The determination by x-ray diffraction methods of the crystal and molecular structures of some coordination compounds: a study of the stereochemistry of pentacoordination

This dissertation in concerned with the stereochemical problem of pentaco-ordination.
The early theoretical work of Pauling, Daudel and Bucher and Linnett gave conflicting predictions about the relative stabilities of the square pryramid and the trigonal bipyramid, and offered no explanation for the many distortions front these idealised geometries observed in pentaco-ordinate complexes. Due to the unsatisfactory description given by these rather simple theories alone, several workers have recently suggested other factors which they consider important in determining the relative stabilities of the possible geometries. They are:-
valency-shell electron-pair repulsions
crystal field stabilization energy
steric requirements of the ligand
packing requirements in the crystal
The crystal and molecular structures of four complexes have been analysed by X-ray diffraction techniques in order to gain further information about the relative importances of these factors in determining the stereochemistry of pentaco-ordinate complexes.
The complexes studied have the following molecular formulae:-
(TTAS)Ag-Co(CO)4 [TTAS = methyl-bis (o-dimethyl arsinophenyl) arsine]
[Pd(TPAS)Cl]ClO4.C6H6 {TPAS = o-phenylene bis-(o-dimethyl arsinophenyl arsine)}
[Co(QP)Cl] BPh4 {QP = tris (o-diphenyl phosphinophenyl) phosphine}
[tlMe21, 1 O-phen] ClO4
These complexes have between twenty-four and eighty-five atoms per asymmetric unit. Their X-ray analyses have been confused both by over-determination (of the phases) in b) and d) and by underdetermination in c). Thus the normal procedures of heavy-atom structure analysis — data processing, the solution of Patterson functions by analytical and superposition methods, Fourier methods and least squares refinement — have involved the study of badly resolved and also in c) pseudo-symmetrical electron density maps, and have been supplemented by the following work involving relatively new techniques.
A study of intensity statistics has been carried out in order to distinguish between any triclinic, monoclinic or orthorhombic space groups of crystals containing several heavy atoms which may be in any combination of special positions. A one-step computer programme has been written to calculate the absolute scale and overall temperature factor, to calculate normalised structure factors and to carry out various probability tests including (N(Z), N(F) and the calculation of theoretical and observed higher moments. A critical discussion of these methods is given.
The successful use of the anomalous dispersion technique of Raman and Ramachandran in the X-ray analysis of [Co(QP)Cl] BPh4 is described. A comparison of the phases obtained with those from refinement by Fourier methods is included, and suggestions are made concerning the general use of this technique.
An attempt to refine phases by the tangent formula in the early stages of the structure analysis of [Co(QP)Cl] BPh4 and the computer programmes written for this purpose are described.
All computer programmes have been written in C-autocode with sections in KDF.9 user code. In each case the overall strategy of the programme is fully discussed, and flow diagrams are given describing the tactics used in detail.
The structure analyses have given final reliability factors of 0.08 to 0.11.
The results of the structure analyses are briefly as follows.
The molecular structure of (TTAS)Ag-Co(Co)4 shows the cobalt has a distorted trigonal bipyramidal co-ordination, with the silver-cobalt single-bond in the apical position and the equatorial carbonyl groups bent towards the silver atom. An explanation for this stereochemistry in terms of valency-shell electron-pair repulsions is suggested. The stabilization resulting from rehybridisation and consequent decrease of non-bonding electron repulsions, and also the importance of inter-molecular contacts are discussed.
The crystal structure of [Pd(TPAS)Cl]ClO4.C6H6 has clathrated benzene molecules with a square pyramidal co-ordination palladium atom. The chlorine occupies a basal plane position, the palladium and four basal atoms are closely coplanor and the apical bond is considerably longer than the basal plane bonds. No oxygen atom from the perchlorate ion is within 3.5Å of the palladium atom, and no hydrogen atom of the ligand occupies the sixth co-ordination position to make the co-ordination octahedral. The preferred stereochemistry is described in terms of crystal field stabilisation.
The distorted trigonal bipyramidal co-ordination of cobalt in [Co(QP)Cl] BPh4 is not due to steric requirements of the ligand. The large distortion from trigonal symmetry giving one equatorial angle of 137,2° is attributed to a Jahn Teller effect.
[TlMe21,10-phen]ClO4 is not pentaco-ordinate. The thallium has a co-ordination best described as pentagonal bipyramidal with one equatorial position vacant. The perchlorate ions are weakly bidentate giving rise to a polymeric structure in the crystal. The rationale of this structure, attributed to important packing requirements, and also the non-linear thallium dimethyl group are discussed.
To conclude, it appears that in some pentaco-ordinate complexes one of the factors discussed In this thesis may clearly be most important in determining the geometry but the preferred stereochemistry will usually result from a delicate balance of several factors.