There are two older reviews that summarize the development of MCD spectroscopy and give examples of the early applications before the mid-1970 s.1,2 Several less comprehensive reviews have appeared more recently and should be consulted for newer applications and examples.3–6By far the most important monograph which deals with the theoretical and symmetry aspects of MCD spectroscopy is the work by Piepho and Schatz.7 This monograph contains an extensive treatment of high symmetry coefficients and the application of the Wigner-Eckart theorem to MCD theory. The text also contains a number of detailed examples. Recent applications of MCD spectroscopy have ranged from investigations of metalloenzymes and metalloproteins in bio- inorganic chemistry to interpreting complicated 4f metal localized transitions in a variety of lanthanide compounds. Forexample, Oganesyan and Thomson,9,10 Neese and Solomon,11 and Pavel and Solomon5have developed models which demonstrate the use of temperature dependent C terms and field saturation studies to probe the spin states in various biomolecular systems.
Mack and Stillman12–17have investigated a number of metal phthalocyanine and metal porphyrin systems and analyzed the absorption and MCD spectra simultaneously by means of a curve fitting algorithm. There have been various studies of lanthanide compounds,18–23 but among the more interesting ones has been the interpretation of spectra for lutetium bisphthalocyanine, Lu(Pc)2, a sandwich complex which exhibits intervalence transitions.24,25MCD spectroscopy has even been employed recently to study the plasmon resonance in colloidal gold nanoparticles.26
Several examples described below from the author’s work27–34are illustrative, but by no means comprehensive. However, they were chosen to convey the flavor of using MCD spectroscopy in order to study electronic transitions and structure in several areas of coordination chemistry ranging from simple complexes to cluster complexes of heavy metals.
2.25.5.2 Atomic MercuryVapor
Although atomic Hg vapor27 is not a coordination compound, the MCD of Hg vapor illustrates the relationship between the atomic Zeeman effect and MCD spectral measurement. Figure 4
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shows the MCD and absorption spectra for Hg vapor in the region of the 1S0(6s2)!3P1(6s6p) atomic transition (253.65 nm) at different field strength. The strong spin-orbit coupling of the Hg atom provides significant singlet-triplet intermixing with the 1S0!1P0 transition, which is located to higher energy (184.957 nm) thereby giving the band at 253.65 nm substantial intensity.
At 0.44 T the MCD spectrum exhibits a very strong positive A term, while the absorption band broadens slightly. The band in the vapor state is very narrow being an atomic transition, so that at highermagnetic fields the absorption band is split into two resolved Zeeman components. At 6.2 T the excited atomic states are no longer degenerate. The absorption reveals two separate Zeeman transitions and theAterm in the MCD now becomes two separateBterms of opposite sign.
This example illustrates clearly the relationship between a degenerate stateA term at low field and the pairof nondegenerate stateBterms when the degeneracy is lifted by the stronger field.
2.25.5.3 BiBr63- Ion
Figure 5presents the absorption and MCD spectra for the octahedral BiBr63ion in acetonitrile solution.28The MCD spectrum shows twoAterms for intense presumably dipole allowed bands.
The lowerenergy positiveAterm is centered at 2.60mm1 (1mm1ẳ104cm1), while the higher 103∆A
0
Figure 4 Electronic absorption (lower curves) and MCD (upper curves) spectra for Hg vapor at different magnetic field strength.28
2 104
0
–
1 104
Figure 5 Electronic absorption (lower curve) and MCD (upper curve) spectra for [Bun4N][BiBr4] in acetonitrile containing a 50-fold excess of Bun4NBrwhich produces the BiBr63ion in solution.28
energy broad negative A term is at 3.7mm1. The former band is assigned as the Bi3þ metal- centered 1S0(6s2)!3P1(6s6p) transition analogous to that for Hg vapor, while the latter is assigned to a ligand-to-metal charge transfer (LMCT) transition from occupied Br 4p orbitals to the LUMO 6pBi3þorbitals. The absorption and the MCD terms for these solution spectra are clearly much broader than for the Hg vapor case and are in keeping with the general character- istics of solution bandwidths (typically 103cm1). The negativeAterm for the BiBr63ion allows a choice between two LMCT transitions to allowed T1u(1T1u) excited states, one each from the t2g5t1uandt1g5t1uexcited configurations, respectively. The assignment is to the LMCT transition
1A1g!T1u(1T1u, t2g Br!t1u Bi3þ), which predicts a negative A term. The alternative assign- ment to the LMCT transition 1A1g!T1u(1T1u, t1g Br!t1u Bi3þ) can be ruled out because it predicts a positiveAterm, which is inconsistent with the observed spectrum.
