Gerard R. Dobson

Octahedral metal carbonyls. XXI. Carbonyl and metal-carbon stretching spectra of monosubstituted group VIB metal carbonyls☆☆☆

Abstract The infrared spectra in solution of thirty-seven complexes of the type LM(CO) 5 (L = Lewis base; M = Cr. Mo, W) have been recorded in the regions 2100–1800 and 700–300 cm −1 . It has been found that the position of the metal-carbon stretching absorption (E-mode) is determined by the identity of the Lewis base donor atom, being lowest for “hard” bases. Within a series of deviatives of ligands containing the same donor atom, however, the expected “reciprocal relationship” between νCO and νMC is observed. The positions of νMC correlate well with complex reactivity via rate-determining dissociation of CO. The results are examined on the basis of current bonding theories.

Solid state structure and reactivity in solution. The crystal and molecular structures of (OC)4W(t-BuS(CH2)nS-t-Bu) (n = 3, 2)

Abstract The crystal and molecular structures of the compounds having composition (OC)4W(t-BuS(CH2)nS-t-Bu) (n = 3,2) were determined from single crystal X-ray diffraction data collected with a computer controlled diffractometer, using Mo-Kα radiation. The data that follow are given in the order n = 3 first, followed by the relevant data for the derivative with n = 2. Space group, P212121 and cell constants: a 9.388(4), b 9.548(2), c 21.989(11) A; D(calc) 1.74 g cm−3 and Z 4 molecules per unit cell. Space group C2/c and cell constants: a 25.568(9), b 8.958(3), c 16.457(2) A and β 95.810(13)°; D(calc) 1.78g cm−3 and Z 8 molecules per unit cell. Both structures were solved by Patterson methods using data corrected for absorption. Final refinements converged to R(F) factors of 4.6 and 6.3%, respectively, for (n = 3 and 2). The W—S distances when n = 3 are 2.574(5) 3.284(7) A. When n = 2, W—S distances are 2.559(5) and 2.565(4) A, the S—W—S angle is 80.8(1)° and the non-bonded S…S distance is 3.323(6) A. Both molecules contain octahedrally distorted WC4S2 fragments which are quite similar in their geometrical details. The overall WC4S2 fragments are normal and their structural characteristics compare well with established values. Comparisons between the structural data and reactivity via ligand exchange (replacement of the bidentate ligand) suggest that observed small but statistically significant differences in molecular geometries for the two complexes exert an appreciable cumulative effect upon reactivity.

Octahedral metal carbonyls. LXXII, Volumes of activation for solvent displacement in (solvent)M(CO)5 transients (M = Cr, Mo, W) generated from M(CO)6 via pulsed laser flash photolysis

Pulsed laser flash photolysis of M(CO)6 (M = Cr, Mo, W) in aliphatic and aromatic hydrocarbon solutions (solvent = n-C7H16 and C6H5X (X = F, Cl, H, CH3)) produces (solvent)M(CO)5 transients, which then react with Lewis bases (L; 1-hexene, piperidine, 2-picoline, pyridine) to afford LM(CO)5 products. The solvent-displacement process has been studied at variable pressures to 150 MPa, and volumes of activation for this process have been determined. These volumes of activation are sensitive to the identity of the coordinated solvent and suggest that bonding of chlorobenzene to Cr takes place via a lone pair of electrons on Cl, but through an isolated olefinic linkage for benzene, fluorobenzene and toluene. Observed volumes of activation also are consistent with the accessibility of an interchange mechanism for solvent displacement of n-heptane and with a dissociative solvent displacement mechanism for the arenes and support an increasing contribution from the solvent interchange process vs. reversible solvent dissociation in the order of the increasing size of the metal atom, Cr<Mo ⋍ W. The observed ability of “[M(CO)5]” to discriminate among various nucleophiles arises in part from the accessibility of these two competitive mechanisms.

OCTAHEDRAL METAL CARBONYLS: XXVI Carbonyl Stretching Spectra and Force Constants for cis-Disubstituted Derivatives of the Group VI-B Metal Carbonyls

Abstract The carbonyl stretching spectra (CS2 solution) of twenty-six cis-L2M(CO)4 complexes (L 2=bidentate chelating ligand bonding through N, P, As or S donor atoms; M=Cr, Mo, W) have been obtained. Two carbonyl stretching force constants and three CO-CO stretch-stretch interaction constants have been determined from the four carbonyl stretching frequencies through an iterative procedure: no relationships among the interaction constants have been assumed. The results are in excellent agreement with force constant data previously derived through isotopic enrichment studies. An approach to the separation of π-bonding and Fenske-DeKock “direct donation” effects in these systems is presented.

Solid state structure and reactivity in solution. 8. Implications of the structure of [1,2-bis(diphenylphosphino)ethane] tetra-carbonylmolybdenum(0) to the chelate effect

Abstract The structure of (diphos)Mo(CO)4 (diphos=1,2- bis(diphenylphosphino)ethane) has been determined by single crystal X-ray crystallography. The crystals are orthorhombic with a=22.600(8), b=16.799(4) and c=14.587(4) A, space group Pbca, Dcalc.=1.45 g cm−3 and Z=8. The structure was solved by the Patterson technique and refined by full-matrix least- squares methods to R=0.044 using 2206 reflections measured on a four-circle diffractometer. The coordination about the central molybdenum atom is a distorted octahedron, the greatest deviation from ideal geometry being the P1-Mo-P2 angle (80.2(1)°). Other distortions are induced through steric interaction between the phenyl and the carbonyl groups. The structure of (diphos)Mo(CO)4 is compared to that previously reported for cis- (MePh2P)2Mo(CO)4; the two molecules differ only that two methyl groups of the latter complex are replaced by the CH2CH2 backbone in the former. The influence of this difference is discussed both in terms of molecular distortions induced by the presence of the chelate ring and their influence on reactivity via chelate ring-opening and -closure.

