All structures show 120° Si-M-Si angles, with only slight deviations of O-M-Si and O-M-O angles from their respective ideal values (90° and 180°) for local D3h point symmetry arising from modelling coordinated THF about a three-fold rotation axis; 1-Pr shows a larger deviation from D3h in the solid state as it crystallizes in P63, in contrast with the P63/m space group of other 1-M. The relative distortions of 1-M were quantified by calculating τ5 parameters, which show the degree of trigonality in a five-coordinate complex ( 5 = ( -)/60; β is the largest and  the second-largest angle in the coordination sphere); τ5 = 1 is ideal trigonal bipyramidal and τ5 = 0 is a square based pyramid. For most of the 1-M series τ5 = 1, whereas for 1-Pr τ5 = 0.98.