Various rules which have been used to rationalise molecular geometries are discussed. After this the results from MNDO calculations on simple carbenes, ethers and amines are presented, and these were designed to test the hypothesis that in the presence of ligands of low electronegativity lone pairs would not fulfil their stereochemical role assigned to them in the VSEPR model. Following on are MNDO calculations on cyclic molecules: the cyclopentadiene ring is planar in silylcyclopentadiene and non-planar in trimethylsilylcyclopentadiene. The calculations attempt to resolve this and in addition the nature of the fluxional exchange. The intermediates which are formed by diazepinium salts when they are protodebrominated are then discussed in the light of MNDO calculations. Single crystal x-ray work was carried out for four heterocyclic molecules. The compounds reported are: 4-phenyl-3-phenylamino-1,2,4-thiadiazolin-5-one; 5-(N-methylthiocarbamoylimino)-4-phenyl-3-phenylamino-4H-l,2,4-thiadiazoline; N,N-bis[2-(5-t-butyl-3H-1,2-dithiol-3-ylidene)ethylidene]hydrazine and N,N-dimethyl-N-[2-(5-t-butyl-3H-1,2-dithiol-3-ylidene)ethylidene]hydrazine. Details of MNDO calculations which were undertaken on molecules related to the x-ray work are then presented, and in the final chapter a discussion of the heterocyclic crystal structures appears. The first appendix reports the structure of a macrocyclic ligand, the second provides a list of publications and the third appendix is a list of the structure factors, least-squares planes and anisotropic temperature parameters for the crystal structure determinations.