As one of the techniques of modifying silver halide grains so that the properties of a silver halide photographic material as a whole show as much improvements as are expected, there is well-known the technique of incorporating a substance other than silver and halide ions into silver halide grains. This art is referred to as "doping technique", the substance incorporated into silver halide grains is referred to as "dopant", and to incorporate a dopant into silver halide grains is referred to as "to dope". In particular, many researches on the techniques of doping transition metal ions have been made. As a result, it is generally recognized that the photographic properties can be modified effectively by transition metal ions got into silver halide grains as a dopant even if the ions are added in a very slight amount.
For heightening the sensitivity of silver halide emulsions, there is known the technique of doping silver halide grains with the metal complexes of the group VIII in the periodic table having cyanide ions as ligands. As dopants having cyanide ions, for instance, JP-B-48-35373 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses hexacyanoferrate complexes, such as potassium ferrocyanide and potassium ferricyanide. However, the effect of that invention is produced in only the cases using the iron ion-containing dopants, irrespective of the species of ligands. JP-B-49-14265 discloses, as a silver halide emulsion having high sensitivity under high illumination intensity, the emulsion comprising silver halide grains which are 0.9 .mu.m or below in grain size, are subjected to the addition of the metal compound in a group VIII in the periodic table in an amount of 10.sup.-6 to 10.sup.-3 mole per mole of silver ions during the formation thereof and is subjected to spectral sensitization with a merocyanine dye. According to this technique, high-speed emulsions can be obtained. In JP-A-5-66511 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and U.S. Pat. No. 5,132,203, it is demonstrated that high-speed materials are obtained by doping the sub-surface layer of silver halide grains with hexacyanoferrate(II). Further, JP-A-2-20853 discloses that silver iodochloride doped with rhenium, ruthenium, osmium and iridium complexes having cyanide ions as ligands can provide high sensitivity on silver halide emulsions. On the other hand, JP-A-121844 discloses the high-speed emulsion comprising light-sensitive silver halide grains each which is constituted of two or more parts different in halide composition and contains divalent iron ions in an amount of at least 10.sup.-7 mole/mole-Ag in the part having the halide composition lowest in band gap energy.
In order to make high-speed emulsions by means of the doping technique, the metal complex of the group VIII in the periodic table, such as hexacyanoiron(II) complexes and hexacyanoruthenium(II) complexes, are frequently employed as dopants. Many of other metal complexes are also used as dopants, and can produce not only the sensitivity increasing effect but also a wide variety of effects including improvement of reciprocity failure and increasing contrast. In U.S. Pat. No. 2,448,060, it is disclosed that the doping with platinum or palladium(III) complexes having halogen ions as ligands can sensitize emulsions. The silver halide emulsions containing cyano complexes of iron(II), iron(III) or cobalt(III) and spectral sensitizing dyes are disclosed in U.S. Pat. No. 3,790,390. The silver halide grains formed in the presence of a rhodium(III) complex having 3, 4, 5 or 6 cyano ligands are disclosed in U.S. Pat. No. 4,847,191. Those patents prove that the dopants can diminish high intensity failure. The silver halide emulsions doped with rhenium, ruthenium, osmium or iridium complexes having at least 4 cyano ligands are disclosed in European Patent 0,336,425, European Patent 0,335,426, JP-A-2-20853 and JP-A-2-20834. It is described therein that the doped silver halide emulsions are improved in storage stability of sensitivity and gradation, and reduced in low intensity failure. European Patent 0,336,427 and JP-A-2-20852 disclose the silver halide emulsions using vanadium, chromium, manganese, iron, ruthenium, osmium, rhenium and iridium complexes having the coordination number of six and containing nitrosyl or thionitrosyl ligands and thereby showing improvement in low intensity reciprocity failure without attended by lowering of medium illumination sensitivity. As the dopants other than transition metal ions, the emulsions doped with bismuth or lead ions are disclosed in U.S. Pat. No. 3,690,888, and the emulsions containing the metal ions of the group XIII or XIV in the periodic table are disclosed in JP-A-7-128778.
