The photochemistry of ruthenium hydrides Ru(depe)(2)H-2, cis-Ru(dppe)(2)H-2 and cis-Ru(dfepe)(2)H-2 [depe = Et(2)PCH(2)CH(2)PEt(2), dppe = Ph(2)PCH(2)CH(2)PPh(2), dfepe = (C2F5)(2)PCH2CH2P(C2F5)(2)] has been studied by matrix isolation at 12 K and laser flash photolysis at ambient temperature. Both techniques yield the 4-coordinate 16-electron RuP4 species. The ethyl and phenyl species, Ru(depe)(2) and Ru(dppe)(2), exhibit very similar UV-visible spectra to Ru(dmpe)(2) [Ru(depe)(2) 475, 580, 735 nm; Ru(dppe)(2) 465, 550, 760 nm in solution]. The spectrum of Ru(dfepe)(2) is blue-shifted relative to the others (380, 450, 620 nm). The comparison of the spectra with that of [Rh(dppe)(2)](+) conclusively establishes a square-planar structure for Ru(depe)(2) and Ru(dppe)(2). The rates of reaction with added ligands are extremely sensitive to substituent. The rate constants for reaction with H-2 are Ru(dfepe)(2) 2.0 x 10(5), Ru(dppe)(2) 2.4(2) x 10(7), Ru(depe)(2) 4.0(4) x 10(8) dm(3) mol(-1) s(-1) compared to 6.8 x 10(9) dm(3) mol(-1) s(-1) for Ru(dmpe)(2). For reaction with CO, the rate constants are Ru(dfepe)(2) 1.1 x 10(4), Ru(dppe)(2) 1.0(2) x 10(7), Ru(depe)(2) 9.1(7) x 10(7) dm(3) mol(-1) s(-1) compared to 4.6 x 10(9) dm(3) mol(-1) s(-)1 for Ru(dmpe)(2). Thus reactivity increases in the order Ru(dfepe)(2) < Ru(dppe)(2) < Ru(depe)(2) < Ru(dmpe)(2) with an overall span of a factor of 34 000 for reaction with H-2 and 418 000 for reaction with CO. The rate constants for reaction of Ru(depe)(2) with C2H4 and Et(3)SiH, and for reaction of Ru(dppe)(2) with C2H4 have also been determined.