(H2O)4] FeCl3(H2O)6 one of four hydrates of ferric chloride, FeCl3(H2O)2.5 cis-[FeCl2(H2O)4]+ 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.

References