Michael R. Rosenthal Bard College ~nnondo~e-on-~udson. New York 12504
I I
I
The Myth of the Non-Coordinating
Advances in theoretical chemistry and a great accumulation of physical and chemical data on coordination complexes have led to a more sophisticated understanding of the structures of these compounds. However, misconceptions are still present in regard to what are referred to as "non-coordinating" or "poorlycoordinating" anions. Certain ions which fit these descriptions in the presence of water are found to he coordinating where water has been rigorously excluded. The use of dehydrating agents such as 2,2-dimethoxypropane or triethylorthoformate allow anhydrous complexes to he prepared that were previously unobtainable, by allowing the preparation of a solution of anhydrous salt to which ligand is then added. This paper is concerned with the chemistry of such anhydrous complexes, studied in the absence of water. The ability to coordinate is a property that is difficult to define. Whether an anion coordinates or not may depend on one or many of a number of terms within an energy cycle or upon kinetic criteria. Detailed analyses are difficult to carry out and have rarely been done. The ligand field splitting, 10 Dq, is not in itself an adequate measure of coordinating ability. It reflects many contributions, among them sigma and pi bonding, spinorbit coupling, and distortions. An illustration of the inadeauacv of usinc 10 Da involves the comnlexes of the f i n t &ansition mecal seriks with pyridine(&). Although ~ v r i d i n ehas a 10 Do value about that of ammonia. it is ex.. tremely difficult to bhtain Mpyen+ (1, 2). One obtains instead species such as Nipyr(ClO4)z even though the 10 Dq value for C 1 0 4 is much lower than that of pyridine. This fact may he due to steric effects, kinetic harriers, the difficulty of stabilizing the large cation, or some other reason. This problem is not found with the ligands ethylenediamine(en), pyrazole, or imidazole. The use of pK. values has not been satisfactory in correlating coordinating ability. This is not surprising when one recognizes the differences in size, polarizahility, charge, and available orbitals between hydrogen ions and transition metal ions. The ability of an anion to coordinate will be determined in part by the competition it receives from the solvent and other bases present, by steric restrictions, and by activation enerw limitations. as the svstem seeks a minimum free energy. Synthesis ban be aibed by adjusting experimental conditions if the pertinent factor can be identified. A coordinated anion is one which more closely approaches the metal ion than a non-coordinated anion. This will affect the metal ion in such a way that its magnetic moment and spectral properties will be significantly different from what they would he if the anion were not coordinated to it. The infrared and nuclear magnetic resonance spectra of the anion itself will often reflect coordination, as will conductance of the complex in solution. Most important, metal-ligand interatomic distances as established by diffraction will shorten upon coordination. The most common means of establishing coordination involves vibrational spectroscopy, in which changes in the symmetry of the anion are reflected in the number and intensity of its fundamental vibrations. However, some horderline cases yield ambiguous vibrational results, some
anions do not change symmetry upon coordination, and scectral splittinas mav he due to chances in crvstalline siw syrnmetrv (2 4 1 . i'nlarieed Haman s~ectrosco;)yoffers mure informarion, but the equipment is expensive. Sinyle crvstal X-rav diffraction nffcrs the onlv de1initk.e answer. but this technique is time-consuming, and requires specialized training and expensive apparatus. Anions have also been found to coordinate through hydrogen-bonding interactions to other ligands in the com(5) plex, affecting the stability. In N i ( i m i d a z ~ l e ) ~ ( N O ~)~ the imidazole N-H hydrogen-bonds to a nitrate oxygen. Other examples include Ni(pyrazo1e)rBrz ( 6 ) , Niens(SOdz(7). and Mg(HzOMNOdz(8). An excellent paper has been written (9) discussing the factors involved in anion coordination versus ligand coordination in the Cu(II-substituted pyrazole system. This paper and the references within i t are excellent sources of studies involving coordination chemistry of "weakly-coordinating" anions. Perchlorate Ian and Tetrafluorabarate Ion Perchlorate ion, Clod-, is a readily available anion which is rarely coordinated when its complexes are prepared in aqueous solution. The earliest well-characterized examples of coordinated perchlorate were found in the early 1960's (10, I I ) , where coordination was detected by changes in the infrared spectrum of C 1 0 4 as the symmetry is lowered from Td to C3,,to CZ" (Fig. 1). These symmetry changes produce significant changes in the infrared spectrum of perchlorate (Table 1).The complexes NiLe(Cl04)z, NiL4(C104)z, and NiLz(C104)z, where L is CH3CN, have been shown (12) to contain ionic, unidentate, and bidentate perchlorate, respectively, by infrared spectroscopy in conjunction with other physical methods, and the sixcoordinate cations were thus characterized. This method was more definitive than earlier conductance (13) and magnetic (14) methods, hut was found to he sometimes misleading. Both C 0 ( P h z m e A s O ) ~ ( C 1 0 ~ ) ~ (15) and C~(2,2'-bipyridyl)~(ClO4)z(16) were postulated by infrared studies to contain one ionic and one unidentate perchlorate. Later work confirmed the former assignment (17) hut left considerable doubt about the latter (12) \--I.
