Gravar-mail: [μ-Bis­(diphenyl­phosphino)methane-1:2κ(2) P:P′]nonacarbonyl-1κ(3) C,2κ(3) C,3κ(3) C-[tris­(4-methyl­phen­yl)arsine-3κAs]-triangulo-triruthenium(0)