The metal complex dope given to silver halide grains, as mentioned above, causes various changes in photographic properties. Most of metal complexes so far used for doping silver halide grains are six-coordinate complexes (i.e., six-coordinated complexes) having an octahedron structure (i.e., an octahedral structure). This is because the six-coordinate complexes having an octahedron structure have been regarded as good dopants for a reason that, as described in J. Phys.: Condens. Matter 9 (1997) 3227-2240, when a six-coordinate metal complex having the octahedron structure, such as hexacyanoferrate(II), is added for doping silver halide grains, the complex ion [AgX.sub.6 ].sup.-5 (X=halogen ion) functions as a unit in silver halide grains to enable partial replacement of the grains by the dopants having the same structure as the aforesaid unit. With respect to the cases where silver halide grains are doped with complexes having coordination structures other than a six-coordinate octahedron structure, [PtCl.sub.4 ].sup.2- and [PdCl.sub.4 ].sup.2- are used for the doping in U.S. Pat. No. 2,448,060, [Pt(CN).sub.4 ].sup.-2, [Pd(CN).sub.4 ].sup.-2 and [Ni(CN).sub.4 ].sup.-2 in JP-A-5-346633, [CoCl.sub.4 ].sup.-2 and [Co(CN).sub.4 ].sup.-2 in JP-A-5-134344, and [Zn(CNO).sub.4 ].sup.-2 in JP-A-4-305644. However, these four-coordinate complexes (i.e., four-coordinated complexes) are each used merely as a member of related compounds for a series of doping tests wherein six-coordinate complexes having an octahedron structure are used as dopants, and a clear concept of doping silver halide with four-coordinate complexes cannot be found in those patents. In other words, it is supposed that those four-coordinate complexes are employed from the viewpoint of changing the species of metal ion or ligand. There are unknown the cases of using four-coordinate complexes other than the above-recited ones for doping silver halide grains.
With respect to the ligands of complexes used for the doping, not only cyanide ion but also ions of diverse chemical species are utilized. Besides cyanide ion, halogen ions are frequently used as ligands. For instance, hexachlororuthenate, hexachloroiridate, hexachlororhodate and hexachlororhenate are disclosed as doping complexes having a [MCl.sub.6 ].sup.n- structure, wherein M is an arbitrary metal, in JP-A-63-184740, JP-A-1-285941, JP-A-2-20852 and JP-A-2-20855. Further, European Patent 0,336,689 and JP-A-2-20855 disclose, as dopants, the six-coordinate rhenium complexes whose ligands are halogeno, nitrosyl, thionitrosyl, cyano, aquo and thiocyano. Furthermore, as emulsions having useful photographic properties, the emulsion wherein is incorporated the six-coordinate transition metal complex having carbonyl as one of the ligands and the emulsion wherein is incorporated the six-coordinate transition metal complex having oxo as two of the ligands are disclosed in JP-A-3-118535 and JP-A-3-118536 respectively. In addition, the cases wherein the complexes having heterocyclic compounds as ligands are used as dopants are disclosed in U.S. Pat. No. 5,360,712.
However, the known cases wherein the complexes having an arbitrary imidazole compound (L') as a ligand are used as dopants are only the cases disclosed in U.S. Pat. No. 5,360,712 cited above wherein [Fe(CN).sub.5 (L')].sup.3- and [Ru(CN).sub.5 (L')].sup.3- are used respectively. While the complexes having six cyano ligands and the above-recited complexes wherein cyanide ions are present as ligands can provide high sensitivity upon emulsions, they inhibit the formation of sensitized nuclei by gold sensitizers, as disclosed in JP-A-8-62761. Therefore, using cyanide ion-free complexes is desired for efficiently making high-speed emulsions. The absence of cyanide ions in complexes is desirable from the viewpoint of the toxicity of cyanide ions, too. No cases are known wherein complexes having no cyanide ions but imidazole compounds as ligands are used as dopants, whether they are four-coordinate or six-coordinate.