2+ 0COCHC12
&j+ (@3jA @ 9 3 . 0 5 ~
2.12 A
H z 0 next nearest
atom at 3.25 A OCOCHClz
(e) (0 (g)
966
Group VIII and Other Transition Metals molecules (g) in the ratio 1 : 2. This is the same ratio of diamagnetic to paramagnetic complexes as in NiBr2(P$2B~)2 mentioned above, and results in a similar intermediate value of p,ff..
We referred in Chapter 3 t o the clathrate compounds based on layers of the composition Ni(CN)2 . NH3. In the clathrates containing C6H6 or C6HsNHZ the layers are directly superposed, with NH3 molecules of different layers directed towards one another, so that the interlayer spacing is 8.3 A and there are large cavities between the layers (Fig. 1.9(b)).(~) In the hydrate and in the anhydrous compound the layers are packed much more closely (interlayer spacing 4.4 A), the N H j molecules of one layer projecting towards the centres of the rings of adjacent layers (pseudo b ~ d ~ - c e n t r e d ) . ( ~ )
Ni(11) forming 4 coplanar bonds. Planar diamagnetic complexes include the N ~ ( c N ) ~ - ion, which has been studied in the K(') and other salts(') (Ni-C,
1.85 A), and molecules such as (a)(3) and (b).(4) The numerous complexes with bidentate ligands include the thio-oxalate ion, (c),(') molecules of the types (d)(6) and (e),(7) and the glyoximes, (f),(') notable for the short intramolecular hydrogen bonds. Many divalent metals form phthalocyanins,(9) M(II) replacing 2 H in C J 2 H l 'N8 (Fig. 27.9).
H H
s 4 7's
I Ni, I NwS' S,N
(el
~ o f ~ m e r i c species include finite molecules and chains. In Ni3L4 ( L = HzN . CH2. CH2. s),(") (g), the three Ni atoms are collinear with a planar arrangement of bonds around each. The whole molecule is twisted owing t o the
(1) ACSc 1964 18 2385
(2) ACSc 1969 23 14,61; AC 1970 B26 361
(3) AC 1968 824 I08 (4) JCS A 1967 1750 (5) JCS 1935 1475 (6) IC 1968 7 2140, 2625;
AC 1969 B25 909, 1294, 1939
(7) ZaC 1968 363 159 (8) AC 1967 22 468 (9) JCS 1937 219
o o @ c N Metal
PIG. 27.9. Molecule of a metal phthalocyanine, C32H1 6N8M.
Group VIII and Other Transition Metals
symmetrical arrangement of the chelates around the central Ni atom and the cis configuration around the terminal Ni atoms. The Ni-Ni distance in this complex is 2.73 A.
Nickel mercaptides are mixtures of insoluble polymeric materials and hexamers.
(11) JACS 1965 87 5251 The molecule of [Ni(SC2H5)2] 6 ( 1 1 ) is shown diagrammatically in Fig. 27.10. The six Ni atoms are coplanar and are bridged by 12 mercaptan groups. The bond arrangement around Ni is square planar, with all angles in the Ni2S2 rings equal to 83', Ni-S, 2.20 A, and Ni-Ni, 2.92 A (mean). The dimethylpyrazine complex, (12) IC 1964 3 1303 N ~ B ~ ~ L , ( ' 2, (h), is an example of a chain in which Ni is 4-coordinated by 2 Br and
by 2 N atoms of the bridging ligands:
- 7' Ni- 1.85 A 4' F . 3 1 A
I "FJ 7-
Br H3C Br
(h) FIG. 27.10. The molecule
INi(SC2Hs)2J 6 (diagrammatic). Departures from exact coplanarity of Ni and its four bonded atoms may occur in unsymmetrical complexes, for example, those containing polydentate ligands. In (13) 1C 1965 4 1726 the (diamagnetic) complex (i),(' 3, there is slight distortion (tetrahedral rather than square pyramidal), the angles SINiNl and SzNiNz being 173'. Large deviations from planarity, leading to paramagnetic tetrahedral complexes, can result if there is steric interference between atoms forming parts of independent ligands. In the tetramethyl dipyrromethenato complex ( j ) ( 1 4 ) there is interference between the CH3* groups, and the angle between the planes of the two ligands is 76'; it is not obvious why this angle is not, 90'. The 3-substituted bis(N-isopropylsalicylaldi-
Group VIII and Other Transition Metals
minato) Ni complexes, (k),(' ') are of interest in this connection, for they fall into (15) AC 1967 22 780 two groups; tetrahedral (y = 3.3 BM) or planar diamagnetic according to the nature
of the substituent in the 3 position. The detailed stereochemistry of these compounds is not readily understandable, for the 3-CH3 compound is planar while the 3-H and 3-C2H, compounds are tetrahedral.
