  Substitution effects of Os for Ru in {\alpha}-RuCl3 are investigated in a
wide composition range of 0 =< x =< 0.67 in Ru1-xOsxCl3 by X-ray and electron
diffraction, magnetic susceptibility, heat capacity, and Raman spectroscopy
measurements. Apart from the Kitaev physics with antiferromagnetic interactions
increasing with x, a rich phase diagram is obtained, which includes an
antiferromagnetic long-range order below 12 K for x =< 0.15, a dome-shaped
spin-singlet dimer phase below 130 K for 0.15 =< x =< 0.40, and a magnetic
short-range order for x > 0.40. A dimerization as similarly observed in
{\alpha}-RuCl3 under high pressure occurs in the spin-singlet phase. It is
suggested that Ru-Os pairs in the solid solutions tend to form dimers with
short bonds and trigger the first-order transition in the presence of
pseudo-threefold rotational symmetry for dimerization around a substituted Os
atom only at low substitutions. This is a rare example of molecular orbital
crystallization induced by elemental substitution in a highly disordered
system. The short-range order at high substitutions may be related to a
random-singlet state stabilized by bond disorder in the honeycomb net.
