The synthesis and properties of a series of unsymmetrical diphosphines o-C6H4((CH2PBu2)-Bu-t)(CH2PR2) are reported where R = C6H5 (L-a); p-CH3C6H4 (L-b); o-CH3C6H4 (L-e); o-CH3CH2C6H4 (L-d); o-CH3OC6H4 (L-e); 2,4,6-(CH3)(3)C6H4 (L-f); CH(CH3)(2) (L-g); or PR2 = PCg (L-h) where PCg is 6-phospha-2,4,8-trioxa-1,3,5,7-tetramethyladamant-6-yl. Hydromethoxycarbonylation of ethene under commercially relevant conditions is catalysed by the Pd complexes of each of the ligands La-h to give methyl propanoate in >99% selectivity with catalytic activities comparable to those obtained with o-C6H4((CH2PBu2)-Bu-t)(2) (L-1) or o-C6H4(CH(2)PCg)(2) (L-5). The catalysts derived from L-c, L-d and L-h are more active than the catalyst derived from L-a or L-1; these ligands have in common, a PR2 donor that is more bulky than the PPh2 present in L-a. Treatment of [PtCl(CH3)(cod)] with La-h gave [PtCl(CH3)(L)] as mixtures of 2 isomers 1a-h and 2a-h. The major isomer in each case was 1a-h with the CH3 ligand trans to the (PBu2)-Bu-t group; the diastereoselectivity of this reaction for products 1a-h ranges from 88% to over 99%. The crystal structures of 1b, 1c and 1e have been determined. Fluxionality associated with chelate ring inversion has been detected for 1c by variable temperature P-31 NMR spectroscopy. The P-31 NMR data are compared for the complexes [PtCl(CH3)(L)], [Pt(CH3)(2)(L)] and [PtCl2(L)] where L = L-h, L-1 or L-5. The crystal structure of [Pt(CH3)(2)(L-h)] (4h) has been determined and shows that PCg is sterically less demanding than (PBu2)-Bu-t in this complex. Treatment of 4h with HCl gave a 1:1 mixture of 1h and 2h that equilibrated over 5 h to a 70:1 mixture. Treatment of an equilibrium mixture of 1h and 2h with isotopically labelled (CO)-C-13 gas gave a single acyl complex [PtCl((COCH3)-C-13)(L-h)] (5h) with retention of configuration at Pt, i.e. the (COCH3)-C-13 is trans to the (PBu2)-Bu-t group. Mechanisms for the CO insertion are discussed which are consistent with the observed stereochemistry. The palladium complexes [PdCl(CH3)(L-h)] (7/8) also reacted with labelled (CO)-C-13 to give a single acyl species [PdCl((COCH3)-C-13)(L-h)]. Treatment of [PdCl((COCH3)-C-13)(L-h)] with MeOH rapidly gave (CH3COOMe)-C-13.