Ni(11) foming 4 tetrahedral bonds. The simplest examples are the complexes N i ~ j - , NiX3L-, NiX2L2, and N~L;, where L is a monodentate and L' is a bidentate ligand. The failure to recognize, until fairly recently, the existence of halide ions N ~ x $ - was due, not to their intrinisic instability, but to their instability in aqueous solution, where X is displaced by the more strongly coordinating H 2 0 molecule. The ~ i ~ 1 : - ion can be formed in melts and studied in solid solutions in, for example, Cs2ZnC14, but Cs2NiC14 dissociates on cooling. On the other hand, Cs3NiC15 (analogous to Cs3(CoCl4)C1) can be prepared from the molten salts and quenched to room temperature, but on slow cooling it breaks down to a mixture of CsCl and CsNiC13 (in which there is octahedral coordination of Ni). By working in alcoholic solution salts of large organic cations can be prepared, for example,
(NEt4)2NiC14 k f f . 9 3'9 BM) and (W3Me)2'''4 (red, peff,, 3'5 BM)' There (1) I c 1966 5 1498 is no Jahn-Teller distortion in the N~CI$' ion, which has a regular tetrahedral shape Ac 1967 23 1064
in ( ~ s $ i ~ ~ e ) , N i ~ l ~ ( ' ) but is slightly flattened (two angles of 107' and four of
(3) JINC 1964 16 2035 111") in the (NEt4) salt,(') presumably the result of crystal packing forces. Ions
(4) IC 1968 2303
Nix:- have been prepared in which X is Cl, Br, I, NCS, or NCO.(') Ions NiX3L- (5) Ic 1968 2629 which have been studied include those in the salts [ N i ~ r ~ ( ~ u i n o l i n e ) l AS^^)(^) and
[Ni13(P$,)] [N(n-C4H9).] . ( I ) In molecules NiX2L2 there may be considerable
(6) JCS A 1968 1413
distortion from regular tetrahedral bond angles as, for example, Br-Ni-Br, 126' in
(7) JCS 1963 3625 N i ~ r ~ ( ~ $ i , ) ~ , ( ~ ) and C1-Ni-C1, 123" and P-Ni-P, 1 11•‹, in N ~ C ~ ~ ( P $ ~ ) , . ( ' ) We
have. already commented on the planar- tetrahedral isomerism of molecules Nix2 L2 and of molecules N ~ L ; containing certain bidentate ligands.
Some more exotic examples of molecules in which Ni forms tetrahedral bonds are shown at (a)-(c).
Croup VIII and Other Transition Metals
(8) HCA 1962 45 647 (9) IC 1968 7 261
(10) JACS 1967 89 5366
(11) NW 1952 39 300;
JACS 1956 78 702
P
P = P ( c ~ H ~ c N ) ~ ( " ) (c)
In Ni4(C0)6 [P(C2H4CN)3] 4 Ni acquires a closed shell configuration if the three metal-metal bonds (2.57 A, compare 2.49 A in the metal) are included with those to the four nearest neighbours (Ni-3 C, 1-89 A, Ni-P, 2.16 A), and for this reason Ni should perhaps be regarded as 7- rather than 4-coordinated in this compound.
The bridged structure assigned to the anion in the red diamagnetic K ~ N ~ ~ ( c N ) , ( ' ' ) does not appear to have been elucidated in detail.