Several cases of unidentate perchlorate have been verified by X-ray diffraction (19 and references therein) as
M---O-C]-O--.M
10 bridging-C,. Figure 1. Modes of perchlorate bonding.
Volume 50, Number 5, May
1973 / 331
Table 1. Vibrations of the Perchlorate Grow Td"ionic
C 3 2 unidentate
Cz.b
hidentate
. .
Figure 3. mull)
'Frequencies are in cm-' and taken from ref. (10). Frequencies are in cm-I and taken from ref. (32)for compounds studied therein. Not observed-outside range of instrument used. "Infrared inactive-all other frequencies are infrared active. All frequencies are Raman active.
.
.
me me me me me me Trimethyltinperchlarate, ( m e ) & n ( ~ i ~(infrared ,) spectrum on
non-coordination is illustrated by the fact that Cu(pyO)r(BF& has heen shown (28) to contain ionic anions a t a Cu-F distance of 3.34 A. I t is tempting to invoke steric effects as an explanation. The tetrafluoroborate anion is similar in its coordinating properties to perchlorate, though it seems slightly less likely to coordinate and its complexes are more easily affected by water, which displaces coordinated tetrafluoroborate. Analogous symmetry changes are expected but few examples are found in the literature. Complexes of pyridine (1, 2, 29) and mesSn(BF4) (30, 31) contain coordinated tetrafluoroborate. A very unusual bit of chemistry involving C104- and BF4- is found when Nipy4(C104)z is suspended in chloroform (it is not soluble) or when Nipy,(BF& is suspended in chloroform or dichloromethane (29). The characteristic blue color of the comnlex disannears and a vellow compound replaces it. The yellow compound in the perchlorate case analvzes for N ~ D V ~ C I O ~ ) ~ . Cand H C uDon I~. setting in moist air changes to blue N ~ ~ Y ~ ( H ~ o ) ~ ( c I o ~ ) ~ . Similar behavior is found for the B F 4 complex. It appears that the solvent shifts the delicate balance between coordinated and uncoordinated anions, possibly by a hy- . drogen-bonding interaction with the anion, similar to those interactions found in pyrazole complexes previously discussed. What is remarkable is that no obvious dissolution occurs-the reaction takes place upon the solid com~ l e xin sus~ension.Such delicate balance involvine ClOnis not unusual for Ni(II) complexes. It has been observed (32) that perchlorate com~lexesform with substituted ethylenediamines only when two or more of the cmrdinated amines contain monomethyl suhstituents. When the N-alkyl groups are larger than methyl, heating of NiL2(H20)2(C104)2 to yield NiLz(C104)~produces a complex that reverted hack to the orieinal hvdrate uDon cmline. A further testimony to t i i s deiicate haiance liesln the studs of Co(diars)~(C104)~ (24) (diars = o-~henylenebisdi. . methylarsine). The compound can he prepared in two fonn-a monoclinic form in which perchlorate is unidentate and the infrared splitting is clearly observable, and a second orthorhombic firm in~whichthe infrared snecfsum implies tetrahedral coordination of the anion, and in which the authors postulate weak coordination, if any a t all. The phenomenon is referred to as "ionic-covalent isomerism." A Ni(I1) complex was also prepared which was isomorphic with the monoclinic form. The infrared spectrum, however, did not indicate perchlorate coordination at room temperature, but did so a t liquid nitrogen temperature. The magnetic moment was 0 a t room temperature. The implication that the M(II)-anion bonding is weaker for Ni than Co is rationalized on the basis of Ni(I1) having more electron density in the dz2 orbital in these spin-paired complexes.
..
