D-orbital Splitting Research Articles

XANES OF MANGANESE COORDINATION COMPLEXES

XANES of Mn compounds of known structure and electronic properties [Mn (II), Mn (III), Mn (IV)] is reported. Preedge, Edge and Post-Edge zone are detailed. Dependence on oxidation state, geometry and nuclearity are discussed. Preedge structure is observed and compared with d-orbital splitting obtained from visible NIR spectroscopy. An important Edge shift has been observed with nuclearity and/or geometry change. Post-Edge differences due to stereochemistry are observed for the Mn (III) O6 chromophore.

Theoretical study of cis-trans isomerization in simple transition-metal complexes

PRDDO calculations on several simple octahedral transition-metal complexes are presented. Relative energies, optimized ge- ometries, and population analyses are discussed for a series of complexes of general formulas M(NH3)4(C0)2 and M(NH3)2(C0)4, M = Cro, Mn+, Fe2+, and Co. Back-bonding stabilizes the cis isomers of the chromium and manganese complexes, but not those of Fe2+ or Cos+. Similar calculations on Cr(CO)2(N2)4 and Cr(CO),(N,), also predict the cis isomers to be favored energetically and are consistent with available experimental evidence. The cis isomers of Cr(C0)2(PH3)4 and Cr(CO),(PH,), are both favored by less than 0.3 kcal/mol, even though the cis conformations of the corresponding amine complexes are strongly favored (6-13 kcal/mol). As reported previously, PH3 can act as a T acceptor even without d orbitals on phosphorus, the acceptor orbital being best described as P-H u*. The energetic manifestation of this effect is a reduction of the cis-trans isomerization energy. Finally, calculations on the conformational preferences of bis(pyridine)bis(acetylacetonato)metal complexes are discussed. The phenomenon of r bonding in organometallic compounds has been studied intensely from both experimental and theoretical perspectives. Some of the most obvious and important conse- quences of r-bonding effects are in the areas of site preferences, conformational stability, and relative bond lengths. A good ex- ample of how r bonding can influence site preference may be found in Hoffmann's work on transition-metal pentacoordination, where the site preferences of r-bonding ligands are detailed for trigonal-bipyramidal and square-pyramidal structures.' Con- formational preferences due to r bonding have been studied in a variety of systems.2 Experimental studies of the effects of r bonding on bond lengths abound in the literature, but some of the clearest results are found in chromium-carbonyl complexes with pho~phine,~ where a distinct trend of long metal-ligand bond lengths trans to strong .rr acceptors can be seen. Here, we present a theoretical study of conformational preferences and relative bond lengths in an isoelectronic and isostructural series of complexes of chromium, manganese, iron, and cobalt. Consider a (hypothetical) complex such as CT(NH~)~(CO)~. The pseudooctahedral coordination leads to the usual d-orbital splittings, with three occupied tp orbitals at low energy and two empty eg orbitals at higher energy (here and throughout this paper we employ octahedral symmetry notation for simplicity). Simple r-bonding arguments lead one to predict that this complex should exist in the cis form, since only that conformation allows r back-bonding to the carbonyls from all three tzg orbitals. Similar conclusions can be reached for systems such as Cr(NH3)4(C0)2 although here all three to orbitals are r bonding in both isomers. Nevertheless, the cis isomer reduces the number of mutually trans carbonyls and thus should be preferred. Indeed, in the absence of steric effects, mixed complexes of carbonyl and other non- or lesser-r-accepting ligands invariably assume the cis conformation. Examples are Cr(C0)2(PH3)4,3 Cr(C0)3(PH3)3,4 Cr(CO),(P- H3)2,' and MO(CO)~(PM~~)~,~ all of which exist in the cis form. AG for the cis-trans isomerization of the last compound is 0.32 kcal/mol, while more bulky phosphines are preferentially trans for steric reasom6 The effects of A bonding on the bond lengths of these and similar complexes are also clear. Thus, in ~is-cr(Co)~(PH~) ~, the Cr-P distance trans to CO is 2.34 A, while the corresponding distance trans to PH, is significantly shorter (2.28 A).3 In this paper, we study the geometric and energetic conse- quences of a bonding in a group of octahedral complexes of general

Molecular orbital calculations on transition metal complexes. Part XXIV. Bis-(π-pyridine)chromium and its cation

INDO SCF MO calculations have been carried out for the formally d 6 system bis-(π-pyridine)chromium, Cr(C 5H 5N) 2, and the corresponding cation. For the neutral molecule the results indicate that the introduction of the nitrogen heteroatom should lead to an ionisation potential about 1.5 eV greater than for the parent bis-(π-benzene) chromium, but as for Cr(C 6H 6) 2 the d-electron ionisation energies follow the ordering a 1(d z 2 ) < e 2(d x 2−y 2 , d xy), and the dominant contributions to the bonding again arise from interactions between the metal e 2(d x 2−y 2 , d xy) and e 1(d xz, d yz) levels and the π-orbitals of the ligand rings. Results for the cation, [Cr(C 5H 5N) 2] +, yield estimates for the one-electron d-orbital splitting parameters, ΔE 1 and ΔE 2, which are quite similar to those found for [Cr(C 6H 6) 2] +, and g ⊥ and A H values comparable to those of the bis-benzene system are expected for the 2A 1(e 2 4a 1) ground state, although the 14N hyperfine interaction should be negligible. Calculations are also made of all the proton hyperfine coupling constants for the experimentally accessible bis-(π-2,6-dimethylpyridine)chromium cation. The calculated splittings of the mainly 3d e 1 and e 2 levels, due to the heteroatom, are in all cases small (<3,000 cm −1), and in contrast to the bis-(π-borabenzene) metal, M(C 5H 5BR) 2, species, the bis-(π-pyridine) derivatives behave in a very similar manner to the homocyclic bis-(π-benzene) systems.