LNi-
Ni(11) forming 5 bonds. The two most symmetrical configurations, trigonal bipyramidal and square (tetragonal) pyramidal, are closely related geometrically, so that descriptions of configurations intermediate between the two extremes are sometimes rather arbitrary. However, complexes with geometries very near to both configurations are found for both diamagnetic and paramagnetic compounds of N~(II), and in one case (p. 966) two distinct configurations of the N ~ ( c N ) ~ - ion are found in the same crystal. The choice of configuration is influenced by the geometry of the ligand if this is polydentate. Apart from the N ~ ( c N ) ~ - ion nothing is known of the structures of ions Nix%-. In salts such as Rb3Ni(N02)5 there may be bridging NO2 groups in an octahedral chain, as in [ ~ i ( e n ) ~ ( N o ~ ) ] + . Complexes with monodentate ligands include the diamagnetic molecule (a) with a nearly ideal trigonal bipyramidal configuration (though with P4(OEt)2 ligands an intermediate configuration is adopted(')) and the paramagnetic complex (b) (p = 3.7 BM).(?)
Complexes with polydentate ligands include the types (c)-(f). We shall not discuss all these cases individually since the configuration often depends on the detailed structure of the (organic) ligand. For example, in derivatives of Schiff bases R . C6H3(0H)CH=N. CH2CHzNEt2 (type (d)) the compound NiL2 may be
-NiL
\Ni A . 2 7 A L
Group VIII and Other Transition Metals
diamagnetic planar, paramagnetic koordinated, or paramagnetic octahedral,
depending on the nature of the substituent R . ( ~ ) The 'tripod-like' tetradentate (3) J ~ c s 1965 87 2059 ligands such as TSP and TAP tend to favour the trigonal bipyramidal configuration,
as in the diamagnetic compounds [Ni(TSP)Cl] and [Ni(TAP)CN] C!lo4.(') (4) IC 1969 8 107 2
References to a number of square pyramidal complexes are given in reference (6). ( 5 ) JACS 1967 89 3424
The structure of N~B~~(TAs),(') in which TAS is the tridentate ligand (6) Ic 19698 1915
is described on p. 980, where it is compared with a 5-coordinated Pd complex with somewhat similar geometry.
Group VIII and Other Transition Metals
(la) IC 1965 4 456
(lb) IC 1969 8 1304
(2) JCS 1963 1309 (3) AC 1966 20 349
(4) JCS 1962 3845
Ni(11) forming 6 octahedral bonds. The simplest examples of octahedrally coordinated ~ i are the crystalline monoxide and the dihalides. Octahedral ~ + complexes mentioned elsewhere include the cation in [Ni(H20)6] SnCl,, the trimeric [Ni(acac)?] , ( l a ) and Ni6(CF3COCHCOCH3) o ( ~ ~ ) 2 ( ~ z 0 ) 2 ( 1 b, both of which are illustrated in Chapter 5 (Fig. 5.9 (p. 166) and Fig. 5.1 1 (p. 167)).
Many octahedral paramagnetic complexes of Ni(11) have been studied, and
f p
N N C S
examples of the general types (a)-(d) include:
(a) Ni(pyraz01e)~Cl~ AC 1969 B25 595
(b) Ni(acac)z(~~r)z IC 1968 7 2316
Ni(en)2 (NCS)2 AC 1963 16 753
(c) [Ni(en)3 1 (No312 AC 1960 13 639
(d) Ni(tren)(NCS)? ACSc 1959 13 2009
In contrast to N i ( t ~ ) ~ C l ~ , which forms finite molecules in which there is an (unexplained) asymmetry (the lengths of the two trans Ni-C1 bonds being 2.40 and 2.52 A),(2) the thiocyanate N ~ ( ~ U ) ~ ( N C S ) ~ consists of infinite chains of edge- sharing octahedra, (e);(3) in contrast to Ni(en)z(NCS)2, type (b), the nitrito compound Ni(e11)~0N0 is built of chains of vertex-sharing octahedra, ( Q . ( ~ )
SCN
Group VIII and Other Transition Metals The situation with regard to diamagnetic octahedral complexes is still unsatis- factory. Halogen compounds N i ( d i a r ~ i n e ) ~ X ~ behave in solution in CH3N02 as uni-univalent electrolytes, but in crystals of the brown form of Ni(diarsine)~I~
there are octahedral molecules of type (b) in which Ni-As is 2.29 A but Ni-I, 3.21 This Ni-I bond length may be compared with 2.55 A in [Ni13(P$3)] - (tetrahedral) and 2-54 A in NiIz [(C6HS)P(C6H4. SCH3)z] (square pyramidal, ref.
