Transition metal chloride complex

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]

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).

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 p_π 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 white	(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.