Figure 2. Sodium perchlorate-bis[N.N' - elhylenebis(sa1icylideneiminatolcopperlll)],omitting hydrogen atoms (X-ray on single crystal).
well as a rather exotic case of bidentate perchlorate (20), in which the Na+ is essentiallv octahedrallv surrounded t ~ yta,o oxygen atoms from t h e perchlorate and two oxygen atoms from each of the Schifi base units (Fir. 2 1 . The problem of identifying bridging perchlorate has proven difficult to solve with vibrational spectroscopy, since the point group of bridging and bidentate perchlorate is identical. An unusual case of bridging perchlorate in aqueous solution has been Dostdated (21) as beine presebt during reactions of fi:amido-fihydroxy dicobalt(11fi com~lexes.A brideine oerchlorate has been nrooosed (22) . . . . in m e s ~ n ( ~ 1 0 4I;la&g ). tin in a five-coordinate trigonal hipyrimidal environment (Fig. 3). A further illustration of the ambiguity in the simple vibrational method is found in complexes of MZ+ and meaPO or meaAs0, MLdC101)+ (23). Although X-ray powder patterns indicate the similarity of these complexes to MLs2+ and thus i m ~ l vcoordinated nerchlorate. the infrared spectra do not 'iidicate the expected splitting. I t was later shown (24) that differing degrees of coordination produce varying effects on the vibrational spectrum. X-ray diffraction is the only reliable probe in such a case. Weak coordination of this sort has been found in several complexes of Cu(I1) with perchlorate and tetrafluoroborate anions, and has been named "semi-coordination" (25). X-ray studies of Cuem(BF4)z (26) have shown small but significant reductions in anion symmetry and a Cu-F distance of 2.56 A. The authors feel that this coordination is stronger than non-bonding but weaker than full coordination, hence "semi-coordination." This phenomenon has been noted in a number of other Cu(I1) complexes (16, 27). The delicate balance between semicoordination and 332 /Journal of Chemical Education
Nitrate Ion The nitrate ion, in spite of its low position in the spectrochemical series near fluoride (33), has been found to coordinate in a variety of environments (Figs. 4, 5). In the late 1950's Gatehouse and his co-workers pointed out (34) that coordination of nitrate lowered its symmetry from Dan to Czy, resulting in the splitting illustrated in Table 2. It should he noted that unidentate nitrate may also he of C , symmetry if the coordination is nonlinear.
Table 2. Vibratiwns of the Nitrate Group
C w bbidentate or bridging
Dm0 ionic
unidentate
ul(A1')* P uz(Azf')**
1050 831
udE')D
1390
ur(E') D
720
bidentate Figure 4. Modes of nitrate bonding.
'Frequencies are in c m l and taken from ref. (37). bFreouenciesare in em-' and taken from ref. (40).
C, Figure 5 . Modes of bridging nitrate banding.
No attempt to distinguish between unidentate and bidentate nitrate was made in the early literature, hut attempts were made to correlate ~ 4 . ~with 1 covalency (35, 36). Anhydrous transition-metal nitrates (37) and tetranitratometallates (36, 38) were studied. Workers studying meaSn(NO3) pointed out (22) that unlike perchlorate, nitrate symmetry is identical (Cz,) in unidentate and bidentate modes. They attempted to identify the mode by comparison to spectra of known structures, and postulated unidentate coordination. One point of view (39) supported the notion. in analoev to carbonate ion. that laree values of u4-u1 were indicati"ons of bidentate nitrate. A more detailed analvsis suoported this view (40) and postulated a a study of bidentate structu& for Sn(NO&. mixed amine complexes containing nitrate which was known to be bidentate indicated (41) the dangers of this approach, for these complexes did not possess the high energv -.un- value oredicted. Further complexities were brought forth when an X-ray diffraction studv of (menP0)2Cn(NOdz indicated (42, 43) symmetric. bideitate nitrate, but possessed an electronic spectrum and magnetism typical of tetrahedral Cn(I1). Similar results were obtained for CO(NO.?)~~(36, 38, 44, 45) where the X-ray study indicated asymmetric bidentate nitrates. The nhenomenon was interoreted as arisine from a distorted qiasi-tetrahedral complex with honds d c rected towards the center of the nitrate erouDs. Neither complex possessed the high energy u 4 vibration. Some studies, such as one performed upon some pure amine complexes (46) correlated infrared and other evidence with coordination mode quite well, hut in general, vibrational correlations often lead to faultv conclusions as to the mode of nitrate coordination, especially when com~ o u n d cs o m ~ a r e ddiffer sienificantlv from one another. A normai coordinate &ialysis on the nitrate ion (47) pointed out that the highest value expected for u 4 in unidentate nitrate and the lowest value expected for u4 in hidentate nitrate would overlap. However, the highest frequency absorption is polarized in bidentate or bridging and depolarized in unidentate nitrate nitrate (u,_,) (u~o...~~,,~,,,). A use of these facts to studv anhvdrous "*. ~~~~~~.~~ metal nitrates (48) found their postulated strictured to be consistent with X-rav data. The authors warned that coupling occurs as one moves from unidentate to bidentate nitrate, invalidating the use of simple correlation schemes as was the custom. A proposal (49) that bidentate nitrate should have two M - 0 stretching vibrations in the fa-infrared while uni-
ow ever,.