( 6 ) of previous section). The nature of the Ni-I bonds is uncertain, for the close approach of I to Ni is prevented by the methyl groups which project above and below the equatorial plane. Structural studies have apparently not been made of compounds such as [Ni(diar~ine)~] (C104), which are often quoted as examples of octahedral spin-paired Ni(11). (The electronic structure of the paramagnetic [Ni(diar~ine)~Cl~] C1, containing one unpaired electron, is uncertain.(6))
Other oxidation states of Ni
The oxidation states other than 11 which are firmly established are 0, I I I , and I V . Ni(0). Only tetrahedral complexes are known; they include Ni(C0)4, Ni(PF3)4, Ni(CNR)4, and the N~(cN):- ion in the yellow potassium salt. Structural information regarding these complexes is summarized in Chapter 22.
N~(III). Compounds of Ni(111) may include NiBr3(PEt3)2, formed by oxida- tion of t r a n ~ - N i B r ~ ( P E t ~ ) ~ , and N i ( d i a r ~ i n e ) ~ C l ~ , which results from the oxidation of the dichloro compound by O2 in the presence of excess C1- ions. Both these compounds have magnetic moments corresponding to 1 unpaired electron, but their structures are not yet known. For the former a trigonal bipyramidal configuration would be consistent with its dipole moment,(') and this structure is likely in view of the structure of the compound ~ i " ' ~ r ~ . P; .0.5 ( N ~ " B ~ ~ P ; ) . c ~ H ~ , ( ' ) in which P'= P(C6H5)(CH3)2. This compound consists of trans planar molecules N ~ B ~ ~ P ; with twice as many trigonal bipyramidal molecules N ~ B ~ ~ P ; in which P
occupies axial positions (Ni-P, 2.27 A, Ni-Br, 2.35 A). (There is a slight distortion of the latter molecules, with one equatorial bond about 0.03 A longer than the other two and an angle of 133" opposite this longer bond. The Ni-P bonds are not significantly different in length from those in the square Ni(11) complex; the Ni-Br bonds are approximately 0.05 A longer.) The diarsine complex is pre- sumably [ N i ( d i a r ~ i n e ) ~ C l ~ ] C1 containing Ni(i11) forming octahedral bonds, though there is some doubt as to whether such octahedral complexes should be regarded as compounds of N~(III) since e.s.r. measurements suggest that they should perhaps be regarded as Ni(11) compounds in which the unpaired electron spends a large part of its time on the As atom.(3)
Structural information about simple N ~ ( I I I ) compounds is limited to those containing F or 0. K3NiF6 has been assigned the K3FeF6 structure and has a moment of 2.5 BM, intermediate between the values for low- and h i g h - ~ ~ i n . ( ~ ) It contains octahedrally coordinated Ni(r11). Oxides LnNi03 formed with the 4f elemknts have the perovskite structure,(5) while NiCr03 has a statistical corundum structure; it is concluded from the magnetic properties that this compound contains high-spin N ~ ( I I I ) . ( ~ )
(5) AC 1964 17 592 (6) JACS 1968 90 1067
(1) ACSc 1963 17 1126 (2) IC 1970 9 453
(3) JACS 1968 90 1067
(4) ZaC 1961 308 179 (5) JSSC 1971 3 582 (6) JAP 1969 40 434
Group VIII and Other Transition Metals
(7) ZaC 1949 258 221
(8) ZaC 1956 286 136
(9) AC 1951 4 148
Ni(1v). The only definite structural information relates to the red salts K Z N ~ F ~ ( ' ) and C S ~ N ~ F ~ . ( ' ) They are diamagnetic and have the K2SiF6 structure, with octahedral coordination of Ni(rv). The oxide BaNi03 has been quoted for many years as an example of a Ni(rv) compound,(9) but there is still doubt about its composition (BaNi02.S?) and the interpretation of its magnetic properties.
The diarsine cation mentioned above can be further oxidized to [Ni(diar~ine)~Cl~] '+ which is isolated as the deep-blue perchlorate, possibly containing Ni(rv).