Octahedral metal carbonyls—XXIII Metal-carbon stretching spectra for disubstituted derivatives of the group VI-B metal carbonyls☆

Abstract The i.r. spectra in solution in the region 700-300 cm−1 for 26 complexes of the type cis-(bidentate)M(CO)4 (bidentate = bidentate chelating ligand bonding through N, P, As or S donor atoms; M = Cr, Mo, W) have been obtained. The metal-carbon stretching absorptions have been identified, where possible. A strong, low energy absorption has been assigned as the antisymmetric stretch of the two axial carbonyls, of B1 symmetry. The positions of these bands have been considered with respect to the observed reactivities of the cis-(bidentate)M(CO)4 complexes via a path involving rate-determining dissociation of CO, and with respect to the positions of other assigned metal-carbon stretching modes for derivatives of the types LM(CO)5 and trans-L2M(CO)4. Relationships among bonding, spectra and reactivity in these complexes are discussed.

Octahedral metal carbonyls : XXXIII. Kinetics and mechanism of reactions of the group VIB metal carbonyls with tetrabutylammonium halides

The Group VIB metal carbonyls, M(CO)6 (M  Cr, Mo, W) react with tetrabutylammonium halides (X = Cl, Br, I) in chlorobenzene to afford the Bu4 N[M(CO)5X] products. For M  Mo and W, the reactions obey a rate law, [M(CO)6]/dt  k2 · [M(CO)6] · [Bu4X], while for M  Cr, an additional halide-independent term, ascribable to rate-determining dissociation of CO from the substrate, also is observed. The observed variations in rate as a function of the identity of the metal atom and the halide suggest that the second-order path involves attack of the halide at the metal atom for Mo and W, but at a carbonyl carbon for Cr. Rates of reaction increase in the order I < Br < Cl, consistent with an important influence of the steric nature of the halide upon the rate.

Octahedral metal carbonyls. 74. Estimates of solvent-metal bond strengths in (solvent)M(CO)5 complexes (solvent=benzene (M=Mo, W) and tetrachloromethane (M=Cr))

Abstract Desolvation in three (solv)M(CO)5 complexes (M=Cr, Mo, W; solv=solvent) produced after pulsed laser flash photolysis in solv/1-hexene mixtures takes place exclusively via dissociative mechanisms. The enthalpies of activation for solvent dissociation, which should closely approximate the solv-M bond strengths, have been determined for the η2-benzene-Mo and η2-benzene-W bonds, in which benzene likely is coordinated to the metal edge-on in a dihapto fashion, and tetrachloromethane-Cr bonds, which involve Cl-to-Cr coordination. These inferred bond strengths are compared to other reported metal-ligand (L) bond strengths derived from kinetics data in solution for LM(CO)5 complexes, and M-alkane bond strengths obtained from time-resolved photoacoustic calorimetric studies.

Octahedral metal carbonyls. : XXXVII. the kinetics and mechanism of reactions of amines with the group VI-B metal carbonyls☆

Abstract Studies of the rates of reaction of the Group VI-B metal hexacarbonyls (M = Cr, Mo, W) with benzylamine, cyclohexylamine and aniline (amine) support a rate law, −d[M(CO) 6 ] /dt = k 1 [M(CO) 6] + k 2 [M9CO)6] [amine]. The rate law is suggestive of two competing mechanisms, with the amine-independent path involving rate-determining dissociation of CO followed by amine uptake (D). The remarkable parallelism in rates for the competing processes for all three metals suggests the same factors to influence the rate of reaction by each path. Activation parameters for reaction of Mo(CO) 6 with benzylamine, and the relative insensitivity of the rate to the identity of a wide variety of incoming nucleophiles for the amine-dependent path support a mechanism (I d , in which M-CO bond breaking is very important. Comparisons of second-order rate constants for reactions of amines, and phosphines and phosphites indicate relatively little τ-interaction in the transition state for substitution reactions of the latter.

SOLID STATE STRUCTURE AND REACTIVITY IN SOLUTION. II. THE CRYSTAL AND MOLECULAR STRUCTURE OF (2,2,7,7-TETRAMETHYL-3,6-DITHIAOCTANE)TETRACARBONYLCHROMIUM(0)

The crystal and molecular structure of the title compound, (DTO)Cr(CO) 4 , has been determined by single-crystal X-ray diffraction techniques. The crystals are monoclinic, a = 9.583(4), b = 13.399(5) and c = 14.809(8) A , β = 101.62(4)°, space group P2 1 /n, d (calc. z = 4) = 1.32 g/cm 3 . The structure was solved by MULTAN and refined by full-matrix least-squares methods to R = 0.05 using 2274 reflections measured on a four circle diffractometer with Mo-Kα radiation. The coordination about the central chromium atom is a distorted octahedron, the greatest deviations from ideal geometry being the angle S1CrS2, 80.6(1)°, and the angle between the two mutually trans carbonyls, 169.8(2)°. Mean metal-ligand bond lenghts are CrS = 2.437(1), CrC( cis ) = 1.863(5) and CrC( trans ) = 1.820(5) A . The influence of the two bulky t-butyl substituents, bonded to S, on the octahedral geometry about Cr, and the possible influence of this molecular distortion on the reactivity of the complex via displacement of the chelating ligand by Lewis bases are discussed.