~
dentate has only one was subsequently (50) shown to be invalid. Even if a simple qualitative method were found to differentiate unidentate and bidentate nitrate, the fact remains that both modes show Cz, and C, symmetry (Fig. 4) and that bidentate nitrate may mimic a complex of higher symmetry in its other molecular properties. Bridging nitrate also exists in several forms (51) as illustrated in Figure 5. The complexity of this problem can be illustrated by considering the pyridine-nitrate complexes. The following complexes have been prepared; NipydN03)zr COPY.?(NO& (1, 2, 52); ZnpydNOdt, ZnpydN03)2, Cdpya(NO&, C d z p ~ d N O d r ,Hgpyz(NOa)z (53); C U P Y Z ( N O ~ Z , Cupy4(N03)z (54). The authors have attempted to mterpret the infrared spectra of these complexes along with mametic. electronic soectral. stoichiometrical. and other dat;? in t&ms of unidentate, bidentate, or bridging nitrate groups. .. - Such assignments are tentative a t best and futile a t worst. came& and his co-workers have recently undertaken a comprehensive X-ray diffraction study of these complexes. They have found (55) that Znpys(N0a)z has a structure that never could have been deduced from infrared spectra. The Zn(I1) ion is surrounded by two equivalent asymmetrically bidentate nitrates and three pyridines. There are four short honds in a square plane consisting of an oxygen atom from each of the nitrates and two pyridine nitrogens, with a pyridine nitrogen above the plane and two long-bonded oxygens below it, forming the sixth point of a quasi-octahedron. They have found (.56), by utilizing a single crystal X-ray analysis, that Copy3(N03)2 is isomorphous, and that Znpyz(N03)z is pseudo-tetrahedral with unidentate nitrate. A related compound, (alphapicoline)zCu(N03)2, is found (57, 58) in two crystalline modifications, one with symmetric bidentate nitrates and one with equivalent asymmetric bidentate nitrates. This compound was previously formulated (42) as bidentate, hut the anion orientation could not have been predicted from infrared spectra. Interestingly, when alpha-picoline is replaced by pyridine, the removal of steric hindrance allows dimerization to occur (59) and the com~ound [Cupyz(NO3)~1~pyis obtained-a centrosymmetric dimer with bridging and asymmetric bidentate nitrate groups both X-ray diffraction studies have been carried out on tetranitratometallates (60). Though the infrared spectraof the Mn(II), Co(II), Ni(II), and Cu(II), complexes are very similar (36), the structures are not identical. The Mn(II), Co(II), and a Zn(I1) complex are isomorphous while the Cu(I1) complex is different. The Ni(I1) structure is not conclusive. The strong tendency of Ni(I1) to six-coordination implies some combination of coordinating nitrates. The similarly intriguing Nipyz(N03)~has not yet yielded its secret to X-ray diffraction (52, 56). A recent review (51) discussed in some detail the factors Volume 50, Number 5, May 1973 / 333
influencing nitrate coordination, and a large numher of verified structures are tabulated therein. It would appear a t this point in time that infrared spectroscopy is of only limited value in determining the mode of nitrate coordination, and that X-ray diffraction is necessary to clearly solve such problems. Other Anions
Many other anions are capable of acting as ligands. Some of these, though occupying relatively low positions in the spectrochemical series are difficult to displace, even by ligands of much higher 10 Dq. The reasons for such hehavior lie within the free energy of formation cycle-ne must consider solvation energies, lattice energies, entropy factors, and kinetic factors as well. Other complications include steric factors, non-bonded interactions, and hydrogen-bonding. Complete understanding of these phenomena is elusive. It was mentioned earlier that it is difficult to obtain hexakispyridine complexes of first row transition metal ions (I, 2). Some authors consider this a steric problem, hut other hexakis complexes are known with large ligands. The stabilization