The structural chemistry of Pd and Pt Planar complexes of Pd(11) and Pt(11)
As long ago as 1893 Werner suggested that the four atoms or groups attached to divalent Pd or Pt atoms in certain molecules were coplanar with the metal atom, this suggestion being made to account for the existence of compounds such as Pt(NH3)'C12 in two isomeric forms. For coplanar bonds there would be cis and
NH3\pt/ Cl and
c1/ \ N H ~
trans forms whereas if the bonds were arranged tetrahedrally there would be only one form of such a molecule. It should be noted that no amount of evidence from chemical reactions, cis- trans isomerism, or optical activity can prove conclusively the coplanar arrangement of four bonds from a metal atom. For example, even if the approximate coplanarity of the four ligands is admitted, this does not prove that the central metal atom lies in the same plane; it could be situated at the vertex of a tetragonal pyramid. In pre-structural days the only way of attempting to prove this unusual bond arrangement was to study geometrical and optical isomerism, and much ingenuity was used in making appropriate compounds. In addition t o the cis-trans isomers of compounds PtX2R2 three isomers of a planar complex Mabcd are possible as compared with only one form of a tetrahedral complex, and all three isomers of [ P ~ ( N H ~ ) ( N H ~ O H ) ( N O ~ X C ~ H ~ N ) ] NO2 have been reported. [Some confusion arose in cases where two forms of a compound have the same empirical formula but different structures, for example, Pt(NH3)2C12 and [Pt(NH3)4](PtC14).] More complex examples of cis-trans isomers include the compounds of Pd and Pt with two molecules of glycine or unsymmetrical glyoximes. A molecule such as Pd(g1y~ine)~ (a), with unsymmetrical rings, would
(a)
be enantiomorphic if the bonds from the metal atom are disposed tetrahedrally, whereas if these bonds are coplanar with the metal atom only cis-trans isomerism is possible. Attempts to resolve certain compounds of this type have been made and have been unsuccessful, but the failure to effect a resolution is clearly of doubtful
Group VIII and Other Transition Metals value as evidence for the disposition of the bonds from the metal atom. This difficulty was overcome in an ingenious way by Mills and Quibell (1935) who prepared a compound which would be optically active if the bonds from the central metal atom were coplanar and non-resolvable if these bonds were arranged tetrahedrally. The following sketches show that if the Pt bonds are coplanar, (b), the ion possesses neither a plane nor a centre of symmetry, whereas if the Pt bonds are tetrahedral, (c), then the planes of the two rings are perpendicular to one another and the ion possesses a plane of symmetry.
The compound,
was resolved into highly stable optical antimers. The corresponding ion containing Pd was also shown, in the same way, to have coplanar rings. Here again it should be noted that the resolution of these compounds is not conclusive proof of the coplanar arrangement of the four Pt or Pd bonds. The metal atom could lie above the plane of the four NH2 groups, and the molecule would still be optically active.
Nevertheless, as was pointed out by Mills and Quibell, certain simpler complexes should be resolvable if the metal bonds were pyramidal.
Dipole moment measurements make possible a distinction between cis and trans planar molecules or ions, the cis isomer having a larger dipole moment and the trans a smaller or zero moment, depending on the nature of the ligands, for example:
Cc
trans PtBr2(Et3P), 0 cis PtBr2(Et3P), 11.2 D PtC12(Pr2S)2 2.35 D PtC12(Pr2S)2 9.5
(In the phosphine compounds the P bonds are tetrahedral, so that the resultant moment of the R3P group is directed along the Pt-P bond, whereas the S bonds in the dialkyl sulphide groups are pyramidal and the resultant moment of the ligand is not directed along the Pt-S bond.) Note that the existence of only one isomer wit4 a high dipole moment is not proof of a planar configuration, for a tetrahedral molecule can also have a high moment. (Nickel con~pounds NiX2L2 do not exhibit cis-trans isomerism, but some exhibit planar-tetrahedral isomerism, as already noted.)
Group VIII and Other Transition Metals
In 1922 the planar structure of the ions P~cI:- and PtC1;- was proved by the determination of the structures of the potassium salts. The coplanar arrangement of four bonds from Pd(11) and Pt(11) has since been demonstrated in many crystals.
