The coordination chemistry of bis(di-ortho-tolylphosphino)methane (dotpm) has been studied. It is an excellent chelating ligand and a range of low-valent mononuclear complexes have been prepared; cis-[M(CO)4(η2-dotpm)] (M = Cr, Mo, W; 1–3), [CpRuCl(η2-dotpm)] (4), and cis-[MX2(η2-dotpm)] (M = Pt, X = Cl, Br, I; 5a–5c, M = Pd, X = Cl; 6). The backbone protons are relatively acidic and can be deprotonated using n-BuLi or LiN(SiMe3)2. Subsequent alkylation by RX (X = halogen; R = Me, Et, CH2Ph) affords cis-[M(CO)4(η2-Rdotpm)] (M = Cr, Mo, W, R = Me; 7–9, M = Mo, W, R = Et, CH2Ph; 12–15), [CpRuCl(η2-Medotpm)] (10), and cis-[PtI2(η2-Medotpm)] (11). Thermolysis of cis-[Mo(CO)4(η2-Medotpm)] (8) yields what is believed to be the coordinately and electronically unsaturated complex [Mo(CO)3(η2-Medotpm)] (16), suggesting that derivatives of dotpm (cone angle 194°) are bulky enough to stabilize a 16-electron complex. Crystal structures of 2, 3, 7–9, 13, and 14 have been determined (diphosphine bite angles ranging from 66.58(3)° to 70.96(5)°.Key words: diphosphine, transition metal, bulky, carbonyl, ortho-tolyl.