of Fepye2+ by the very large anion, Fe4(C0)13~- (61), suggests that the problem may lie with the difficulty of stabilizing the crystal lattice. The problem is complicated in this case by the need to exclude water rigorously, since it replaces pyridines on the z-axis easily, in spite of HzO's low 10 Dq value relative to pyridine. The salts of the very weakly coordinating anions are barely soluble or are insoluble in the non-aqueous solvents thus necessary in synthesis. The compound Nipy6(PF6)~has been recently reported (62) in which infrared spectral evidence indicated that the PF6- was ionic and thus the Nipysz+ cation was obtained. This compound was prepared in aqueous solution followed by five days of drying in vacuo. It may he that the PF6anion is just weak enough a coordinator, relative to pyridine, to allow the Nipye2+ cation to he stabilized in a solid lattice. An X-ray diffraction study of this compound would he illuminating. The authors also prepared Nipy4(PFs)z and Ni(4-mepy)r(PF& in which the PF6- anion was found to be ionic, as well as Cupyn(PFs)~,a compound in which the P F s is found to he semi-coordinated. A numher of common anions have been known for many years to coordinate with some ease, often in aqueous solution. These include carbonate and bicarbonate (63-66), phosphate (631, sulfate (63), acetate (63, 65). chromate (67). sulfite (631, oxalate (65, 68), and nitrite (63). All of the above anions are oxygen donors except nitrite ion, which may also function as a nitrogen donor. More esoteric examples have appeared recently, furthering the notion that most any anion is coordinating under the proper conditions. The perrhenate ion, Reon-, has been shown to have coordinating properties approximately equal to that of perchlorate (69-71). Infrared spectra have been interpreted in terms of unidentate coordination as the symmetry is lowered from Td to (23". NO X-ray data have been reported and no hidentate coordination has been suggested. Infrared evidence has been presented (31) to show that the anions in meaSn(AsF6) and meaSn(ShF6) are coordinated to tin. The authors interpret the A s F o as being hidentate bridging in a trans manner while the S h F 6 is thought to he bidentate bridging in a cis manner. It seems that any fluoro-anion is a possible coordinating anion. Evidence has been found for coordinated SiFe2- (72) in Nipy4(SiF6) and semi-coordinated PFG- has been previously mentioned (62). Complexes of Ni(I1) with a tetradentate Schiff base amine in which the anion is B H c have been interpreted in terms of coordinated horohvdride ion (73). . . Bv. comoarison of the infrared spectra o f t h e complexes to spectra of hydrogen-bridged horohydrides of beryllium and alumi334
/ Journal of Chemical Education
--.-, Flgure 6 . R h [ P ( O C H d 3 J 2 BPhl (X-ray on single crystal)
num and to spectra of ionic horohydrides such as NaBH4, it was concluded that the complexes contained bidentate horohydride ion. If there is any doubt whether any anion exists that is tmlv noncoordinatine. consider an anion lone used to ~minimize anion coordkation to achieve full ligand coordination, tetra~henvlhorate.The tetranhenvlhorate ion has been found ti coordinate through its pi Hystem to metal ions (Fig. 6) on the basis of an X-ray diffraction study (74) and by an extensive infrared and nuclear magnetic resonance spectroscopy study (75). I t is clear that the notion of the non-coordinating anion should he put to rest alongside the notion of the non-coordinatiug solvent (76-78).
-
~~
~~~
Literature Cited Rosenthal. M . R.. andDrspo. R. S..lnorg C h e m . 4.840119651. Herlocker. D . W.. and Rorenthal. M . R.. lnora. Chim. Aclo, 4.501 11910l. R o s . S. D.. Specbochim. Aclo, IR. 221 11962). E..lnorp. Chem.. 3. ,134 119641. Griffith~.J. R.,andIrish. Santoro. A . Mirhell. A. D.. Zocchi. M . . and Rolmann. C. W.. A m C ~ y r f . .B25. 812(19691. (6) Mishell. A.D., Reimann. C. W.. and Ssntoro. A..Aelo C w t . B25.595(L969). (7) Mazhar-UI-Haque. Caurhlin. C. N.. and Emerson. K.. Inor