Binary compounds (chlorides, oxides, sulphides, etc.) are dealt with elsewhere; here we give examples of finite molecules and ions.
Pd(11) compounds
The structures of square planar mononuclear complexes have been established by diffraction studies of compounds such as K2PdC14, [Pd(NH3)4]C12. H 2 0 , and numerous complex cyanides (many of which are isostructural with the Ni and Pt analogues), for example, Ca[Pd(CN)4] . 5 H 2 0 and Na2 [Pd(CN)4] . 3 H20. Corn- pounds having points of special interest include the following. In [Pd(en)2]- [ ~ d ( e n ) ~ ~ ~ ~ ] ( ' ) both ions are square planar, and the S 2 0 3 ligand is bonded through S, (a). The dissolution of Pd in HN03 followed by treatment with NH3 does not give the expected [Pd(NH3)4] (N03)2 but the bright-yellow compound [Pd(NH3)3N02] [Pd(NH3)4] (NO3)4 which contains two kinds of planar ion in the proportion of 2 : I . ( ~ ) The n-propyl rnercaptide is h e ~ a r n e r i c , ( ~ ) like the Ni compound illustrated in Fig. 27.10, and the molecule (b) is ' b a ~ i n - s h a ~ e d ' . ( ~ ) In the
2,2'-dipyridyl imine complex (c),(') in which there is interference between the HX atoms, the bond arrangement around Pd remains square planar and the ligands distort, in contrast to the Ni methenato complex mentioned earlier (p. 968).
Palladium forms complexes such as the dithio-oxalate ion, Pd(N2S2H)2, glyoximes, and phthalocyanine which are structurally similar to the compounds of Ni and Pt, and also numerous bridged compounds. Early X-ray studies established the planar trans configuration of (d) and (e):
but there have been more recent studies of bridged compounds of Pt to which we refer shortly.
Group VIII and Other Transition Metals Examples of the rare compounds in which Pd(11) does not form square coplanar
bonds include [PdAlCl4(C6H6)] and [PdA12C17(C6H6)] which are formed when ( 6 ) JACS 970 92 289
AlC13, A1 metal, and PdClz are reacted together in boiling benzene.(6) The molecules have the structures sketched at (f) and (g). These molecules are notable
for the short Pd-Pd bonds, which are shorter than in the metal (2.75 A), and long Pd-C1 bonds.
Compounds o f Pt(11)
There is no evidence for the formation of more than four bonds (which are ( I ) JCS A 1969 485 coplanar) except in the cases noted on pp. 980 and 981. As in the case of Pt(1v)
(p. 983) the formulae of certain compounds suggest other coordination numbers, but structural studies have confirmed 4-coordination. In [ P t ( a c a ~ ) ~ C l ] K,(') (a), 5-coordination is avoided by coordination to C, and an X-ray study of the compound originally formulated as [Pt(NH3)4(CH3CN)2] C12 . H 2 0 shows that
'CH 3
(2) JINC 1962 24 801
this is not an example of 6-coordinated Pt(11) but is { P ~ ( N H ~ ) ~ [CH3 . C(NH2). NH] 2) C12 . H ~ o . ( ~ ) There is planar Ccoordination of the
metal by two NH3 and two acetamidine molecules, (b). (3) JINC 1962 24 791, 797;
The compounds of Ni, Pd, and Pt of the type M(diar~ine)~X, appear to behave *c 1517
in solution as uni-univalent electrolytes, indicating a close association of one X halogen atom with the metal, [M(diar~ine)~X] +. In the crystalline state(3) they , form very distorted octahedral units with Ni-As, 2.3 A, and Pd(Pt)-As, 2.4 A, but
M-X very much longer than a normal covalent bond. In P t ( d i a r ~ i n e ) ~ C l ~ the length of $e Pt-Cl bond is 4.16 A, suggesting C1- ions resting on the four CH3 groups projecting out of the equatorial plane of the two diarsine molecules. The shorter
( 7 )
M-I distances (3.2,3.4, and 3.5 A for the Ni, Pd, and Pt compounds) may be due to
considerable polarization of the I - ions; they are intermediate between the covalent x