2.25.5.4 M(Et-Xan)2(M=NiII, PdII, and PtII; Et-Xan=C2H5OCS2-)
The ethyl xanthato ligand forms bis chelate square complexes with NiII, PdII, and PtII.29 These complexes are characterized by intense UV–vis spectra which contain a number of bands.
The absorption and MCD spectra in acetonitrile solution are shown in Figure 6. Because the symmetry is low (approximately D2h) the MCD features are allBterms. As can be seen from the figure the MCD spectra presents more individual features than the absorption spectra and, therefore, contains potentially more information. The assignment of these spectra was made in terms of LMCT transitions involving the S donorlone pairs to the empty d* (dx2y2) orbital.
At high energy, transitions that are believed to have metal-centered 5d ! 6p character were identified forthe PtIIand NiIIcomplex, with a possibility forthe PdIIcomplex as well. The MCD forallowed metal localized transitions are often more intense than LMCT orligand-localized transitions.
2.25.5.5 Pt(P(But)3)2
The Pt(P(But)3)2complex30,31 is a lineartwo-coordinate Pt0complex with a 5d10 metal electron configuration. This complex exhibits a rich spectrum in the 2.4–5.0mm1region.Figure 7 shows the absorption and MCD in 2-methylpentane solution at 295 and 80 K (glass). There are three clearpositive Aterms which are ascribed to metal-to-ligand charge transfer (MLCT) transitions to u states, which possess some 5d ! 6p metal-centered character. The MLCT character is supported by a large blue shift in the spectra for the isoelectronic and isostructural Au(P(But)3)2þ complex ion. A careful consideration of the strong spin-orbit coupling for Pt0and AuIshows that the excited MLCT/d ! p states are spread over 1.0–1.5mm1. The detailed assignment of the spectra lead to the conclusion that the predominantly 5d MOs of the complex are very close in energy with ordering 2gþ(dz2)g(dx2y2,dxy)> g(dxz,dyz). One of these, g(dx2y2,dxy), is non- bonding by symmetry, so that the 5d orbitals must have minimal involvement in metal–ligand bonding, even though they are very active spectroscopically.
2.25.5.6 Pt(AuPPh3)82+ and Au(AuPPh3)83+ Cluster Complexes
The measurement of MCD spectra as a companion to absorption spectra can often reveal more spectral features because of the bisignate nature of the MCD. This is illustrated nicely by the spectra obtained for the Pt(AuPPh3)82þand Au(AuPPh3)83þclustercomplexes embedded in thin poly(methyl methacrylate) plastic films shown in Figure 8.32–34 These complexes have metal- centered crown (D4d) and centered icosahedral fragment (D2h) structures, respectively.
The absorption spectra show a number of features, but the MCD spectra reveal an even greater complexity. The transitions are assigned to intraframework metal–metal transitions among MOs constructed from the peripheral Au 6svalence orbitals. The low-energy portion of the MCD for the Pt(AuPPh3)82þ complex is more intense and differs significantly from that of the Au(Au(PPh3)83þcomplex and is attributed to additional transitions from the occupied 5dorbitals on the center Pt to the gold-based framework LUMOs.
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