Application of the angular overlap model to the interpretation of the electronic spectra of some copper(II) and nickel(II) acetylacetonate complexes

The angular overlap model has been used to calculate σ- and π;-bonding parameters for three copper complexes and one nickel complex of the type M(acac)2(acac = the acetylacetonate anion or a closely related ligand). The results suggest that the orbital energy sequence in the copper complexes is probably dxydz2 > dx2–y2 > dxz∼dyz, but that in the nickel complex the stronger metal-ligand interaction causes the sequence dxydx2–y2 > dz2 > dxz∼dyz. The parameters estimated for the diamagnetic nickel complex can successfully be used to predict the d-orbital splittings observed for three paramagnetic nickel acetylacetonate complexes.

K-Absorption Spectral Investigations on Some Cobalt Complexes Involving Sulphur-Ligands

K-absorption edges of Cobalt in some complexes involving sulphur-ligands have been examined. The ligand-field splitting of 3d-orbitals is found to show up in the low energy region of the K-edge. These complexes have large 10 Dq values as estimated from electronic spectra. Edge-widths and edge-shifts have been measured and their correlation respectively to coordination stoichiometry and valency of the metal ion has been discussed. Splitting of the K-absorption maximum has been observed.

Ligand field splittings in non-cubic complexes V. Analysis of the d-d spectra of the gaseous dichlorides of the first transition series

The d-d spectra of gaseous VCl 2, CrCl 2, FeCl 2, CoCl 2, NiCl 2, and CuCl 2 are discussed in terms of ligand field theory. The Tanabe-Sugano matrices for linear (D ∞h) d 3, d 7, and d 8 systems are given and the d-orbital splitting parameters are determined for each molecule. The results can be satisfactorily rationalized by empirical MO calculations, based on the angular overlap model. Alternative assignments of the spectra of VCl 2, CoCl 2, and CuCl 2 are discussed in detail.

Ligand-field splittings in noncubic complexes. Part VII. Calcium copper(II) acetate hexahydrate

An angular overlap treatment with electrostatic corrections is applied to the d-orbital splitting in calcium copper(II) acetate hexahydrate CaCu(AcO)4,6H2O. All parameters used in the calculations are taken from previous results for tetragonal copper(II)–oxygen system. The relative energies of the d orbitals are predicted to be dxy > dx2-y2 > dz2 > dxz,yz and the calculated d–d transition energies are in excellent agreement with the observed spectrum. The effects of spin–orbit coupling and the binding of the distant oxygen ligands are discussed. It is concluded that these four distant atoms make relatively little contribution to the d-orbital splitting and their binding to the metal is largely electrostatic. The distortion of the four short-bonded oxygen atoms from square coplanarity also has a small effect on the d-orbital energies.

Ligand-field splittings in noncubic complexes. Part VI. Comparison of the crystal field and angular overlap treatments for eight-co-ordinate D 2d systems

The angular overlap method is applied to the d-orbital splitting in eight-co-ordinate complexes of D2d symmetry. The effects of varying bond angles and relative bond lengths are discussed, and the results compared with those obtained by other workers using the crystal-field model. The angular overlap model is shown to yield qualitatively similar results to the crystal-field treatment when proper account is taken of π-bonding. The two methods are compared critically, and it is concluded that the angular overlap model is preferable since it takes explicit account of σ- and π-covalence and gives apparently reasonable results without much additional labour. The choice of parameters in the angular overlap model is less arbitrary, and the crystal-field model may exaggerate the effects of relatively distant ligands on the d-orbitals.

Thermal dehydration of some 8-quinolinol chelate hydrates

The thermal dehydration of the 8-quinolinol chelate hydrates of Ni(II), Co(II), Cu(II), Cd(II), Mn(II), Mg(II), and Zn(II) was studied by thermogravimetry, differential scanning calorimetry, h.t.r.s. and d.r.s. Calorimetric data indicated that the heats of dehydration were correlatable with crystal field and electrostatic theory. The thermogravimetric data, applied in a different type of solid state kinetics equation, indicated that there was a difference in type of water bonding between that in the magnesium(II) complex and the other complexes. Changes in reflectance spectra upon dehydration were ascribed to possible changes in coordination for the complexes of Co(II), Cu(II), and Ni(II), involving the d-orbital splittings, and to changes in coordination affecting charge transfer in the complexes of Mg(II), Zn(II), and Cd(II).

The splitting of d-orbitals in less symmetrical molecules containing a cyclopentadienyl ligand

The splitting of d orbitals in an electrostatic field due to a cyclopentadienyl ring and four ligands arranged at the corners of a square is investigated. It is shown that, although the symmetry of the molecule is Cs, the effective symmetry for the d1 and d2 systems is G4v. The linear combinations for the d2 system in C1v symmetry are given and the matrix elements for the secular determinant are listed. The results may be applied to a discussion of the bonding in tricarbonyl-π-cyclopentadienylethyl-molybdenum, whose structure is related to the idealized model considered.